1 //===- Reg2Mem.cpp - Convert registers to allocas -------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file demotes all registers to memory references. It is intended to be 10 // the inverse of PromoteMemoryToRegister. By converting to loads, the only 11 // values live across basic blocks are allocas and loads before phi nodes. 12 // It is intended that this should make CFG hacking much easier. 13 // To make later hacking easier, the entry block is split into two, such that 14 // all introduced allocas and nothing else are in the entry block. 15 // 16 //===----------------------------------------------------------------------===// 17 18 #include "llvm/Transforms/Scalar/Reg2Mem.h" 19 #include "llvm/ADT/Statistic.h" 20 #include "llvm/Analysis/LoopInfo.h" 21 #include "llvm/IR/BasicBlock.h" 22 #include "llvm/IR/CFG.h" 23 #include "llvm/IR/Dominators.h" 24 #include "llvm/IR/Function.h" 25 #include "llvm/IR/InstIterator.h" 26 #include "llvm/IR/Instructions.h" 27 #include "llvm/IR/PassManager.h" 28 #include "llvm/InitializePasses.h" 29 #include "llvm/Pass.h" 30 #include "llvm/Transforms/Scalar.h" 31 #include "llvm/Transforms/Utils.h" 32 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 33 #include "llvm/Transforms/Utils/Local.h" 34 #include <list> 35 using namespace llvm; 36 37 #define DEBUG_TYPE "reg2mem" 38 39 STATISTIC(NumRegsDemoted, "Number of registers demoted"); 40 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted"); 41 42 static bool valueEscapes(const Instruction &Inst) { 43 const BasicBlock *BB = Inst.getParent(); 44 for (const User *U : Inst.users()) { 45 const Instruction *UI = cast<Instruction>(U); 46 if (UI->getParent() != BB || isa<PHINode>(UI)) 47 return true; 48 } 49 return false; 50 } 51 52 static bool runPass(Function &F) { 53 // Insert all new allocas into entry block. 54 BasicBlock *BBEntry = &F.getEntryBlock(); 55 assert(pred_empty(BBEntry) && 56 "Entry block to function must not have predecessors!"); 57 58 // Find first non-alloca instruction and create insertion point. This is 59 // safe if block is well-formed: it always have terminator, otherwise 60 // we'll get and assertion. 61 BasicBlock::iterator I = BBEntry->begin(); 62 while (isa<AllocaInst>(I)) ++I; 63 64 CastInst *AllocaInsertionPoint = new BitCastInst( 65 Constant::getNullValue(Type::getInt32Ty(F.getContext())), 66 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I); 67 68 // Find the escaped instructions. But don't create stack slots for 69 // allocas in entry block. 70 std::list<Instruction*> WorkList; 71 for (Instruction &I : instructions(F)) 72 if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I)) 73 WorkList.push_front(&I); 74 75 // Demote escaped instructions 76 NumRegsDemoted += WorkList.size(); 77 for (Instruction *I : WorkList) 78 DemoteRegToStack(*I, false, AllocaInsertionPoint); 79 80 WorkList.clear(); 81 82 // Find all phi's 83 for (BasicBlock &BB : F) 84 for (auto &Phi : BB.phis()) 85 WorkList.push_front(&Phi); 86 87 // Demote phi nodes 88 NumPhisDemoted += WorkList.size(); 89 for (Instruction *I : WorkList) 90 DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint); 91 92 return true; 93 } 94 95 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) { 96 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F); 97 auto *LI = &AM.getResult<LoopAnalysis>(F); 98 unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI)); 99 bool Changed = runPass(F); 100 if (N == 0 && !Changed) 101 return PreservedAnalyses::all(); 102 PreservedAnalyses PA; 103 PA.preserve<DominatorTreeAnalysis>(); 104 PA.preserve<LoopAnalysis>(); 105 return PA; 106 } 107 108 namespace { 109 struct RegToMemLegacy : public FunctionPass { 110 static char ID; // Pass identification, replacement for typeid 111 RegToMemLegacy() : FunctionPass(ID) { 112 initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry()); 113 } 114 115 void getAnalysisUsage(AnalysisUsage &AU) const override { 116 AU.addRequiredID(BreakCriticalEdgesID); 117 AU.addPreservedID(BreakCriticalEdgesID); 118 } 119 120 bool runOnFunction(Function &F) override { 121 if (F.isDeclaration() || skipFunction(F)) 122 return false; 123 return runPass(F); 124 } 125 }; 126 } // namespace 127 128 char RegToMemLegacy::ID = 0; 129 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem", 130 "Demote all values to stack slots", false, false) 131 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges) 132 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem", 133 "Demote all values to stack slots", false, false) 134 135 // createDemoteRegisterToMemory - Provide an entry point to create this pass. 136 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID; 137 FunctionPass *llvm::createDemoteRegisterToMemoryPass() { 138 return new RegToMemLegacy(); 139 } 140