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llvm/lib/Transforms/Utils/SimplifyCFG.cpp
Dan Gohman 4bb31bfaa3 Fix a grammaro. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99917 91177308-0d34-0410-b5e6-96231b3b80d8
2010-03-30 20:04:57 +00:00

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//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "simplifycfg"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Type.h"
#include "llvm/DerivedTypes.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include <algorithm>
#include <functional>
#include <set>
#include <map>
using namespace llvm;
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
namespace {
class SimplifyCFGOpt {
const TargetData *const TD;
ConstantInt *GetConstantInt(Value *V);
Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values);
Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values);
bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
std::vector<ConstantInt*> &Values);
Value *isValueEqualityComparison(TerminatorInst *TI);
BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI,
std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases);
bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
BasicBlock *Pred);
bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI);
public:
explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {}
bool run(BasicBlock *BB);
};
}
/// SafeToMergeTerminators - Return true if it is safe to merge these two
/// terminator instructions together.
///
static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
if (SI1 == SI2) return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if *that* PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
if (SI1Succs.count(*I))
for (BasicBlock::iterator BBI = (*I)->begin();
isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB))
return false;
}
return true;
}
/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
/// now be entries in it from the 'NewPred' block. The values that will be
/// flowing into the PHI nodes will be the same as those coming in from
/// ExistPred, an existing predecessor of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred) {
assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
PHINode *PN;
for (BasicBlock::iterator I = Succ->begin();
(PN = dyn_cast<PHINode>(I)); ++I)
PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred);
}
/// GetIfCondition - Given a basic block (BB) with two predecessors (and
/// presumably PHI nodes in it), check to see if the merge at this block is due
/// to an "if condition". If so, return the boolean condition that determines
/// which entry into BB will be taken. Also, return by references the block
/// that will be entered from if the condition is true, and the block that will
/// be entered if the condition is false.
///
///
static Value *GetIfCondition(BasicBlock *BB,
BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
"Function can only handle blocks with 2 predecessors!");
BasicBlock *Pred1 = *pred_begin(BB);
BasicBlock *Pred2 = *++pred_begin(BB);
// We can only handle branches. Other control flow will be lowered to
// branches if possible anyway.
if (!isa<BranchInst>(Pred1->getTerminator()) ||
!isa<BranchInst>(Pred2->getTerminator()))
return 0;
BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
// Eliminate code duplication by ensuring that Pred1Br is conditional if
// either are.
if (Pred2Br->isConditional()) {
// If both branches are conditional, we don't have an "if statement". In
// reality, we could transform this case, but since the condition will be
// required anyway, we stand no chance of eliminating it, so the xform is
// probably not profitable.
if (Pred1Br->isConditional())
return 0;
std::swap(Pred1, Pred2);
std::swap(Pred1Br, Pred2Br);
}
if (Pred1Br->isConditional()) {
// If we found a conditional branch predecessor, make sure that it branches
// to BB and Pred2Br. If it doesn't, this isn't an "if statement".
if (Pred1Br->getSuccessor(0) == BB &&
Pred1Br->getSuccessor(1) == Pred2) {
IfTrue = Pred1;
IfFalse = Pred2;
} else if (Pred1Br->getSuccessor(0) == Pred2 &&
Pred1Br->getSuccessor(1) == BB) {
IfTrue = Pred2;
IfFalse = Pred1;
} else {
// We know that one arm of the conditional goes to BB, so the other must
// go somewhere unrelated, and this must not be an "if statement".
return 0;
}
// The only thing we have to watch out for here is to make sure that Pred2
// doesn't have incoming edges from other blocks. If it does, the condition
// doesn't dominate BB.
if (++pred_begin(Pred2) != pred_end(Pred2))
return 0;
return Pred1Br->getCondition();
}
// Ok, if we got here, both predecessors end with an unconditional branch to
// BB. Don't panic! If both blocks only have a single (identical)
// predecessor, and THAT is a conditional branch, then we're all ok!
if (pred_begin(Pred1) == pred_end(Pred1) ||
++pred_begin(Pred1) != pred_end(Pred1) ||
pred_begin(Pred2) == pred_end(Pred2) ||
++pred_begin(Pred2) != pred_end(Pred2) ||
*pred_begin(Pred1) != *pred_begin(Pred2))
return 0;
// Otherwise, if this is a conditional branch, then we can use it!
BasicBlock *CommonPred = *pred_begin(Pred1);
if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
assert(BI->isConditional() && "Two successors but not conditional?");
if (BI->getSuccessor(0) == Pred1) {
IfTrue = Pred1;
IfFalse = Pred2;
} else {
IfTrue = Pred2;
IfFalse = Pred1;
}
return BI->getCondition();
}
return 0;
}
/// DominatesMergePoint - If we have a merge point of an "if condition" as
/// accepted above, return true if the specified value dominates the block. We
/// don't handle the true generality of domination here, just a special case
/// which works well enough for us.
///
/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
/// see if V (which must be an instruction) is cheap to compute and is
/// non-trapping. If both are true, the instruction is inserted into the set
/// and true is returned.
static bool DominatesMergePoint(Value *V, BasicBlock *BB,
std::set<Instruction*> *AggressiveInsts) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I) {
// Non-instructions all dominate instructions, but not all constantexprs
// can be executed unconditionally.
if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
if (C->canTrap())
return false;
return true;
}
BasicBlock *PBB = I->getParent();
// We don't want to allow weird loops that might have the "if condition" in
// the bottom of this block.
if (PBB == BB) return false;
// If this instruction is defined in a block that contains an unconditional
// branch to BB, then it must be in the 'conditional' part of the "if
// statement".
if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
if (!AggressiveInsts) return false;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if it's a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
if (!I->isSafeToSpeculativelyExecute())
return false;
switch (I->getOpcode()) {
default: return false; // Cannot hoist this out safely.
case Instruction::Load: {
// We have to check to make sure there are no instructions before the
// load in its basic block, as we are going to hoist the loop out to
// its predecessor.
BasicBlock::iterator IP = PBB->begin();
while (isa<DbgInfoIntrinsic>(IP))
IP++;
if (IP != BasicBlock::iterator(I))
return false;
break;
}
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
case Instruction::ICmp:
break; // These are all cheap and non-trapping instructions.
}
// Okay, we can only really hoist these out if their operands are not
// defined in the conditional region.
for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
if (!DominatesMergePoint(*i, BB, 0))
return false;
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts->insert(I);
}
return true;
}
/// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr
/// and PointerNullValue. Return NULL if value is not a constant int.
ConstantInt *SimplifyCFGOpt::GetConstantInt(Value *V) {
// Normal constant int.
ConstantInt *CI = dyn_cast<ConstantInt>(V);
if (CI || !TD || !isa<Constant>(V) || !V->getType()->isPointerTy())
return CI;
// This is some kind of pointer constant. Turn it into a pointer-sized
// ConstantInt if possible.
const IntegerType *PtrTy = TD->getIntPtrType(V->getContext());
// Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
if (isa<ConstantPointerNull>(V))
return ConstantInt::get(PtrTy, 0);
// IntToPtr const int.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
if (CE->getOpcode() == Instruction::IntToPtr)
if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
// The constant is very likely to have the right type already.
if (CI->getType() == PtrTy)
return CI;
else
return cast<ConstantInt>
(ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
}
return 0;
}
/// GatherConstantSetEQs - Given a potentially 'or'd together collection of
/// icmp_eq instructions that compare a value against a constant, return the
/// value being compared, and stick the constant into the Values vector.
