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* Remove all cfg simplification stuff for a new cfg simplify pass (todo)

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* Convert to worklist instead of iterative algorithm

llvm-svn: 2510
This commit is contained in:
Chris Lattner 2002-05-07 04:24:11 +00:00
parent 6b0fe18eb2
commit 4a830e1a4a
1 changed files with 94 additions and 339 deletions
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433
lib/Transforms/Scalar/DCE.cpp
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@ -1,38 +1,24 @@
//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
// This file implements dead code elimination and basic block merging.
// This file implements dead inst elimination and dead code elimination.
//
// Specifically, this:
// * removes definitions with no uses
// * removes basic blocks with no predecessors
// * merges a basic block into its predecessor if there is only one and the
// predecessor only has one successor.
// * Eliminates PHI nodes for basic blocks with a single predecessor
// * Eliminates a basic block that only contains an unconditional branch
// * Eliminates function prototypes that are not referenced
//
// TODO: This should REALLY be worklist driven instead of iterative. Right now,
// we scan linearly through values, removing unused ones as we go. The problem
// is that this may cause other earlier values to become unused. To make sure
// that we get them all, we iterate until things stop changing. Instead, when
// removing a value, recheck all of its operands to see if they are now unused.
// Piece of cake, and more efficient as well.
//
// Note, this is not trivial, because we have to worry about invalidating
// iterators. :(
// Dead Inst Elimination performs a single pass over the function removing
// instructions that are obviously dead. Dead Code Elimination is similar, but
// it rechecks instructions that were used by removed instructions to see if
// they are newly dead.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/DCE.h"
#include "llvm/Module.h"
#include "llvm/GlobalVariable.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
#include "llvm/Pass.h"
#include "Support/STLExtras.h"
#include <algorithm>
#include "llvm/InstrTypes.h"
#include "llvm/Function.h"
#include "llvm/Support/InstIterator.h"
#include <set>
static inline bool isInstDead(Instruction *I) {
return I->use_empty() && !I->hasSideEffects() && !isa<TerminatorInst>(I);
}
// dceInstruction - Inspect the instruction at *BBI and figure out if it's
// [trivially] dead. If so, remove the instruction and update the iterator
@ -42,339 +28,108 @@
bool dceInstruction(BasicBlock::InstListType &BBIL,
BasicBlock::iterator &BBI) {
// Look for un"used" definitions...
if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() &&
!isa<TerminatorInst>(*BBI)) {
if (isInstDead(*BBI)) {
delete BBIL.remove(BBI); // Bye bye
return true;
}
return false;
}
static inline bool RemoveUnusedDefs(BasicBlock::InstListType &Vals) {
bool Changed = false;
for (BasicBlock::InstListType::iterator DI = Vals.begin();
DI != Vals.end(); )
if (dceInstruction(Vals, DI))
Changed = true;
else
++DI;
return Changed;
//===----------------------------------------------------------------------===//
// DeadInstElimination pass implementation
//
namespace {
struct DeadInstElimination : public BasicBlockPass {
const char *getPassName() const { return "Dead Instruction Elimination"; }
virtual bool runOnBasicBlock(BasicBlock *BB) {
BasicBlock::InstListType &Vals = BB->getInstList();
bool Changed = false;
for (BasicBlock::iterator DI = Vals.begin(); DI != Vals.end(); )
if (dceInstruction(Vals, DI))
Changed = true;
else
++DI;
return Changed;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.preservesCFG();
}
};
}
struct DeadInstElimination : public BasicBlockPass {
const char *getPassName() const { return "Dead Instruction Elimination"; }
virtual bool runOnBasicBlock(BasicBlock *BB) {
return RemoveUnusedDefs(BB->getInstList());
}
};
Pass *createDeadInstEliminationPass() {
return new DeadInstElimination();
}
// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only
// a single predecessor. This means that the PHI node must only have a single
// RHS value and can be eliminated.
