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/ADT/Statistic.h" 19 #include "llvm/IR/BasicBlock.h" 20 #include "llvm/IR/CFG.h" 21 #include "llvm/IR/Function.h" 22 #include "llvm/IR/Instructions.h" 23 #include "llvm/IR/LLVMContext.h" 24 #include "llvm/IR/Module.h" 25 #include "llvm/InitializePasses.h" 26 #include "llvm/Pass.h" 27 #include "llvm/Transforms/Scalar.h" 28 #include "llvm/Transforms/Utils.h" 29 #include "llvm/Transforms/Utils/Local.h" 30 #include <list> 31 using namespace llvm; 32 33 #define DEBUG_TYPE "reg2mem" 34 35 STATISTIC(NumRegsDemoted, "Number of registers demoted"); 36 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted"); 37 38 namespace { 39 struct RegToMem : public FunctionPass { 40 static char ID; // Pass identification, replacement for typeid 41 RegToMem() : FunctionPass(ID) { 42 initializeRegToMemPass(*PassRegistry::getPassRegistry()); 43 } 44 45 void getAnalysisUsage(AnalysisUsage &AU) const override { 46 AU.addRequiredID(BreakCriticalEdgesID); 47 AU.addPreservedID(BreakCriticalEdgesID); 48 } 49 50 bool valueEscapes(const Instruction *Inst) const { 51 const BasicBlock *BB = Inst->getParent(); 52 for (const User *U : Inst->users()) { 53 const Instruction *UI = cast<Instruction>(U); 54 if (UI->getParent() != BB || isa<PHINode>(UI)) 55 return true; 56 } 57 return false; 58 } 59 60 bool runOnFunction(Function &F) override; 61 }; 62 } 63 64 char RegToMem::ID = 0; 65 INITIALIZE_PASS_BEGIN(RegToMem, "reg2mem", "Demote all values to stack slots", 66 false, false) 67 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges) 68 INITIALIZE_PASS_END(RegToMem, "reg2mem", "Demote all values to stack slots", 69 false, false) 70 71 bool RegToMem::runOnFunction(Function &F) { 72 if (F.isDeclaration() || skipFunction(F)) 73 return false; 74 75 // Insert all new allocas into entry block. 76 BasicBlock *BBEntry = &F.getEntryBlock(); 77 assert(pred_empty(BBEntry) && 78 "Entry block to function must not have predecessors!"); 79 80 // Find first non-alloca instruction and create insertion point. This is 81 // safe if block is well-formed: it always have terminator, otherwise 82 // we'll get and assertion. 83 BasicBlock::iterator I = BBEntry->begin(); 84 while (isa<AllocaInst>(I)) ++I; 85 86 CastInst *AllocaInsertionPoint = new BitCastInst( 87 Constant::getNullValue(Type::getInt32Ty(F.getContext())), 88 Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I); 89 90 // Find the escaped instructions. But don't create stack slots for 91 // allocas in entry block. 92 std::list<Instruction*> WorkList; 93 for (BasicBlock &ibb : F) 94 for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie; 95 ++iib) { 96 if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) && 97 valueEscapes(&*iib)) { 98 WorkList.push_front(&*iib); 99 } 100 } 101 102 // Demote escaped instructions 103 NumRegsDemoted += WorkList.size(); 104 for (Instruction *ilb : WorkList) 105 DemoteRegToStack(*ilb, false, AllocaInsertionPoint); 106 107 WorkList.clear(); 108 109 // Find all phi's 110 for (BasicBlock &ibb : F) 111 for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie; 112 ++iib) 113 if (isa<PHINode>(iib)) 114 WorkList.push_front(&*iib); 115 116 // Demote phi nodes 117 NumPhisDemoted += WorkList.size(); 118 for (Instruction *ilb : WorkList) 119 DemotePHIToStack(cast<PHINode>(ilb), AllocaInsertionPoint); 120 121 return true; 122 } 123 124 125 // createDemoteRegisterToMemory - Provide an entry point to create this pass. 126 char &llvm::DemoteRegisterToMemoryID = RegToMem::ID; 127 FunctionPass *llvm::createDemoteRegisterToMemoryPass() { 128 return new RegToMem(); 129 } 130