src/llvm/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 #include "llvm/Transforms/Scalar.h"
  18 #include "llvm/ADT/DepthFirstIterator.h"
  19 #include "llvm/ADT/SmallPtrSet.h"
  20 #include "llvm/ADT/SmallVector.h"
  21 #include "llvm/ADT/Statistic.h"
  22 #include "llvm/IR/BasicBlock.h"
  23 #include "llvm/IR/CFG.h"
  24 #include "llvm/IR/InstIterator.h"
  25 #include "llvm/IR/Instructions.h"
  26 #include "llvm/IR/IntrinsicInst.h"
  27 #include "llvm/Pass.h"
  28 using namespace llvm;
  29
  30 #define DEBUG_TYPE "adce"
  31
  32 STATISTIC(NumRemoved, "Number of instructions removed");
  33
  34 namespace {
  35   struct ADCE : public FunctionPass {
  36     static char ID; // Pass identification, replacement for typeid
  37     ADCE() : FunctionPass(ID) {
  38       initializeADCEPass(*PassRegistry::getPassRegistry());
  39     }
  40
  41     bool runOnFunction(Function& F) override;
  42
  43     void getAnalysisUsage(AnalysisUsage& AU) const override {
  44       AU.setPreservesCFG();
  45     }
  46
  47   };
  48 }
  49
  50 char ADCE::ID = 0;
  51 INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
  52
  53 bool ADCE::runOnFunction(Function& F) {
  54   if (skipOptnoneFunction(F))
  55     return false;
  56
  57   SmallPtrSet<Instruction*, 128> alive;
  58   SmallVector<Instruction*, 128> worklist;
  59
  60   // Collect the set of "root" instructions that are known live.
  61   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  62     if (isa<TerminatorInst>(I.getInstructionIterator()) ||
  63         isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
  64         isa<LandingPadInst>(I.getInstructionIterator()) ||
  65         I->mayHaveSideEffects()) {
  66       alive.insert(I.getInstructionIterator());
  67       worklist.push_back(I.getInstructionIterator());
  68     }
  69
  70   // Propagate liveness backwards to operands.
  71   while (!worklist.empty()) {
  72     Instruction* curr = worklist.pop_back_val();
  73     for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
  74          OI != OE; ++OI)
  75       if (Instruction* Inst = dyn_cast<Instruction>(OI))
  76         if (alive.insert(Inst).second)
  77           worklist.push_back(Inst);
  78   }
  79
  80   // The inverse of the live set is the dead set.  These are those instructions
  81   // which have no side effects and do not influence the control flow or return
  82   // value of the function, and may therefore be deleted safely.
  83   // NOTE: We reuse the worklist vector here for memory efficiency.
  84   for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
  85     if (!alive.count(I.getInstructionIterator())) {
  86       worklist.push_back(I.getInstructionIterator());
  87       I->dropAllReferences();
  88     }
  89
  90   for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(),
  91        E = worklist.end(); I != E; ++I) {
  92     ++NumRemoved;
  93     (*I)->eraseFromParent();
  94   }
  95
  96   return !worklist.empty();
  97 }
  98
  99 FunctionPass *llvm::createAggressiveDCEPass() {
 100   return new ADCE();
 101 }