2008-05-29 08:45:13 +00:00
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//===- DCE.cpp - Code to perform dead code elimination --------------------===//
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2005-04-21 23:48:37 +00:00
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//
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2003-10-20 19:43:21 +00:00
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// The LLVM Compiler Infrastructure
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//
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2007-12-29 20:36:04 +00:00
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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2005-04-21 23:48:37 +00:00
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//
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2003-10-20 19:43:21 +00:00
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//===----------------------------------------------------------------------===//
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2001-06-30 06:39:11 +00:00
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//
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2008-05-29 08:45:13 +00:00
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// This file implements the Aggressive Dead Code Elimination pass. This pass
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// optimistically assumes that all instructions are dead until proven otherwise,
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// allowing it to eliminate dead computations that other DCE passes do not
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// catch, particularly involving loop computations.
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2001-06-30 06:39:11 +00:00
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//
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//===----------------------------------------------------------------------===//
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2006-12-19 21:40:18 +00:00
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#define DEBUG_TYPE "adce"
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2002-05-07 20:03:00 +00:00
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#include "llvm/Transforms/Scalar.h"
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2004-04-10 06:53:09 +00:00
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#include "llvm/Instructions.h"
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2008-05-29 08:45:13 +00:00
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#include "llvm/Pass.h"
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2007-02-05 23:32:05 +00:00
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#include "llvm/Support/Compiler.h"
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2008-05-29 08:45:13 +00:00
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#include "llvm/Support/InstIterator.h"
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#include "llvm/ADT/Statistic.h"
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2008-07-02 17:32:04 +00:00
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#include "llvm/ADT/DenseSet.h"
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2008-06-23 06:13:12 +00:00
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#include "llvm/ADT/SmallVector.h"
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2008-05-29 08:45:13 +00:00
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2003-12-19 09:08:34 +00:00
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using namespace llvm;
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2003-11-11 22:41:34 +00:00
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2008-05-29 08:45:13 +00:00
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STATISTIC(NumRemoved, "Number of instructions removed");
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2002-05-06 17:27:57 +00:00
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2006-12-19 21:40:18 +00:00
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namespace {
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2008-05-29 08:45:13 +00:00
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struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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ADCE() : FunctionPass((intptr_t)&ID) {}
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2008-07-02 17:32:04 +00:00
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DenseSet<Instruction*> alive;
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2008-06-23 06:13:12 +00:00
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SmallVector<Instruction*, 1024> worklist;
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2008-05-29 08:45:13 +00:00
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virtual bool runOnFunction(Function& F);
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virtual void getAnalysisUsage(AnalysisUsage& AU) const {
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AU.setPreservesCFG();
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2002-07-30 00:22:34 +00:00
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}
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2008-05-29 08:45:13 +00:00
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};
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2003-06-24 23:02:45 +00:00
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}
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2008-05-29 08:45:13 +00:00
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char ADCE::ID = 0;
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static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
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2003-06-24 23:02:45 +00:00
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2008-05-29 08:45:13 +00:00
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bool ADCE::runOnFunction(Function& F) {
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2008-06-23 06:13:12 +00:00
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alive.clear();
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worklist.clear();
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2008-05-16 04:34:51 +00:00
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2008-05-29 08:45:13 +00:00
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// Collect the set of "root" instructions that are known live.
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (isa<TerminatorInst>(I.getInstructionIterator()) ||
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I->mayWriteToMemory()) {
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alive.insert(I.getInstructionIterator());
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worklist.push_back(I.getInstructionIterator());
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2001-09-09 22:26:47 +00:00
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}
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2008-05-16 04:34:51 +00:00
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2008-05-29 08:45:13 +00:00
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// Propagate liveness backwards to operands.
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while (!worklist.empty()) {
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Instruction* curr = worklist.back();
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worklist.pop_back();
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for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
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OI != OE; ++OI)
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if (Instruction* Inst = dyn_cast<Instruction>(OI))
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if (alive.insert(Inst))
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worklist.push_back(Inst);
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2001-09-09 22:26:47 +00:00
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}
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2008-05-29 08:45:13 +00:00
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// The inverse of the live set is the dead set. These are those instructions
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// which have no side effects and do not influence the control flow or return
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// value of the function, and may therefore be deleted safely.
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2008-06-23 06:13:12 +00:00
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// NOTE: We reuse the worklist vector here for memory efficiency.
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2008-05-29 08:45:13 +00:00
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for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
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if (!alive.count(I.getInstructionIterator())) {
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2008-06-23 06:13:12 +00:00
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worklist.push_back(I.getInstructionIterator());
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2008-05-29 08:45:13 +00:00
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I->dropAllReferences();
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2003-06-24 23:02:45 +00:00
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}
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2008-05-29 08:45:13 +00:00
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2008-06-23 06:13:12 +00:00
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for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
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E = worklist.end(); I != E; ++I) {
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2008-05-29 08:45:13 +00:00
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NumRemoved++;
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(*I)->eraseFromParent();
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2004-12-12 23:49:37 +00:00
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}
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2008-05-29 08:45:13 +00:00
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2008-06-23 06:13:12 +00:00
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return !worklist.empty();
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2001-09-09 22:26:47 +00:00
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}
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2008-05-29 08:45:13 +00:00
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FunctionPass *llvm::createAggressiveDCEPass() {
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return new ADCE();
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2008-05-29 14:38:23 +00:00
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}
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