Value *SimplifyCFGOpt::
GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values) {
if (Instruction *Inst = dyn_cast<Instruction>(V)) {
if (Inst->getOpcode() == Instruction::ICmp &&
cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_EQ) {
if (ConstantInt *C = GetConstantInt(Inst->getOperand(1))) {
Values.push_back(C);
return Inst->getOperand(0);
} else if (ConstantInt *C = GetConstantInt(Inst->getOperand(0))) {
Values.push_back(C);
return Inst->getOperand(1);
}
} else if (Inst->getOpcode() == Instruction::Or) {
if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
if (LHS == RHS)
return LHS;
}
}
return 0;
}
/// GatherConstantSetNEs - Given a potentially 'and'd together collection of
/// setne instructions that compare a value against a constant, return the value
/// being compared, and stick the constant into the Values vector.
Value *SimplifyCFGOpt::
GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values) {
if (Instruction *Inst = dyn_cast<Instruction>(V)) {
if (Inst->getOpcode() == Instruction::ICmp &&
cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_NE) {
if (ConstantInt *C = GetConstantInt(Inst->getOperand(1))) {
Values.push_back(C);
return Inst->getOperand(0);
} else if (ConstantInt *C = GetConstantInt(Inst->getOperand(0))) {
Values.push_back(C);
return Inst->getOperand(1);
}
} else if (Inst->getOpcode() == Instruction::And) {
if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
if (LHS == RHS)
return LHS;
}
}
return 0;
}
/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
/// bunch of comparisons of one value against constants, return the value and
/// the constants being compared.
bool SimplifyCFGOpt::GatherValueComparisons(Instruction *Cond, Value *&CompVal,
std::vector<ConstantInt*> &Values) {
if (Cond->getOpcode() == Instruction::Or) {
CompVal = GatherConstantSetEQs(Cond, Values);
// Return true to indicate that the condition is true if the CompVal is
// equal to one of the constants.
return true;
} else if (Cond->getOpcode() == Instruction::And) {
CompVal = GatherConstantSetNEs(Cond, Values);
// Return false to indicate that the condition is false if the CompVal is
// equal to one of the constants.
return false;
}
return false;
}
static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) {
Instruction* Cond = 0;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cond = dyn_cast<Instruction>(SI->getCondition());
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
}
TI->eraseFromParent();
if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond);
}
/// isValueEqualityComparison - Return true if the specified terminator checks
/// to see if a value is equal to constant integer value.
Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) {
Value *CV = 0;
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// Do not permit merging of large switch instructions into their
// predecessors unless there is only one predecessor.
if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()),
pred_end(SI->getParent())) <= 128)
CV = SI->getCondition();
} else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional() && BI->getCondition()->hasOneUse())
if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
ICI->getPredicate() == ICmpInst::ICMP_NE) &&
GetConstantInt(ICI->getOperand(1)))
CV = ICI->getOperand(0);
// Unwrap any lossless ptrtoint cast.
if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext()))
if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV))
CV = PTII->getOperand(0);
return CV;
}
/// GetValueEqualityComparisonCases - Given a value comparison instruction,
/// decode all of the 'cases' that it represents and return the 'default' block.
BasicBlock *SimplifyCFGOpt::
GetValueEqualityComparisonCases(TerminatorInst *TI,
std::vector<std::pair<ConstantInt*,
BasicBlock*> > &Cases) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cases.reserve(SI->getNumCases());
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
return SI->getDefaultDest();
}
BranchInst *BI = cast<BranchInst>(TI);
ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1)),
BI->getSuccessor(ICI->getPredicate() ==
ICmpInst::ICMP_NE)));
return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
}
/// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries
/// in the list that match the specified block.
static void EliminateBlockCases(BasicBlock *BB,
std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
for (unsigned i = 0, e = Cases.size(); i != e; ++i)
if (Cases[i].second == BB) {
Cases.erase(Cases.begin()+i);
--i; --e;
}
}
/// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
/// well.
static bool
ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
// Make V1 be smaller than V2.
if (V1->size() > V2->size())
std::swap(V1, V2);
if (V1->size() == 0) return false;
if (V1->size() == 1) {
// Just scan V2.
ConstantInt *TheVal = (*V1)[0].first;
for (unsigned i = 0, e = V2->size(); i != e; ++i)
if (TheVal == (*V2)[i].first)
return true;
}
// Otherwise, just sort both lists and compare element by element.
std::sort(V1->begin(), V1->end());
std::sort(V2->begin(), V2->end());
unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
while (i1 != e1 && i2 != e2) {
if ((*V1)[i1].first == (*V2)[i2].first)
return true;
if ((*V1)[i1].first < (*V2)[i2].first)
++i1;
else
++i2;
}
return false;
}
/// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
/// terminator instruction and its block is known to only have a single
/// predecessor block, check to see if that predecessor is also a value
/// comparison with the same value, and if that comparison determines the
/// outcome of this comparison. If so, simplify TI. This does a very limited
/// form of jump threading.
bool SimplifyCFGOpt::
SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
BasicBlock *Pred) {
Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
if (!PredVal) return false; // Not a value comparison in predecessor.
Value *ThisVal = isValueEqualityComparison(TI);
assert(ThisVal && "This isn't a value comparison!!");
if (ThisVal != PredVal) return false; // Different predicates.
// Find out information about when control will move from Pred to TI's block.
std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
PredCases);
EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
// Find information about how control leaves this block.
std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
// If TI's block is the default block from Pred's comparison, potentially
// simplify TI based on this knowledge.
if (PredDef == TI->getParent()) {
// If we are here, we know that the value is none of those cases listed in
// PredCases. If there are any cases in ThisCases that are in PredCases, we
// can simplify TI.
if (ValuesOverlap(PredCases, ThisCases)) {
if (isa<BranchInst>(TI)) {
// Okay, one of the successors of this condbr is dead. Convert it to a
// uncond br.
assert(ThisCases.size() == 1 && "Branch can only have one case!");
// Insert the new branch.
Instruction *NI = BranchInst::Create(ThisDef, TI);
(void) NI;
// Remove PHI node entries for the dead edge.
ThisCases[0].second->removePredecessor(TI->getParent());
DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
EraseTerminatorInstAndDCECond(TI);
return true;
} else {
SwitchInst *SI = cast<SwitchInst>(TI);
// Okay, TI has cases that are statically dead, prune them away.
SmallPtrSet<Constant*, 16> DeadCases;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
DeadCases.insert(PredCases[i].first);
DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI);
for (unsigned i = SI->getNumCases()-1; i != 0; --i)
if (DeadCases.count(SI->getCaseValue(i))) {
SI->getSuccessor(i)->removePredecessor(TI->getParent());
SI->removeCase(i);
}
DEBUG(dbgs() << "Leaving: " << *TI << "\n");
return true;
}
}
} else {
// Otherwise, TI's block must correspond to some matched value. Find out
// which value (or set of values) this is.
ConstantInt *TIV = 0;
BasicBlock *TIBB = TI->getParent();
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].second == TIBB) {
if (TIV == 0)
TIV = PredCases[i].first;
else
return false; // Cannot handle multiple values coming to this block.
}
assert(TIV && "No edge from pred to succ?");
// Okay, we found the one constant that our value can be if we get into TI's
// BB. Find out which successor will unconditionally be branched to.
BasicBlock *TheRealDest = 0;
for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
if (ThisCases[i].first == TIV) {
TheRealDest = ThisCases[i].second;
break;
}
// If not handled by any explicit cases, it is handled by the default case.
if (TheRealDest == 0) TheRealDest = ThisDef;
// Remove PHI node entries for dead edges.
