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/LLVMContext.h" 28 #include "llvm/IR/Module.h" 29 #include "llvm/IR/PassManager.h" 30 #include "llvm/InitializePasses.h" 31 #include "llvm/Pass.h" 32 #include "llvm/Transforms/Scalar.h" 33 #include "llvm/Transforms/Utils.h" 34 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 35 #include "llvm/Transforms/Utils/Local.h" 36 #include <list> 37 using namespace llvm; 38 39 #define DEBUG_TYPE "reg2mem" 40 41 STATISTIC(NumRegsDemoted, "Number of registers demoted"); 42 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted"); 43 44 static bool valueEscapes(const Instruction &Inst) { 45 const BasicBlock *BB = Inst.getParent(); 46 for (const User *U : Inst.users()) { 47 const Instruction *UI = cast<Instruction>(U); 48 if (UI->getParent() != BB || isa<PHINode>(UI)) 49 return true; 50 } 51 return false; 52 } 53 54 static bool runPass(Function &F) { 55 // Insert all new allocas into entry block. 56 BasicBlock *BBEntry = &F.getEntryBlock(); 57 assert(pred_empty(BBEntry) && 58 "Entry block to function must not have predecessors!"); 59 60 // Find first non-alloca instruction and create insertion point. This is 61 // safe if block is well-formed: it always have terminator, otherwise 62 // we'll get and assertion. 63 BasicBlock::iterator I = BBEntry->begin(); 64 while (isa<AllocaInst>(I)) ++I; 65 66 CastInst *AllocaInsertionPoint = new BitCastInst( 67 Constant::getNullValue(Type::getInt32Ty(F.getContext())), 68 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I); 69 70 // Find the escaped instructions. But don't create stack slots for 71 // allocas in entry block. 72 std::list<Instruction*> WorkList; 73 for (Instruction &I : instructions(F)) 74 if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I)) 75 WorkList.push_front(&I); 76 77 // Demote escaped instructions 78 NumRegsDemoted += WorkList.size(); 79 for (Instruction *I : WorkList) 80 DemoteRegToStack(*I, false, AllocaInsertionPoint); 81 82 WorkList.clear(); 83 84 // Find all phi's 85 for (BasicBlock &BB : F) 86 for (auto &Phi : BB.phis()) 87 WorkList.push_front(&Phi); 88 89 // Demote phi nodes 90 NumPhisDemoted += WorkList.size(); 91 for (Instruction *I : WorkList) 92 DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint); 93 94 return true; 95 } 96 97 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) { 98 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F); 99 auto *LI = &AM.getResult<LoopAnalysis>(F); 100 unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI)); 101 bool Changed = runPass(F); 102 if (N == 0 && !Changed) 103 return PreservedAnalyses::all(); 104 PreservedAnalyses PA; 105 PA.preserve<DominatorTreeAnalysis>(); 106 PA.preserve<LoopAnalysis>(); 107 return PA; 108 } 109 110 namespace { 111 struct RegToMemLegacy : public FunctionPass { 112 static char ID; // Pass identification, replacement for typeid 113 RegToMemLegacy() : FunctionPass(ID) { 114 initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry()); 115 } 116 117 void getAnalysisUsage(AnalysisUsage &AU) const override { 118 AU.addRequiredID(BreakCriticalEdgesID); 119 AU.addPreservedID(BreakCriticalEdgesID); 120 } 121 122 bool runOnFunction(Function &F) override { 123 if (F.isDeclaration() || skipFunction(F)) 124 return false; 125 return runPass(F); 126 } 127 }; 128 } // namespace 129 130 char RegToMemLegacy::ID = 0; 131 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem", 132 "Demote all values to stack slots", false, false) 133 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges) 134 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem", 135 "Demote all values to stack slots", false, false) 136 137 // createDemoteRegisterToMemory - Provide an entry point to create this pass. 138 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID; 139 FunctionPass *llvm::createDemoteRegisterToMemoryPass() { 140 return new RegToMemLegacy(); 141 } 142