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