BasicBlock *CheckEdge = TheRealDest;
for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
if (*SI != CheckEdge)
(*SI)->removePredecessor(TIBB);
else
CheckEdge = 0;
// Insert the new branch.
Instruction *NI = BranchInst::Create(TheRealDest, TI);
(void) NI;
DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
<< "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
EraseTerminatorInstAndDCECond(TI);
return true;
}
return false;
}
namespace {
/// ConstantIntOrdering - This class implements a stable ordering of constant
/// integers that does not depend on their address. This is important for
/// applications that sort ConstantInt's to ensure uniqueness.
struct ConstantIntOrdering {
bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
return LHS->getValue().ult(RHS->getValue());
}
};
}
/// FoldValueComparisonIntoPredecessors - The specified terminator is a value
/// equality comparison instruction (either a switch or a branch on "X == c").
/// See if any of the predecessors of the terminator block are value comparisons
/// on the same value. If so, and if safe to do so, fold them together.
bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
BasicBlock *BB = TI->getParent();
Value *CV = isValueEqualityComparison(TI); // CondVal
assert(CV && "Not a comparison?");
bool Changed = false;
SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.pop_back_val();
// See if the predecessor is a comparison with the same value.
TerminatorInst *PTI = Pred->getTerminator();
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
// Figure out which 'cases' to copy from SI to PSI.
std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
// Based on whether the default edge from PTI goes to BB or not, fill in
// PredCases and PredDefault with the new switch cases we would like to
// build.
SmallVector<BasicBlock*, 8> NewSuccessors;
if (PredDefault == BB) {
// If this is the default destination from PTI, only the edges in TI
// that don't occur in PTI, or that branch to BB will be activated.
std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].second != BB)
PTIHandled.insert(PredCases[i].first);
else {
// The default destination is BB, we don't need explicit targets.
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i; --e;
}
// Reconstruct the new switch statement we will be building.
if (PredDefault != BBDefault) {
PredDefault->removePredecessor(Pred);
PredDefault = BBDefault;
NewSuccessors.push_back(BBDefault);
}
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (!PTIHandled.count(BBCases[i].first) &&
BBCases[i].second != BBDefault) {
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].second);
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].second == BB) {
PTIHandled.insert(PredCases[i].first);
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i; --e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (PTIHandled.count(BBCases[i].first)) {
// If this is one we are capable of getting...
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].second);
PTIHandled.erase(BBCases[i].first);// This constant is taken care of
}
// If there are any constants vectored to BB that TI doesn't handle,
// they must go to the default destination of TI.
for (std::set<ConstantInt*, ConstantIntOrdering>::iterator I =
PTIHandled.begin(),
E = PTIHandled.end(); I != E; ++I) {
PredCases.push_back(std::make_pair(*I, BBDefault));
NewSuccessors.push_back(BBDefault);
}
}
// Okay, at this point, we know which new successor Pred will get. Make
// sure we update the number of entries in the PHI nodes for these
// successors.
for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
// Convert pointer to int before we switch.
if (CV->getType()->isPointerTy()) {
assert(TD && "Cannot switch on pointer without TargetData");
CV = new PtrToIntInst(CV, TD->getIntPtrType(CV->getContext()),
"magicptr", PTI);
}
// Now that the successors are updated, create the new Switch instruction.
SwitchInst *NewSI = SwitchInst::Create(CV, PredDefault,
PredCases.size(), PTI);
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
NewSI->addCase(PredCases[i].first, PredCases[i].second);
EraseTerminatorInstAndDCECond(PTI);
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = 0;
for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
if (NewSI->getSuccessor(i) == BB) {
if (InfLoopBlock == 0) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock = BasicBlock::Create(BB->getContext(),
"infloop", BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
}
NewSI->setSuccessor(i, InfLoopBlock);
}
Changed = true;
}
}
return Changed;
}
// isSafeToHoistInvoke - If we would need to insert a select that uses the
// value of this invoke (comments in HoistThenElseCodeToIf explain why we
// would need to do this), we can't hoist the invoke, as there is nowhere
// to put the select in this case.
static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
Instruction *I1, Instruction *I2) {
for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
PHINode *PN;
for (BasicBlock::iterator BBI = SI->begin();
(PN = dyn_cast<PHINode>(BBI)); ++BBI) {
Value *BB1V = PN->getIncomingValueForBlock(BB1);
Value *BB2V = PN->getIncomingValueForBlock(BB2);
if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) {
return false;
}
}
}
return true;
}
/// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
/// BB2, hoist any common code in the two blocks up into the branch block. The
/// caller of this function guarantees that BI's block dominates BB1 and BB2.
static bool HoistThenElseCodeToIf(BranchInst *BI) {
// This does very trivial matching, with limited scanning, to find identical
// instructions in the two blocks. In particular, we don't want to get into
// O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
// such, we currently just scan for obviously identical instructions in an
// identical order.
BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
BasicBlock::iterator BB1_Itr = BB1->begin();
BasicBlock::iterator BB2_Itr = BB2->begin();
Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++;
while (isa<DbgInfoIntrinsic>(I1))
I1 = BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = BB2_Itr++;
if (I1->getOpcode() != I2->getOpcode() || isa<PHINode>(I1) ||
!I1->isIdenticalToWhenDefined(I2) ||
(isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)))
return false;
// If we get here, we can hoist at least one instruction.
BasicBlock *BIParent = BI->getParent();
do {
// If we are hoisting the terminator instruction, don't move one (making a
// broken BB), instead clone it, and remove BI.
if (isa<TerminatorInst>(I1))
goto HoistTerminator;
// For a normal instruction, we just move one to right before the branch,
// then replace all uses of the other with the first. Finally, we remove
// the now redundant second instruction.
BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
if (!I2->use_empty())
I2->replaceAllUsesWith(I1);
I1->intersectOptionalDataWith(I2);
BB2->getInstList().erase(I2);
I1 = BB1_Itr++;
while (isa<DbgInfoIntrinsic>(I1))
I1 = BB1_Itr++;
I2 = BB2_Itr++;
while (isa<DbgInfoIntrinsic>(I2))
I2 = BB2_Itr++;
} while (I1->getOpcode() == I2->getOpcode() &&
I1->isIdenticalToWhenDefined(I2));
return true;
HoistTerminator:
// It may not be possible to hoist an invoke.
if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
return true;
// Okay, it is safe to hoist the terminator.
Instruction *NT = I1->clone();
BIParent->getInstList().insert(BI, NT);
if (!NT->getType()->isVoidTy()) {
I1->replaceAllUsesWith(NT);
I2->replaceAllUsesWith(NT);
NT->takeName(I1);
}
// Hoisting one of the terminators from our successor is a great thing.
// Unfortunately, the successors of the if/else blocks may have PHI nodes in
// them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
// nodes, so we insert select instruction to compute the final result.
std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
PHINode *PN;
for (BasicBlock::iterator BBI = SI->begin();
(PN = dyn_cast<PHINode>(BBI)); ++BBI) {
Value *BB1V = PN->getIncomingValueForBlock(BB1);
Value *BB2V = PN->getIncomingValueForBlock(BB2);
if (BB1V != BB2V) {
// These values do not agree. Insert a select instruction before NT
// that determines the right value.
SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
if (SI == 0)
SI = SelectInst::Create(BI->getCondition(), BB1V, BB2V,
BB1V->getName()+"."+BB2V->getName(), NT);
// Make the PHI node use the select for all incoming values for BB1/BB2
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
PN->setIncomingValue(i, SI);
}
}
}
// Update any PHI nodes in our new successors.
for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
AddPredecessorToBlock(*SI, BIParent, BB1);
EraseTerminatorInstAndDCECond(BI);
return true;
}
/// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1
/// and an BB2 and the only successor of BB1 is BB2, hoist simple code
/// (for now, restricted to a single instruction that's side effect free) from
/// the BB1 into the branch block to speculatively execute it.
static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
// Only speculatively execution a single instruction (not counting the
// terminator) for now.
