lib/Transforms/Scalar/ADCE.cpp

   1 //===- DCE.cpp - Code to perform dead code elimination --------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements the Aggressive Dead Code Elimination pass.  This pass
  11 // optimistically assumes that all instructions are dead until proven otherwise,
  12 // allowing it to eliminate dead computations that other DCE passes do not
  13 // catch, particularly involving loop computations.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #define DEBUG_TYPE "adce"
  18 #include "llvm/Transforms/Scalar.h"
  19 #include "llvm/BasicBlock.h"
  20 #include "llvm/Instructions.h"
  21 #include "llvm/IntrinsicInst.h"
  22 #include "llvm/Pass.h"
  23 #include "llvm/Support/CFG.h"
  24 #include "llvm/Support/InstIterator.h"
  25 #include "llvm/ADT/DepthFirstIterator.h"
  26 #include "llvm/ADT/SmallPtrSet.h"
  27 #include "llvm/ADT/SmallVector.h"
  28 #include "llvm/ADT/Statistic.h"
  29 using namespace llvm;
  30
  31 STATISTIC(NumRemoved, "Number of instructions removed");
  32
  33 namespace {
  34   struct ADCE : public FunctionPass {
  35     static char ID; // Pass identification, replacement for typeid
  36     ADCE() : FunctionPass(&ID) {}
  37
  38     virtual bool runOnFunction(Function& F);
  39
  40     virtual void getAnalysisUsage(AnalysisUsage& AU) const {
  41       AU.setPreservesCFG();
  42     }
  43
  44   };
  45 }
  46
  47 char ADCE::ID = 0;
  48 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
  49
  50 bool ADCE::runOnFunction(Function& F) {
  51   SmallPtrSet<Instruction*, 128> alive;
  52   SmallVector<Instruction*, 128> worklist;
  53
  54   // Collect the set of "root" instructions that are known live.
  55   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  56     if (isa<TerminatorInst>(I.getInstructionIterator()) ||
  57         isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
  58         I->mayHaveSideEffects()) {
  59       alive.insert(I.getInstructionIterator());
  60       worklist.push_back(I.getInstructionIterator());
  61     }
  62
  63   // Propagate liveness backwards to operands.
  64   while (!worklist.empty()) {
  65     Instruction* curr = worklist.back();
  66     worklist.pop_back();
  67
  68     for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
  69          OI != OE; ++OI)
  70       if (Instruction* Inst = dyn_cast<Instruction>(OI))
  71         if (alive.insert(Inst))
  72           worklist.push_back(Inst);
  73   }
  74
  75   // The inverse of the live set is the dead set.  These are those instructions
  76   // which have no side effects and do not influence the control flow or return
  77   // value of the function, and may therefore be deleted safely.
  78   // NOTE: We reuse the worklist vector here for memory efficiency.
  79   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  80     if (!alive.count(I.getInstructionIterator())) {
  81       worklist.push_back(I.getInstructionIterator());
  82       I->dropAllReferences();
  83     }
  84
  85   for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
  86        E = worklist.end(); I != E; ++I) {
  87     NumRemoved++;
  88     (*I)->eraseFromParent();
  89   }
  90
  91   return !worklist.empty();
  92 }
  93
  94 FunctionPass *llvm::createAggressiveDCEPass() {
  95   return new ADCE();
  96 }