//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
//
// This file implements "aggressive" dead code elimination. ADCE is DCe where
// values are assumed to be dead until proven otherwise. This is similar to
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Constant.h"
+#include "llvm/Instructions.h"
#include "llvm/Type.h"
+#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/PostDominators.h"
-#include "llvm/iTerminators.h"
-#include "llvm/iPHINode.h"
-#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
-#include "Support/Debug.h"
-#include "Support/DepthFirstIterator.h"
-#include "Support/Statistic.h"
-#include "Support/STLExtras.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/STLExtras.h"
#include <algorithm>
+using namespace llvm;
namespace {
Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
+ Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed");
//===----------------------------------------------------------------------===//
// ADCE Class
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ // We require that all function nodes are unified, because otherwise code
+ // can be marked live that wouldn't necessarily be otherwise.
+ AU.addRequired<UnifyFunctionExitNodes>();
+ AU.addRequired<AliasAnalysis>();
AU.addRequired<PostDominatorTree>();
AU.addRequired<PostDominanceFrontier>();
}
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
- DEBUG(std::cerr << "Insn Live: " << I);
+ DEBUG(std::cerr << "Insn Live: " << *I);
LiveSet.insert(I);
WorkList.push_back(I);
}
inline void markTerminatorLive(const BasicBlock *BB) {
- DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator());
+ DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
}
};
RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
} // End of anonymous namespace
-Pass *createAggressiveDCEPass() { return new ADCE(); }
+Pass *llvm::createAggressiveDCEPass() { return new ADCE(); }
void ADCE::markBlockAlive(BasicBlock *BB) {
// Mark the basic block as being newly ALIVE... and mark all branches that
// #arguments != #predecessors, so we remove them now.
//
PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
-
+
// Delete the instruction...
- I = BB->getInstList().erase(I);
+ ++I;
+ BB->getInstList().erase(PN);
Changed = true;
+ ++NumInstRemoved;
} else {
++I;
}
bool ADCE::doADCE() {
bool MadeChanges = false;
+ AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+
+ // Iterate over all invokes in the function, turning invokes into calls if
+ // they cannot throw.
+ for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
+ if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
+ if (Function *F = II->getCalledFunction())
+ if (AA.onlyReadsMemory(F)) {
+ // The function cannot unwind. Convert it to a call with a branch
+ // after it to the normal destination.
+ std::vector<Value*> Args(II->op_begin()+3, II->op_end());
+ std::string Name = II->getName(); II->setName("");
+ Instruction *NewCall = new CallInst(F, Args, Name, II);
+ II->replaceAllUsesWith(NewCall);
+ new BranchInst(II->getNormalDest(), II);
+
+ // Update PHI nodes in the unwind destination
+ II->getUnwindDest()->removePredecessor(BB);
+ BB->getInstList().erase(II);
+
+ if (NewCall->use_empty()) {
+ BB->getInstList().erase(NewCall);
+ ++NumCallRemoved;
+ }
+ }
+
// Iterate over all of the instructions in the function, eliminating trivially
// dead instructions, and marking instructions live that are known to be
// needed. Perform the walk in depth first order so that we avoid marking any
// instructions live in basic blocks that are unreachable. These blocks will
// be eliminated later, along with the instructions inside.
//
- for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
- BBI != BBE; ++BBI) {
+ std::set<BasicBlock*> ReachableBBs;
+ for (df_ext_iterator<BasicBlock*>
+ BBI = df_ext_begin(&Func->front(), ReachableBBs),
+ BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
- if (II->mayWriteToMemory() || isa<ReturnInst>(II) || isa<UnwindInst>(II)){
- markInstructionLive(II);
- ++II; // Increment the inst iterator if the inst wasn't deleted
- } else if (isInstructionTriviallyDead(II)) {
+ Instruction *I = II++;
+ if (CallInst *CI = dyn_cast<CallInst>(I)) {
+ Function *F = CI->getCalledFunction();
+ if (F && AA.onlyReadsMemory(F)) {
+ if (CI->use_empty()) {
+ BB->getInstList().erase(CI);
+ ++NumCallRemoved;
+ }
+ } else {
+ markInstructionLive(I);
+ }
+ } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
+ isa<UnwindInst>(I)) {
+ markInstructionLive(I);
+ } else if (isInstructionTriviallyDead(I)) {
// Remove the instruction from it's basic block...