Instruction *HInst = NULL;
Instruction *Term = BB1->getTerminator();
for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end();
BBI != BBE; ++BBI) {
Instruction *I = BBI;
// Skip debug info.
if (isa<DbgInfoIntrinsic>(I)) continue;
if (I == Term) break;
if (!HInst)
HInst = I;
else
return false;
}
if (!HInst)
return false;
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
if (isa<Instruction>(BrCond) &&
cast<Instruction>(BrCond)->getOpcode() == Instruction::FCmp)
return false;
// If BB1 is actually on the false edge of the conditional branch, remember
// to swap the select operands later.
bool Invert = false;
if (BB1 != BI->getSuccessor(0)) {
assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?");
Invert = true;
}
// Turn
// BB:
// %t1 = icmp
// br i1 %t1, label %BB1, label %BB2
// BB1:
// %t3 = add %t2, c
// br label BB2
// BB2:
// =>
// BB:
// %t1 = icmp
// %t4 = add %t2, c
// %t3 = select i1 %t1, %t2, %t3
switch (HInst->getOpcode()) {
default: return false; // Not safe / profitable to hoist.
case Instruction::Add:
case Instruction::Sub:
// Not worth doing for vector ops.
if (HInst->getType()->isVectorTy())
return false;
break;
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::LShr:
case Instruction::AShr:
// Don't mess with vector operations.
if (HInst->getType()->isVectorTy())
return false;
break; // These are all cheap and non-trapping instructions.
}
// If the instruction is obviously dead, don't try to predicate it.
if (HInst->use_empty()) {
HInst->eraseFromParent();
return true;
}
// Can we speculatively execute the instruction? And what is the value
// if the condition is false? Consider the phi uses, if the incoming value
// from the "if" block are all the same V, then V is the value of the
// select if the condition is false.
BasicBlock *BIParent = BI->getParent();
SmallVector<PHINode*, 4> PHIUses;
Value *FalseV = NULL;
BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end();
UI != E; ++UI) {
// Ignore any user that is not a PHI node in BB2. These can only occur in
// unreachable blocks, because they would not be dominated by the instr.
PHINode *PN = dyn_cast<PHINode>(UI);
if (!PN || PN->getParent() != BB2)
return false;
PHIUses.push_back(PN);
Value *PHIV = PN->getIncomingValueForBlock(BIParent);
if (!FalseV)
FalseV = PHIV;
else if (FalseV != PHIV)
return false; // Inconsistent value when condition is false.
}
assert(FalseV && "Must have at least one user, and it must be a PHI");
// Do not hoist the instruction if any of its operands are defined but not
// used in this BB. The transformation will prevent the operand from
// being sunk into the use block.
for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
i != e; ++i) {
Instruction *OpI = dyn_cast<Instruction>(*i);
if (OpI && OpI->getParent() == BIParent &&
!OpI->isUsedInBasicBlock(BIParent))
return false;
}
// If we get here, we can hoist the instruction. Try to place it
// before the icmp instruction preceding the conditional branch.
BasicBlock::iterator InsertPos = BI;
if (InsertPos != BIParent->begin())
--InsertPos;
// Skip debug info between condition and branch.
while (InsertPos != BIParent->begin() && isa<DbgInfoIntrinsic>(InsertPos))
--InsertPos;
if (InsertPos == BrCond && !isa<PHINode>(BrCond)) {
SmallPtrSet<Instruction *, 4> BB1Insns;
for(BasicBlock::iterator BB1I = BB1->begin(), BB1E = BB1->end();
BB1I != BB1E; ++BB1I)
BB1Insns.insert(BB1I);
for(Value::use_iterator UI = BrCond->use_begin(), UE = BrCond->use_end();
UI != UE; ++UI) {
Instruction *Use = cast<Instruction>(*UI);
if (BB1Insns.count(Use)) {
// If BrCond uses the instruction that place it just before
// branch instruction.
InsertPos = BI;
break;
}
}
} else
InsertPos = BI;
BIParent->getInstList().splice(InsertPos, BB1->getInstList(), HInst);
// Create a select whose true value is the speculatively executed value and
// false value is the previously determined FalseV.
SelectInst *SI;
if (Invert)
SI = SelectInst::Create(BrCond, FalseV, HInst,
FalseV->getName() + "." + HInst->getName(), BI);
else
SI = SelectInst::Create(BrCond, HInst, FalseV,
HInst->getName() + "." + FalseV->getName(), BI);
// Make the PHI node use the select for all incoming values for "then" and
// "if" blocks.
for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) {
PHINode *PN = PHIUses[i];
for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j)
if (PN->getIncomingBlock(j) == BB1 ||
PN->getIncomingBlock(j) == BIParent)
PN->setIncomingValue(j, SI);
}
++NumSpeculations;
return true;
}
/// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
/// across this block.
static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
BranchInst *BI = cast<BranchInst>(BB->getTerminator());
unsigned Size = 0;
for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
if (isa<DbgInfoIntrinsic>(BBI))
continue;
if (Size > 10) return false; // Don't clone large BB's.
++Size;
// We can only support instructions that do not define values that are
// live outside of the current basic block.
for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
UI != E; ++UI) {
Instruction *U = cast<Instruction>(*UI);
if (U->getParent() != BB || isa<PHINode>(U)) return false;
}
// Looks ok, continue checking.
}
return true;
}
/// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
/// that is defined in the same block as the branch and if any PHI entries are
/// constants, thread edges corresponding to that entry to be branches to their
/// ultimate destination.
static bool FoldCondBranchOnPHI(BranchInst *BI) {
BasicBlock *BB = BI->getParent();
PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
// NOTE: we currently cannot transform this case if the PHI node is used
// outside of the block.
if (!PN || PN->getParent() != BB || !PN->hasOneUse())
return false;
// Degenerate case of a single entry PHI.
if (PN->getNumIncomingValues() == 1) {
FoldSingleEntryPHINodes(PN->getParent());
return true;
}
// Now we know that this block has multiple preds and two succs.
if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
// Okay, this is a simple enough basic block. See if any phi values are
// constants.
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
ConstantInt *CB;
if ((CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i))) &&
CB->getType()->isIntegerTy(1)) {
// Okay, we now know that all edges from PredBB should be revectored to
// branch to RealDest.
BasicBlock *PredBB = PN->getIncomingBlock(i);
BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
if (RealDest == BB) continue; // Skip self loops.
// The dest block might have PHI nodes, other predecessors and other
// difficult cases. Instead of being smart about this, just insert a new
// block that jumps to the destination block, effectively splitting
// the edge we are about to create.
BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(),
RealDest->getName()+".critedge",
RealDest->getParent(), RealDest);
BranchInst::Create(RealDest, EdgeBB);
PHINode *PN;
for (BasicBlock::iterator BBI = RealDest->begin();
(PN = dyn_cast<PHINode>(BBI)); ++BBI) {
Value *V = PN->getIncomingValueForBlock(BB);
PN->addIncoming(V, EdgeBB);
}
// BB may have instructions that are being threaded over. Clone these
// instructions into EdgeBB. We know that there will be no uses of the
// cloned instructions outside of EdgeBB.
BasicBlock::iterator InsertPt = EdgeBB->begin();
std::map<Value*, Value*> TranslateMap; // Track translated values.
for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
} else {
// Clone the instruction.
