llvm/lib/Transforms/Utils/SimplifyCFG.cpp

294 lines
12 KiB
C++

//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constant.h"
#include "llvm/Intrinsics.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/iOther.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <functional>
// PropagatePredecessors - This gets "Succ" ready to have the predecessors from
// "BB". This is a little tricky because "Succ" has PHI nodes, which need to
// have extra slots added to them to hold the merge edges from BB's
// predecessors, and BB itself might have had PHI nodes in it. This function
// returns true (failure) if the Succ BB already has a predecessor that is a
// predecessor of BB and incoming PHI arguments would not be discernable.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
if (!isa<PHINode>(Succ->front()))
return false; // We can make the transformation, no problem.
// If there is more than one predecessor, and there are PHI nodes in
// the successor, then we need to add incoming edges for the PHI nodes
//
const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
// Check to see if one of the predecessors of BB is already a predecessor of
// Succ. If so, we cannot do the transformation if there are any PHI nodes
// with incompatible values coming in from the two edges!
//
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
// Loop over all of the PHI nodes checking to see if there are
// incompatible values coming in.
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
// Loop up the entries in the PHI node for BB and for *PI if the values
// coming in are non-equal, we cannot merge these two blocks (instead we
// should insert a conditional move or something, then merge the
// blocks).
int Idx1 = PN->getBasicBlockIndex(BB);
int Idx2 = PN->getBasicBlockIndex(*PI);
assert(Idx1 != -1 && Idx2 != -1 &&
"Didn't have entries for my predecessors??");
if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
return true; // Values are not equal...
}
}
// Loop over all of the PHI nodes in the successor BB
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
Value *OldVal = PN->removeIncomingValue(BB, false);
assert(OldVal && "No entry in PHI for Pred BB!");
// If this incoming value is one of the PHI nodes in BB...
if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
PHINode *OldValPN = cast<PHINode>(OldVal);
for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
End = BBPreds.end(); PredI != End; ++PredI) {
PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
}
} else {
for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
End = BBPreds.end(); PredI != End; ++PredI) {
// Add an incoming value for each of the new incoming values...
PN->addIncoming(OldVal, *PredI);
}
}
}
return false;
}
// 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 SimplifyCFG(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()->front() != BB && "Can't Simplify entry block!");
// Check to see if the first instruction in this block is just an
// 'llvm.unwind'. If so, replace any invoke instructions which use this as an
// exception destination with call instructions.
//
if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->getTerminator()))
if (BB->begin() == BasicBlock::iterator(UI)) { // Empty block?
std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.back();
if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
if (II->getExceptionalDest() == BB) {
// Insert a new branch instruction before the invoke, because this
// is now a fall through...
BranchInst *BI = new BranchInst(II->getNormalDest(), II);
Pred->getInstList().remove(II); // Take out of symbol table
// Insert the call now...
std::vector<Value*> Args(II->op_begin()+3, II->op_end());
CallInst *CI = new CallInst(II->getCalledValue(), Args,
II->getName(), BI);
// If the invoke produced a value, the Call now does instead
II->replaceAllUsesWith(CI);
delete II;
Changed = true;
}
Preds.pop_back();
}
}
// Remove basic blocks that have no predecessors... which are unreachable.
if (pred_begin(BB) == pred_end(BB) &&
!BB->hasConstantReferences()) {
//cerr << "Removing BB: \n" << BB;
// Loop through all of our successors and make sure they know that one
// of their predecessors is going away.
for_each(succ_begin(BB), succ_end(BB),
std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
while (!BB->empty()) {
Instruction &I = BB->back();
// If this instruction is used, replace uses with an arbitrary
// constant value. Because control flow can't get here, we don't care
// what we replace the value with. Note that since this block is
// unreachable, and all values contained within it must dominate their
// uses, that all uses will eventually be removed.
if (!I.use_empty())
// Make all users of this instruction reference the constant instead
I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
// Remove the instruction from the basic block
BB->getInstList().pop_back();
}
M->getBasicBlockList().erase(BB);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB);
// Check to see if this block has no non-phi instructions and only a single
// successor. If so, replace references to this basic block with references
// to the successor.
succ_iterator SI(succ_begin(BB));
if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
while (isa<PHINode>(*BBI)) ++BBI;
if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
if (Succ != BB) { // Arg, don't hurt infinite loops!
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
// Be careful though, if this transformation fails (returns true) then
// we cannot do this transformation!
//
if (!PropagatePredecessorsForPHIs(BB, Succ)) {
//cerr << "Killing Trivial BB: \n" << BB;
std::string OldName = BB->getName();
std::vector<BasicBlock*>
OldSuccPreds(pred_begin(Succ), pred_end(Succ));
// Move all PHI nodes in BB to Succ if they are alive, otherwise
// delete them.
while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
if (PN->use_empty())
BB->getInstList().erase(BB->begin()); // Nuke instruction...
else {
// The instruction is alive, so this means that Succ must have
// *ONLY* had BB as a predecessor, and the PHI node is still valid
// now. Simply move it into Succ, because we know that BB
// strictly dominated Succ.
BB->getInstList().remove(BB->begin());
Succ->getInstList().push_front(PN);
// We need to add new entries for the PHI node to account for
// predecessors of Succ that the PHI node does not take into
// account. At this point, since we know that BB dominated succ,
// this means that we should any newly added incoming edges should
// use the PHI node as the value for these edges, because they are
// loop back edges.
for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
if (OldSuccPreds[i] != BB)
PN->addIncoming(PN, OldSuccPreds[i]);
}
// Everything that jumped to BB now goes to Succ...
BB->replaceAllUsesWith(Succ);
// Delete the old basic block...
M->getBasicBlockList().erase(BB);
if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
Succ->setName(OldName);
//cerr << "Function after removal: \n" << M;
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 (!BB->hasConstantReferences()) {
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;
}
BasicBlock *OnlySucc = 0;
if (OnlyPred && OnlyPred != BB && // Don't break self loops
OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
// Check to see if there is only one distinct successor...
succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
OnlySucc = BB;
for (; SI != SE; ++SI)
if (*SI != OnlySucc) {
OnlySucc = 0; // There are multiple distinct successors!
break;
}
}
if (OnlySucc) {
//cerr << "Merging: " << BB << "into: " << OnlyPred;
TerminatorInst *Term = OnlyPred->getTerminator();
// Resolve any PHI nodes at the start of the block. They are all
// guaranteed to have exactly one entry if they exist, unless there are
// multiple duplicate (but guaranteed to be equal) entries for the
// incoming edges. This occurs when there are multiple edges from
// OnlyPred to OnlySucc.
//
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
PN->replaceAllUsesWith(PN->getIncomingValue(0));
BB->getInstList().pop_front(); // Delete the phi node...
}
// Delete the unconditional branch from the predecessor...
OnlyPred->getInstList().pop_back();
// Move all definitions in the succecessor to the predecessor...
OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
// Make all PHI nodes that refered to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(OnlyPred);
std::string OldName = BB->getName();
// Erase basic block from the function...
M->getBasicBlockList().erase(BB);
// Inherit predecessors name if it exists...
if (!OldName.empty() && !OnlyPred->hasName())
OnlyPred->setName(OldName);
return true;
}
}
return Changed;
}