ADCE.cpp revision dce4a407a24b04eebc6a376f8e62b41aaa7b071f
178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch//===- DCE.cpp - Code to perform dead code elimination --------------------===// 278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// 378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// The LLVM Compiler Infrastructure 478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// 578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// This file is distributed under the University of Illinois Open Source 678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// License. See LICENSE.TXT for details. 778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// 878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch//===----------------------------------------------------------------------===// 978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// 1078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// This file implements the Aggressive Dead Code Elimination pass. This pass 1178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// optimistically assumes that all instructions are dead until proven otherwise, 1278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// allowing it to eliminate dead computations that other DCE passes do not 1378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// catch, particularly involving loop computations. 1478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch// 1578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch//===----------------------------------------------------------------------===// 1678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 1778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/Transforms/Scalar.h" 1878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/ADT/DepthFirstIterator.h" 1978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/ADT/SmallPtrSet.h" 2078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/ADT/SmallVector.h" 2178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/ADT/Statistic.h" 2278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/IR/BasicBlock.h" 2378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/IR/CFG.h" 2478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/IR/InstIterator.h" 2578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/IR/Instructions.h" 2678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/IR/IntrinsicInst.h" 2778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#include "llvm/Pass.h" 2878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdochusing namespace llvm; 2978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 3078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch#define DEBUG_TYPE "adce" 3178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 3278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben MurdochSTATISTIC(NumRemoved, "Number of instructions removed"); 3378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 3478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdochnamespace { 3578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch struct ADCE : public FunctionPass { 3678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch static char ID; // Pass identification, replacement for typeid 3778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch ADCE() : FunctionPass(ID) { 3878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch initializeADCEPass(*PassRegistry::getPassRegistry()); 3978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 4078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 4178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch bool runOnFunction(Function& F) override; 4278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 4378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch void getAnalysisUsage(AnalysisUsage& AU) const override { 4478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch AU.setPreservesCFG(); 4578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 4678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 4778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch }; 4878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch} 4978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 5078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdochchar ADCE::ID = 0; 5178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben MurdochINITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false) 5278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 5378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdochbool ADCE::runOnFunction(Function& F) { 5478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch if (skipOptnoneFunction(F)) 5578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch return false; 5678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 5778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch SmallPtrSet<Instruction*, 128> alive; 5878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch SmallVector<Instruction*, 128> worklist; 5978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 6078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // Collect the set of "root" instructions that are known live. 6178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 6278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch if (isa<TerminatorInst>(I.getInstructionIterator()) || 6378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch isa<DbgInfoIntrinsic>(I.getInstructionIterator()) || 6478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch isa<LandingPadInst>(I.getInstructionIterator()) || 6578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch I->mayHaveSideEffects()) { 6678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch alive.insert(I.getInstructionIterator()); 6778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch worklist.push_back(I.getInstructionIterator()); 6878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 6978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 7078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // Propagate liveness backwards to operands. 7178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch while (!worklist.empty()) { 7278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch Instruction* curr = worklist.pop_back_val(); 7378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end(); 7478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch OI != OE; ++OI) 7578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch if (Instruction* Inst = dyn_cast<Instruction>(OI)) 7678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch if (alive.insert(Inst)) 7778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch worklist.push_back(Inst); 7878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 7978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 8078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // The inverse of the live set is the dead set. These are those instructions 8178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // which have no side effects and do not influence the control flow or return 8278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // value of the function, and may therefore be deleted safely. 8378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch // NOTE: We reuse the worklist vector here for memory efficiency. 8478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I) 8578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch if (!alive.count(I.getInstructionIterator())) { 8678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch worklist.push_back(I.getInstructionIterator()); 8778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch I->dropAllReferences(); 8878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 8978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 9078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(), 9178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch E = worklist.end(); I != E; ++I) { 9278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch ++NumRemoved; 9378901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch (*I)->eraseFromParent(); 9478901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch } 9578901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 9678901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch return !worklist.empty(); 9778901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch} 9878901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch 9978901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben MurdochFunctionPass *llvm::createAggressiveDCEPass() { 10078901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch return new ADCE(); 10178901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch} 10278901d17b47ef1f8d6d0a89eaf37f9523ba1de85Ben Murdoch