Instruction *N = BBI->clone();
if (BBI->hasName()) N->setName(BBI->getName()+".c");
// Update operands due to translation.
for (User::op_iterator i = N->op_begin(), e = N->op_end();
i != e; ++i) {
std::map<Value*, Value*>::iterator PI =
TranslateMap.find(*i);
if (PI != TranslateMap.end())
*i = PI->second;
}
// Check for trivial simplification.
if (Constant *C = ConstantFoldInstruction(N)) {
TranslateMap[BBI] = C;
delete N; // Constant folded away, don't need actual inst
} else {
// Insert the new instruction into its new home.
EdgeBB->getInstList().insert(InsertPt, N);
if (!BBI->use_empty())
TranslateMap[BBI] = N;
}
}
}
// Loop over all of the edges from PredBB to BB, changing them to branch
// to EdgeBB instead.
TerminatorInst *PredBBTI = PredBB->getTerminator();
for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
if (PredBBTI->getSuccessor(i) == BB) {
BB->removePredecessor(PredBB);
PredBBTI->setSuccessor(i, EdgeBB);
}
// Recurse, simplifying any other constants.
return FoldCondBranchOnPHI(BI) | true;
}
}
return false;
}
/// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry
/// PHI node, see if we can eliminate it.
static bool FoldTwoEntryPHINode(PHINode *PN) {
// Ok, this is a two entry PHI node. Check to see if this is a simple "if
// statement", which has a very simple dominance structure. Basically, we
// are trying to find the condition that is being branched on, which
// subsequently causes this merge to happen. We really want control
// dependence information for this check, but simplifycfg can't keep it up
// to date, and this catches most of the cases we care about anyway.
//
BasicBlock *BB = PN->getParent();
BasicBlock *IfTrue, *IfFalse;
Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
if (!IfCond) return false;
// Okay, we found that we can merge this two-entry phi node into a select.
// Doing so would require us to fold *all* two entry phi nodes in this block.
// At some point this becomes non-profitable (particularly if the target
// doesn't support cmov's). Only do this transformation if there are two or
// fewer PHI nodes in this block.
unsigned NumPhis = 0;
for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
if (NumPhis > 2)
return false;
DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: "
<< IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
// Loop over the PHI's seeing if we can promote them all to select
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
std::set<Instruction*> AggressiveInsts;
BasicBlock::iterator AfterPHIIt = BB->begin();
while (isa<PHINode>(AfterPHIIt)) {
PHINode *PN = cast<PHINode>(AfterPHIIt++);
if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) {
if (PN->getIncomingValue(0) != PN)
PN->replaceAllUsesWith(PN->getIncomingValue(0));
else
PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
} else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
&AggressiveInsts) ||
!DominatesMergePoint(PN->getIncomingValue(1), BB,
&AggressiveInsts)) {
return false;
}
}
// If we all PHI nodes are promotable, check to make sure that all
// instructions in the predecessor blocks can be promoted as well. If
// not, we won't be able to get rid of the control flow, so it's not
// worth promoting to select instructions.
BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
PN = cast<PHINode>(BB->begin());
BasicBlock *Pred = PN->getIncomingBlock(0);
if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
IfBlock1 = Pred;
DomBlock = *pred_begin(Pred);
for (BasicBlock::iterator I = Pred->begin();
!isa<TerminatorInst>(I); ++I)
if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control
// flow, so the xform is not worth it.
return false;
}
}
Pred = PN->getIncomingBlock(1);
if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
IfBlock2 = Pred;
DomBlock = *pred_begin(Pred);
for (BasicBlock::iterator I = Pred->begin();
!isa<TerminatorInst>(I); ++I)
if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control
// flow, so the xform is not worth it.
return false;
}
}
// If we can still promote the PHI nodes after this gauntlet of tests,
// do all of the PHI's now.
// Move all 'aggressive' instructions, which are defined in the
// conditional parts of the if's up to the dominating block.
if (IfBlock1) {
DomBlock->getInstList().splice(DomBlock->getTerminator(),
IfBlock1->getInstList(),
IfBlock1->begin(),
IfBlock1->getTerminator());
}
if (IfBlock2) {
DomBlock->getInstList().splice(DomBlock->getTerminator(),
IfBlock2->getInstList(),
IfBlock2->begin(),
IfBlock2->getTerminator());
}
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
// Change the PHI node into a select instruction.
Value *TrueVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
Value *NV = SelectInst::Create(IfCond, TrueVal, FalseVal, "", AfterPHIIt);
PN->replaceAllUsesWith(NV);
NV->takeName(PN);
BB->getInstList().erase(PN);
}
return true;
}
/// isTerminatorFirstRelevantInsn - Return true if Term is very first
/// instruction ignoring Phi nodes and dbg intrinsics.
static bool isTerminatorFirstRelevantInsn(BasicBlock *BB, Instruction *Term) {
BasicBlock::iterator BBI = Term;
while (BBI != BB->begin()) {
--BBI;
if (!isa<DbgInfoIntrinsic>(BBI))
break;
}
if (isa<PHINode>(BBI) || &*BBI == Term || isa<DbgInfoIntrinsic>(BBI))
return true;
return false;
}
/// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes
/// to two returning blocks, try to merge them together into one return,
/// introducing a select if the return values disagree.
static bool SimplifyCondBranchToTwoReturns(BranchInst *BI) {
assert(BI->isConditional() && "Must be a conditional branch");
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
// Check to ensure both blocks are empty (just a return) or optionally empty
// with PHI nodes. If there are other instructions, merging would cause extra
// computation on one path or the other.
if (!isTerminatorFirstRelevantInsn(TrueSucc, TrueRet))
return false;
if (!isTerminatorFirstRelevantInsn(FalseSucc, FalseRet))
return false;
// Okay, we found a branch that is going to two return nodes. If
// there is no return value for this function, just change the
// branch into a return.
if (FalseRet->getNumOperands() == 0) {
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
ReturnInst::Create(BI->getContext(), 0, BI);
EraseTerminatorInstAndDCECond(BI);
return true;
}
// Otherwise, figure out what the true and false return values are
// so we can insert a new select instruction.
Value *TrueValue = TrueRet->getReturnValue();
Value *FalseValue = FalseRet->getReturnValue();
// Unwrap any PHI nodes in the return blocks.
if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
if (TVPN->getParent() == TrueSucc)
TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
if (FVPN->getParent() == FalseSucc)
FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
// In order for this transformation to be safe, we must be able to
// unconditionally execute both operands to the return. This is
// normally the case, but we could have a potentially-trapping
// constant expression that prevents this transformation from being
// safe.
if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
if (TCV->canTrap())
return false;
if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
if (FCV->canTrap())
return false;
// Okay, we collected all the mapped values and checked them for sanity, and
// defined to really do this transformation. First, update the CFG.
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
// Insert select instructions where needed.
Value *BrCond = BI->getCondition();
if (TrueValue) {
// Insert a select if the results differ.
if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
} else if (isa<UndefValue>(TrueValue)) {
TrueValue = FalseValue;
} else {
TrueValue = SelectInst::Create(BrCond, TrueValue,
FalseValue, "retval", BI);
}
}
Value *RI = !TrueValue ?
ReturnInst::Create(BI->getContext(), BI) :
ReturnInst::Create(BI->getContext(), TrueValue, BI);
(void) RI;
DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
<< "\n " << *BI << "NewRet = " << *RI
<< "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc);
EraseTerminatorInstAndDCECond(BI);
return true;
}
/// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch,
/// and if a predecessor branches to us and one of our successors, fold the
/// setcc into the predecessor and use logical operations to pick the right
/// destination.
bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
BasicBlock *BB = BI->getParent();
Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
if (Cond == 0) return false;
// Only allow this if the condition is a simple instruction that can be
// executed unconditionally. It must be in the same block as the branch, and
// must be at the front of the block.
