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       initializeADCEPass(*PassRegistry::getPassRegistry());
  38     }
  39
  40     virtual bool runOnFunction(Function& F);
  41
  42     virtual void getAnalysisUsage(AnalysisUsage& AU) const {
  43       AU.setPreservesCFG();
  44     }
  45
  46   };
  47 }
  48
  49 char ADCE::ID = 0;
  50 INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
  51
  52 bool ADCE::runOnFunction(Function& F) {
  53   SmallPtrSet<Instruction*, 128> alive;
  54   SmallVector<Instruction*, 128> worklist;
  55
  56   // Collect the set of "root" instructions that are known live.
  57   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  58     if (isa<TerminatorInst>(I.getInstructionIterator()) ||
  59         isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
  60         I->mayHaveSideEffects()) {
  61       alive.insert(I.getInstructionIterator());
  62       worklist.push_back(I.getInstructionIterator());
  63     }
  64
  65   // Propagate liveness backwards to operands.
  66   while (!worklist.empty()) {
  67     Instruction* curr = worklist.pop_back_val();
  68
  69     for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
  70          OI != OE; ++OI)
  71       if (Instruction* Inst = dyn_cast<Instruction>(OI))
  72         if (alive.insert(Inst))
  73           worklist.push_back(Inst);
  74   }
  75
  76   // The inverse of the live set is the dead set.  These are those instructions
  77   // which have no side effects and do not influence the control flow or return
  78   // value of the function, and may therefore be deleted safely.
  79   // NOTE: We reuse the worklist vector here for memory efficiency.
  80   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  81     if (!alive.count(I.getInstructionIterator())) {
  82       worklist.push_back(I.getInstructionIterator());
  83       I->dropAllReferences();
  84     }
  85
  86   for (SmallVector<Instruction*, 1024>::iterator I = worklist.begin(),
  87        E = worklist.end(); I != E; ++I) {
  88     ++NumRemoved;
  89     (*I)->eraseFromParent();
  90   }
  91
  92   return !worklist.empty();
  93 }
  94
  95 FunctionPass *llvm::createAggressiveDCEPass() {
  96   return new ADCE();
  97 }