//===----------------------------------------------------------------------===//
// DeadCodeElimination pass implementation
//
// This routine is very simple because we know that PHI nodes must be the first
// things in a basic block, if they are present.
//
static bool RemoveSingularPHIs(BasicBlock *BB) {
pred_iterator PI(pred_begin(BB));
if (PI == pred_end(BB) || ++PI != pred_end(BB))
return false; // More than one predecessor...
Instruction *I = BB->front();
if (!isa<PHINode>(I)) return false; // No PHI nodes
//cerr << "Killing PHIs from " << BB;
//cerr << "Pred #0 = " << *pred_begin(BB);
//cerr << "Function == " << BB->getParent();
do {
PHINode *PN = cast<PHINode>(I);
assert(PN->getNumOperands() == 2 && "PHI node should only have one value!");
Value *V = PN->getOperand(0);
PN->replaceAllUsesWith(V); // Replace PHI node with its single value.
delete BB->getInstList().remove(BB->begin());
I = BB->front();
} while (isa<PHINode>(I));
return true; // Yes, we nuked at least one phi node
}
static void ReplaceUsesWithConstant(Instruction *I) {
// Make all users of this instruction reference the constant instead
I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
}
// PropogatePredecessors - 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. This function returns true (failure) if the Succ BB already
// has a predecessor that is a predecessor of BB.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
// 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!
//
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
PI != PE; ++PI) {
if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end())
return true;
}
BasicBlock::iterator I = Succ->begin();
do { // Loop over all of the PHI nodes in the successor BB
PHINode *PN = cast<PHINode>(*I);
Value *OldVal = PN->removeIncomingValue(BB);
assert(OldVal && "No entry in PHI for Pred BB!");
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);
}
++I;
} while (isa<PHINode>(*I));
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, and returns an
// iterator that designates the first element remaining after the block that
// was deleted.
//
// WARNING: The entry node of a function may not be simplified.
//
bool SimplifyCFG(Function::iterator &BBIt) {
BasicBlock *BB = *BBIt;
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!");
// 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()) ReplaceUsesWithConstant(I);
// Remove the instruction from the basic block
delete BB->getInstList().pop_back();
}
delete M->getBasicBlocks().remove(BBIt);
return true;
}
// Check to see if this block has no instructions and only a single
// successor. If so, replace block references with successor.
succ_iterator SI(succ_begin(BB));
if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
if (BB->front()->isTerminator()) { // Terminator is the only instruction!
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
//cerr << "Killing Trivial BB: \n" << BB;
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 (!isa<PHINode>(Succ->front()) ||
!PropogatePredecessorsForPHIs(BB, Succ)) {
BB->replaceAllUsesWith(Succ);
BB = M->getBasicBlocks().remove(BBIt);
if (BB->hasName() && !Succ->hasName()) // Transfer name if we can
Succ->setName(BB->getName());
delete BB; // Delete basic block
//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 (!isa<PHINode>(BB->front()) && !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
// 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: " << Pred;
TerminatorInst *Term = OnlyPred->getTerminator();
// Delete the unconditional branch from the predecessor...
BasicBlock::iterator DI = OnlyPred->end();
delete OnlyPred->getInstList().remove(--DI); // Destroy branch
// Move all definitions in the succecessor to the predecessor...
std::vector<Instruction*> Insts(BB->begin(), BB->end());
BB->getInstList().remove(BB->begin(), BB->end());
OnlyPred->getInstList().insert(OnlyPred->end(),
Insts.begin(), Insts.end());
// Remove basic block from the function... and advance iterator to the
// next valid block...
M->getBasicBlocks().remove(BBIt);
// Make all PHI nodes that refered to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(OnlyPred);
// Inherit predecessors name if it exists...
if (BB->hasName() && !OnlyPred->hasName())
OnlyPred->setName(BB->getName());
delete BB; // You ARE the weakest link... goodbye
return true;
}
}
return false;
}
static bool DoDCEPass(Function *F) {
Function::iterator BBIt, BBEnd = F->end();
if (F->begin() == BBEnd) return false; // Nothing to do
bool Changed = false;
// Loop through now and remove instructions that have no uses...
for (BBIt = F->begin(); BBIt != BBEnd; ++BBIt) {
Changed |= RemoveUnusedDefs((*BBIt)->getInstList());
Changed |= RemoveSingularPHIs(*BBIt);
}
// Loop over all of the basic blocks (except the first one) and remove them
// if they are unneeded...