BasicBlock::iterator FrontIt = BB->front();
// Ignore dbg intrinsics.
while(isa<DbgInfoIntrinsic>(FrontIt))
++FrontIt;
if ((!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
Cond->getParent() != BB || &*FrontIt != Cond || !Cond->hasOneUse()) {
return false;
}
// Make sure the instruction after the condition is the cond branch.
BasicBlock::iterator CondIt = Cond; ++CondIt;
// Ingore dbg intrinsics.
while(isa<DbgInfoIntrinsic>(CondIt))
++CondIt;
if (&*CondIt != BI) {
assert (!isa<DbgInfoIntrinsic>(CondIt) && "Hey do not forget debug info!");
return false;
}
// Cond is known to be a compare or binary operator. Check to make sure that
// neither operand is a potentially-trapping constant expression.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
if (CE->canTrap())
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
if (CE->canTrap())
return false;
// Finally, don't infinitely unroll conditional loops.
BasicBlock *TrueDest = BI->getSuccessor(0);
BasicBlock *FalseDest = BI->getSuccessor(1);
if (TrueDest == BB || FalseDest == BB)
return false;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
BasicBlock *PredBlock = *PI;
BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
// Check that we have two conditional branches. If there is a PHI node in
// the common successor, verify that the same value flows in from both
// blocks.
if (PBI == 0 || PBI->isUnconditional() ||
!SafeToMergeTerminators(BI, PBI))
continue;
Instruction::BinaryOps Opc;
bool InvertPredCond = false;
if (PBI->getSuccessor(0) == TrueDest)
Opc = Instruction::Or;
else if (PBI->getSuccessor(1) == FalseDest)
Opc = Instruction::And;
else if (PBI->getSuccessor(0) == FalseDest)
Opc = Instruction::And, InvertPredCond = true;
else if (PBI->getSuccessor(1) == TrueDest)
Opc = Instruction::Or, InvertPredCond = true;
else
continue;
DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
// If we need to invert the condition in the pred block to match, do so now.
if (InvertPredCond) {
Value *NewCond =
BinaryOperator::CreateNot(PBI->getCondition(),
PBI->getCondition()->getName()+".not", PBI);
PBI->setCondition(NewCond);
BasicBlock *OldTrue = PBI->getSuccessor(0);
BasicBlock *OldFalse = PBI->getSuccessor(1);
PBI->setSuccessor(0, OldFalse);
PBI->setSuccessor(1, OldTrue);
}
// Clone Cond into the predecessor basic block, and or/and the
// two conditions together.
Instruction *New = Cond->clone();
PredBlock->getInstList().insert(PBI, New);
New->takeName(Cond);
Cond->setName(New->getName()+".old");
Value *NewCond = BinaryOperator::Create(Opc, PBI->getCondition(),
New, "or.cond", PBI);
PBI->setCondition(NewCond);
if (PBI->getSuccessor(0) == BB) {
AddPredecessorToBlock(TrueDest, PredBlock, BB);
PBI->setSuccessor(0, TrueDest);
}
if (PBI->getSuccessor(1) == BB) {
AddPredecessorToBlock(FalseDest, PredBlock, BB);
PBI->setSuccessor(1, FalseDest);
}
return true;
}
return false;
}
/// SimplifyCondBranchToCondBranch - If we have a conditional branch as a
/// predecessor of another block, this function tries to simplify it. We know
/// that PBI and BI are both conditional branches, and BI is in one of the
/// successor blocks of PBI - PBI branches to BI.
static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
assert(PBI->isConditional() && BI->isConditional());
BasicBlock *BB = BI->getParent();
// If this block ends with a branch instruction, and if there is a
// predecessor that ends on a branch of the same condition, make
// this conditional branch redundant.
if (PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
// Okay, the outcome of this conditional branch is statically
// knowable. If this block had a single pred, handle specially.
if (BB->getSinglePredecessor()) {
// Turn this into a branch on constant.
bool CondIsTrue = PBI->getSuccessor(0) == BB;
BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
CondIsTrue));
return true; // Nuke the branch on constant.
}
// Otherwise, if there are multiple predecessors, insert a PHI that merges
// in the constant and simplify the block result. Subsequent passes of
// simplifycfg will thread the block.
if (BlockIsSimpleEnoughToThreadThrough(BB)) {
PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()),
BI->getCondition()->getName() + ".pr",
BB->begin());
// Okay, we're going to insert the PHI node. Since PBI is not the only
// predecessor, compute the PHI'd conditional value for all of the preds.
// Any predecessor where the condition is not computable we keep symbolic.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
PBI != BI && PBI->isConditional() &&
PBI->getCondition() == BI->getCondition() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
bool CondIsTrue = PBI->getSuccessor(0) == BB;
NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
CondIsTrue), *PI);
} else {
NewPN->addIncoming(BI->getCondition(), *PI);
}
BI->setCondition(NewPN);
return true;
}
}
// If this is a conditional branch in an empty block, and if any
// predecessors is a conditional branch to one of our destinations,
// fold the conditions into logical ops and one cond br.
BasicBlock::iterator BBI = BB->begin();
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(BBI))
++BBI;
if (&*BBI != BI)
return false;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
if (CE->canTrap())
return false;
int PBIOp, BIOp;
if (PBI->getSuccessor(0) == BI->getSuccessor(0))
PBIOp = BIOp = 0;
else if (PBI->getSuccessor(0) == BI->getSuccessor(1))
PBIOp = 0, BIOp = 1;
else if (PBI->getSuccessor(1) == BI->getSuccessor(0))
PBIOp = 1, BIOp = 0;
else if (PBI->getSuccessor(1) == BI->getSuccessor(1))
PBIOp = BIOp = 1;
else
return false;
// Check to make sure that the other destination of this branch
// isn't BB itself. If so, this is an infinite loop that will
// keep getting unwound.
if (PBI->getSuccessor(PBIOp) == BB)
return false;
// Do not perform this transformation if it would require
// insertion of a large number of select instructions. For targets
// without predication/cmovs, this is a big pessimization.
BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
unsigned NumPhis = 0;
for (BasicBlock::iterator II = CommonDest->begin();
isa<PHINode>(II); ++II, ++NumPhis)
if (NumPhis > 2) // Disable this xform.
return false;
// Finally, if everything is ok, fold the branches to logical ops.
BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
<< "AND: " << *BI->getParent());
// If OtherDest *is* BB, then BB is a basic block with a single conditional
// branch in it, where one edge (OtherDest) goes back to itself but the other
// exits. We don't *know* that the program avoids the infinite loop
// (even though that seems likely). If we do this xform naively, we'll end up
// recursively unpeeling the loop. Since we know that (after the xform is
// done) that the block *is* infinite if reached, we just make it an obviously
// infinite loop with no cond branch.
if (OtherDest == BB) {
// Insert it at the end of the function, because it's either code,
// or it won't matter if it's hot. :)
BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(),
"infloop", BB->getParent());
BranchInst::Create(InfLoopBlock, InfLoopBlock);
OtherDest = InfLoopBlock;
}
DEBUG(dbgs() << *PBI->getParent()->getParent());
// BI may have other predecessors. Because of this, we leave
// it alone, but modify PBI.
// Make sure we get to CommonDest on True&True directions.