- II = BB->getInstList().erase(II);
+ BB->getInstList().erase(I);
++NumInstRemoved;
- MadeChanges = true;
- } else {
- ++II; // Increment the inst iterator if the inst wasn't deleted
}
}
}
// transformations safely.
//
PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
- if (DT[&Func->getEntryNode()] == 0) {
+ if (DT[&Func->getEntryBlock()] == 0) {
WorkList.clear();
return MadeChanges;
}
+ // Scan the function marking blocks without post-dominance information as
+ // live. Blocks without post-dominance information occur when there is an
+ // infinite loop in the program. Because the infinite loop could contain a
+ // function which unwinds, exits or has side-effects, we don't want to delete
+ // the infinite loop or those blocks leading up to it.
+ for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
+ if (DT[I] == 0)
+ for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
+ markInstructionLive((*PI)->getTerminator());
+
+
+
DEBUG(std::cerr << "Processing work list\n");
// AliveBlocks - Set of basic blocks that we know have instructions that are
WorkList.pop_back();
BasicBlock *BB = I->getParent();
+ if (!ReachableBBs.count(BB)) continue;
if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
AliveBlocks.insert(BB); // Block is now ALIVE!
markBlockAlive(BB); // Make it so now!
// Loop over all of the instructions in the function, telling dead
// instructions to drop their references. This is so that the next sweep
// over the program can safely delete dead instructions without other dead
- // instructions still refering to them.
+ // instructions still referring to them.
//
dropReferencesOfDeadInstructionsInLiveBlock(I);
//
if (!AliveBlocks.count(&Func->front())) {
BasicBlock *NewEntry = new BasicBlock();
- NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
+ new BranchInst(&Func->front(), NewEntry);
Func->getBasicBlockList().push_front(NewEntry);
AliveBlocks.insert(NewEntry); // This block is always alive!
LiveSet.insert(NewEntry->getTerminator()); // The branch is live
if (LastNode == 0) { // No postdominator!
// Call RemoveSuccessor to transmogrify the terminator instruction
// to not contain the outgoing branch, or to create a new
- // terminator if the form fundementally changes (ie unconditional
- // branch to return). Note that this will change a branch into an
- // infinite loop into a return instruction!
+ // terminator if the form fundamentally changes (i.e.,
+ // unconditional branch to return). Note that this will change a
+ // branch into an infinite loop into a return instruction!
//
RemoveSuccessor(TI, i);
// Now loop over all of the instructions in the basic block, telling
// dead instructions to drop their references. This is so that the next
// sweep over the program can safely delete dead instructions without
- // other dead instructions still refering to them.
+ // other dead instructions still referring to them.
//
dropReferencesOfDeadInstructionsInLiveBlock(BB);
}
}
// Delete the old terminator instruction...
+ const Type *TermTy = BB->getTerminator()->getType();
+ if (TermTy != Type::VoidTy)
+ BB->getTerminator()->replaceAllUsesWith(
+ Constant::getNullValue(TermTy));
BB->getInstList().pop_back();
const Type *RetTy = Func->getReturnType();
- BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
- Constant::getNullValue(RetTy) : 0));
+ new ReturnInst(RetTy != Type::VoidTy ?
+ Constant::getNullValue(RetTy) : 0, BB);
}
}
for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
if (!LiveSet.count(II)) { // Is this instruction alive?
// Nope... remove the instruction from it's basic block...
+ if (isa<CallInst>(II))
+ ++NumCallRemoved;
+ else
+ ++NumInstRemoved;
II = BI->getInstList().erase(II);
- ++NumInstRemoved;
MadeChanges = true;
} else {
++II;
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