//
for (BBIt = F->begin(), ++BBIt; BBIt != F->end(); ) {
if (SimplifyCFG(BBIt)) {
Changed = true;
} else {
++BBIt;
}
}
return Changed;
}
// Remove unused global values - This removes unused global values of no
// possible value. This currently includes unused function prototypes and
// unitialized global variables.
//
static bool RemoveUnusedGlobalValues(Module *Mod) {
bool Changed = false;
for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) {
Function *Meth = *MI;
if (Meth->isExternal() && Meth->use_size() == 0) {
// No references to prototype?
//cerr << "Removing function proto: " << Meth->getName() << endl;
delete Mod->getFunctionList().remove(MI); // Remove prototype
// Remove moves iterator to point to the next one automatically
Changed = true;
} else {
++MI; // Skip prototype in use.
}
}
for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) {
GlobalVariable *GV = *GI;
if (!GV->hasInitializer() && GV->use_size() == 0) {
// No references to uninitialized global variable?
//cerr << "Removing global var: " << GV->getName() << endl;
delete Mod->getGlobalList().remove(GI);
// Remove moves iterator to point to the next one automatically
Changed = true;
} else {
++GI;
}
}
return Changed;
}
namespace {
struct DeadCodeElimination : public FunctionPass {
struct DCE : public FunctionPass {
const char *getPassName() const { return "Dead Code Elimination"; }
// Pass Interface...
virtual bool doInitialization(Module *M) {
return RemoveUnusedGlobalValues(M);
virtual bool runOnFunction(Function *F);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.preservesCFG();
}
};
}
bool DCE::runOnFunction(Function *F) {
// Start out with all of the instructions in the worklist...
std::vector<Instruction*> WorkList(inst_begin(F), inst_end(F));
std::set<Instruction*> DeadInsts;
// Loop over the worklist finding instructions that are dead. If they are
// dead make them drop all of their uses, making other instructions
// potentially dead, and work until the worklist is empty.
//
while (!WorkList.empty()) {
Instruction *I = WorkList.back();
WorkList.pop_back();
// It is possible that we may require multiple passes over the code to fully
// eliminate dead code. Iterate until we are done.
//
virtual bool runOnFunction(Function *F) {
bool Changed = false;
while (DoDCEPass(F)) Changed = true;
return Changed;
if (isInstDead(I)) { // If the instruction is dead...
// Loop over all of the values that the instruction uses, if there are
// instructions being used, add them to the worklist, because they might
// go dead after this one is removed.
//
for (User::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI)
if (Instruction *Used = dyn_cast<Instruction>(*UI))
WorkList.push_back(Used);
// Tell the instruction to let go of all of the values it uses...
I->dropAllReferences();
// Keep track of this instruction, because we are going to delete it later
DeadInsts.insert(I);
}
virtual bool doFinalization(Module *M) {
return RemoveUnusedGlobalValues(M);
}
};
}
// If we found no dead instructions, we haven't changed the function...
if (DeadInsts.empty()) return false;
// Otherwise, loop over the program, removing and deleting the instructions...
for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) {
BasicBlock::InstListType &BBIL = (*I)->getInstList();
for (BasicBlock::iterator BI = BBIL.begin(); BI != BBIL.end(); )
if (DeadInsts.count(*BI)) { // Is this instruction dead?
delete BBIL.remove(BI); // Yup, remove and delete inst
} else { // This instruction is not dead
++BI; // Continue on to the next one...
}
}
return true;
}
Pass *createDeadCodeEliminationPass() {
return new DeadCodeElimination();
return new DCE();
}