Value *PBICond = PBI->getCondition();
if (PBIOp)
PBICond = BinaryOperator::CreateNot(PBICond,
PBICond->getName()+".not",
PBI);
Value *BICond = BI->getCondition();
if (BIOp)
BICond = BinaryOperator::CreateNot(BICond,
BICond->getName()+".not",
PBI);
// Merge the conditions.
Value *Cond = BinaryOperator::CreateOr(PBICond, BICond, "brmerge", PBI);
// Modify PBI to branch on the new condition to the new dests.
PBI->setCondition(Cond);
PBI->setSuccessor(0, CommonDest);
PBI->setSuccessor(1, OtherDest);
// OtherDest may have phi nodes. If so, add an entry from PBI's
// block that are identical to the entries for BI's block.
PHINode *PN;
for (BasicBlock::iterator II = OtherDest->begin();
(PN = dyn_cast<PHINode>(II)); ++II) {
Value *V = PN->getIncomingValueForBlock(BB);
PN->addIncoming(V, PBI->getParent());
}
// We know that the CommonDest already had an edge from PBI to
// it. If it has PHIs though, the PHIs may have different
// entries for BB and PBI's BB. If so, insert a select to make
// them agree.
for (BasicBlock::iterator II = CommonDest->begin();
(PN = dyn_cast<PHINode>(II)); ++II) {
Value *BIV = PN->getIncomingValueForBlock(BB);
unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
Value *PBIV = PN->getIncomingValue(PBBIdx);
if (BIV != PBIV) {
// Insert a select in PBI to pick the right value.
Value *NV = SelectInst::Create(PBICond, PBIV, BIV,
PBIV->getName()+".mux", PBI);
PN->setIncomingValue(PBBIdx, NV);
}
}
DEBUG(dbgs() << "INTO: " << *PBI->getParent());
DEBUG(dbgs() << *PBI->getParent()->getParent());
// This basic block is probably dead. We know it has at least
// one fewer predecessor.
return true;
}
bool SimplifyCFGOpt::run(BasicBlock *BB) {
bool Changed = false;
Function *M = BB->getParent();
assert(BB && BB->getParent() && "Block not embedded in function!");
assert(BB->getTerminator() && "Degenerate basic block encountered!");
assert(&BB->getParent()->getEntryBlock() != BB &&
"Can't Simplify entry block!");
// Remove basic blocks that have no predecessors... or that just have themself
// as a predecessor. These are unreachable.
if (pred_begin(BB) == pred_end(BB) || BB->getSinglePredecessor() == BB) {
DEBUG(dbgs() << "Removing BB: \n" << *BB);
DeleteDeadBlock(BB);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB);
// Check for and eliminate duplicate PHI nodes in this block.
Changed |= EliminateDuplicatePHINodes(BB);
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
if (PN->getNumIncomingValues() == 2)
Changed |= FoldTwoEntryPHINode(PN);
// If this is a returning block with only PHI nodes in it, fold the return
// instruction into any unconditional branch predecessors.
//
// If any predecessor is a conditional branch that just selects among
// different return values, fold the replace the branch/return with a select
// and return.
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
if (isTerminatorFirstRelevantInsn(BB, BB->getTerminator())) {
// Find predecessors that end with branches.
SmallVector<BasicBlock*, 8> UncondBranchPreds;
SmallVector<BranchInst*, 8> CondBranchPreds;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
TerminatorInst *PTI = (*PI)->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
if (BI->isUnconditional())
UncondBranchPreds.push_back(*PI);
else
CondBranchPreds.push_back(BI);
}
}
// If we found some, do the transformation!
if (!UncondBranchPreds.empty()) {
while (!UncondBranchPreds.empty()) {
BasicBlock *Pred = UncondBranchPreds.pop_back_val();
DEBUG(dbgs() << "FOLDING: " << *BB
<< "INTO UNCOND BRANCH PRED: " << *Pred);
Instruction *UncondBranch = Pred->getTerminator();
// Clone the return and add it to the end of the predecessor.
Instruction *NewRet = RI->clone();
Pred->getInstList().push_back(NewRet);
// If the return instruction returns a value, and if the value was a
// PHI node in "BB", propagate the right value into the return.
for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
i != e; ++i)
if (PHINode *PN = dyn_cast<PHINode>(*i))
if (PN->getParent() == BB)
*i = PN->getIncomingValueForBlock(Pred);
// Update any PHI nodes in the returning block to realize that we no
// longer branch to them.
BB->removePredecessor(Pred);
Pred->getInstList().erase(UncondBranch);
}
// If we eliminated all predecessors of the block, delete the block now.
if (pred_begin(BB) == pred_end(BB))
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
// Check out all of the conditional branches going to this return
// instruction. If any of them just select between returns, change the
// branch itself into a select/return pair.
while (!CondBranchPreds.empty()) {
BranchInst *BI = CondBranchPreds.pop_back_val();
// Check to see if the non-BB successor is also a return block.
if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
SimplifyCondBranchToTwoReturns(BI))
return true;
}
}
} else if (isa<UnwindInst>(BB->begin())) {
// Check to see if the first instruction in this block is just an unwind.
// If so, replace any invoke instructions which use this as an exception
// destination with call instructions.
//
SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.back();
if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
if (II->getUnwindDest() == BB) {
// Insert a new branch instruction before the invoke, because this
// is now a fall through.
BranchInst *BI = BranchInst::Create(II->getNormalDest(), II);
Pred->getInstList().remove(II); // Take out of symbol table
// Insert the call now.
SmallVector<Value*,8> Args(II->op_begin(), II->op_end()-3);
CallInst *CI = CallInst::Create(II->getCalledValue(),
Args.begin(), Args.end(),
II->getName(), BI);
CI->setCallingConv(II->getCallingConv());
CI->setAttributes(II->getAttributes());
// If the invoke produced a value, the Call now does instead.
II->replaceAllUsesWith(CI);
delete II;
Changed = true;
}
Preds.pop_back();
}
// If this block is now dead, remove it.
if (pred_begin(BB) == pred_end(BB)) {
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
if (isValueEqualityComparison(SI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
return SimplifyCFG(BB) || 1;
// If the block only contains the switch, see if we can fold the block
// away into any preds.
BasicBlock::iterator BBI = BB->begin();
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(BBI))
++BBI;
if (SI == &*BBI)
if (FoldValueComparisonIntoPredecessors(SI))
return SimplifyCFG(BB) || 1;
}
} else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
if (BI->isUnconditional()) {
BasicBlock::iterator BBI = BB->getFirstNonPHI();
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(BBI))
++BBI;
if (BBI->isTerminator()) // Terminator is the only non-phi instruction!
if (TryToSimplifyUncondBranchFromEmptyBlock(BB))
return true;
} else { // Conditional branch
if (isValueEqualityComparison(BI)) {
// If we only have one predecessor, and if it is a branch on this value,
// see if that predecessor totally determines the outcome of this
// switch.
if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
return SimplifyCFG(BB) | true;
// This block must be empty, except for the setcond inst, if it exists.
// Ignore dbg intrinsics.
BasicBlock::iterator I = BB->begin();
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(I))
++I;
if (&*I == BI) {
if (FoldValueComparisonIntoPredecessors(BI))
return SimplifyCFG(BB) | true;
} else if (&*I == cast<Instruction>(BI->getCondition())){
++I;
// Ignore dbg intrinsics.
while (isa<DbgInfoIntrinsic>(I))
++I;
if(&*I == BI) {
if (FoldValueComparisonIntoPredecessors(BI))
return SimplifyCFG(BB) | true;
}
}
}
// If this is a branch on a phi node in the current block, thread control
// through this block if any PHI node entries are constants.
if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
if (PN->getParent() == BI->getParent())
if (FoldCondBranchOnPHI(BI))
return SimplifyCFG(BB) | true;
// If this basic block is ONLY a setcc and a branch, and if a predecessor
// branches to us and one of our successors, fold the setcc into the
// predecessor and use logical operations to pick the right destination.
if (FoldBranchToCommonDest(BI))
return SimplifyCFG(BB) | true;
// Scan predecessor blocks for conditional branches.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
if (PBI != BI && PBI->isConditional())
if (SimplifyCondBranchToCondBranch(PBI, BI))
return SimplifyCFG(BB) | true;
}
} else if (isa<UnreachableInst>(BB->getTerminator())) {
// If there are any instructions immediately before the unreachable that can
// be removed, do so.
Instruction *Unreachable = BB->getTerminator();
while (Unreachable != BB->begin()) {
BasicBlock::iterator BBI = Unreachable;
--BBI;
// Do not delete instructions that can have side effects, like calls
// (which may never return) and volatile loads and stores.
if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI)) break;
if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
if (SI->isVolatile())
break;
if (LoadInst *LI = dyn_cast<LoadInst>(BBI))
if (LI->isVolatile())
break;
// Delete this instruction
BB->getInstList().erase(BBI);
Changed = true;
}
// If the unreachable instruction is the first in the block, take a gander
// at all of the predecessors of this instruction, and simplify them.
if (&BB->front() == Unreachable) {
SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
TerminatorInst *TI = Preds[i]->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (BI->isUnconditional()) {
if (BI->getSuccessor(0) == BB) {
new UnreachableInst(TI->getContext(), TI);
TI->eraseFromParent();
Changed = true;
}
} else {
if (BI->getSuccessor(0) == BB) {
BranchInst::Create(BI->getSuccessor(1), BI);
EraseTerminatorInstAndDCECond(BI);
} else if (BI->getSuccessor(1) == BB) {
BranchInst::Create(BI->getSuccessor(0), BI);
EraseTerminatorInstAndDCECond(BI);
Changed = true;
}
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
if (SI->getSuccessor(i) == BB) {
BB->removePredecessor(SI->getParent());
SI->removeCase(i);
--i; --e;
Changed = true;
}
// If the default value is unreachable, figure out the most popular
// destination and make it the default.
if (SI->getSuccessor(0) == BB) {
std::map<BasicBlock*, unsigned> Popularity;
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
Popularity[SI->getSuccessor(i)]++;
// Find the most popular block.
unsigned MaxPop = 0;
BasicBlock *MaxBlock = 0;
for (std::map<BasicBlock*, unsigned>::iterator
I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
if (I->second > MaxPop) {
MaxPop = I->second;
MaxBlock = I->first;
}
}
if (MaxBlock) {
// Make this the new default, allowing us to delete any explicit
// edges to it.
SI->setSuccessor(0, MaxBlock);
Changed = true;
// If MaxBlock has phinodes in it, remove MaxPop-1 entries from
// it.
if (isa<PHINode>(MaxBlock->begin()))
for (unsigned i = 0; i != MaxPop-1; ++i)
MaxBlock->removePredecessor(SI->getParent());
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
if (SI->getSuccessor(i) == MaxBlock) {
SI->removeCase(i);
--i; --e;
}
}
}
} else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
if (II->getUnwindDest() == BB) {
// Convert the invoke to a call instruction. This would be a good
// place to note that the call does not throw though.
BranchInst *BI = BranchInst::Create(II->getNormalDest(), II);
II->removeFromParent(); // Take out of symbol table
// Insert the call now...
SmallVector<Value*, 8> Args(II->op_begin(), II->op_end()-3);
CallInst *CI = CallInst::Create(II->getCalledValue(),
Args.begin(), Args.end(),
II->getName(), BI);
CI->setCallingConv(II->getCallingConv());
CI->setAttributes(II->getAttributes());
// If the invoke produced a value, the call does now instead.
II->replaceAllUsesWith(CI);
delete II;
Changed = true;
}
}
}
// If this block is now dead, remove it.
if (pred_begin(BB) == pred_end(BB)) {
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
}
}
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
//
if (MergeBlockIntoPredecessor(BB))
return true;
// Otherwise, if this block only has a single predecessor, and if that block
// is a conditional branch, see if we can hoist any code from this block up
// into our predecessor.
pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
BasicBlock *OnlyPred = *PI++;
for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
if (*PI != OnlyPred) {
OnlyPred = 0; // There are multiple different predecessors...
break;
}
if (OnlyPred)
if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
if (BI->isConditional()) {
// Get the other block.
BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
PI = pred_begin(OtherBB);
++PI;
if (PI == pred_end(OtherBB)) {
// We have a conditional branch to two blocks that are only reachable
// from the condbr. We know that the condbr dominates the two blocks,
// so see if there is any identical code in the "then" and "else"
// blocks. If so, we can hoist it up to the branching block.
Changed |= HoistThenElseCodeToIf(BI);
} else {
BasicBlock* OnlySucc = NULL;
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
SI != SE; ++SI) {
if (!OnlySucc)
OnlySucc = *SI;
else if (*SI != OnlySucc) {
OnlySucc = 0; // There are multiple distinct successors!
break;
}
}
if (OnlySucc == OtherBB) {
// If BB's only successor is the other successor of the predecessor,
// i.e. a triangle, see if we can hoist any code from this block up
// to the "if" block.
Changed |= SpeculativelyExecuteBB(BI, BB);
}
}
}
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
// Change br (X == 0 | X == 1), T, F into a switch instruction.
if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
Instruction *Cond = cast<Instruction>(BI->getCondition());
// If this is a bunch of seteq's or'd together, or if it's a bunch of
// 'setne's and'ed together, collect them.
Value *CompVal = 0;
std::vector<ConstantInt*> Values;
bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
if (CompVal) {
// There might be duplicate constants in the list, which the switch
// instruction can't handle, remove them now.
std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
// Figure out which block is which destination.
BasicBlock *DefaultBB = BI->getSuccessor(1);
BasicBlock *EdgeBB = BI->getSuccessor(0);
if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
// Convert pointer to int before we switch.
if (CompVal->getType()->isPointerTy()) {
assert(TD && "Cannot switch on pointer without TargetData");
CompVal = new PtrToIntInst(CompVal,
TD->getIntPtrType(CompVal->getContext()),
"magicptr", BI);
}
// Create the new switch instruction now.
SwitchInst *New = SwitchInst::Create(CompVal, DefaultBB,
Values.size(), BI);
// Add all of the 'cases' to the switch instruction.
for (unsigned i = 0, e = Values.size(); i != e; ++i)
New->addCase(Values[i], EdgeBB);
// We added edges from PI to the EdgeBB. As such, if there were any
// PHI nodes in EdgeBB, they need entries to be added corresponding to
// the number of edges added.
for (BasicBlock::iterator BBI = EdgeBB->begin();
isa<PHINode>(BBI); ++BBI) {
PHINode *PN = cast<PHINode>(BBI);
Value *InVal = PN->getIncomingValueForBlock(*PI);
for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
PN->addIncoming(InVal, *PI);
}
// Erase the old branch instruction.
EraseTerminatorInstAndDCECond(BI);
return true;
}
}
return Changed;
}
/// SimplifyCFG - This function is used to do simplification of a CFG. For
/// example, it adjusts branches to branches to eliminate the extra hop, it
/// eliminates unreachable basic blocks, and does other "peephole" optimization
/// of the CFG. It returns true if a modification was made.
///
/// WARNING: The entry node of a function may not be simplified.
///
bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) {
return SimplifyCFGOpt(TD).run(BB);
}
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