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//===- Reg2Mem.cpp - Convert registers to allocas -------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file demotes all registers to memory references. It is intended to be
// the inverse of PromoteMemoryToRegister. By converting to loads, the only
// values live across basic blocks are allocas and loads before phi nodes.
// It is intended that this should make CFG hacking much easier.
// To make later hacking easier, the entry block is split into two, such that
// all introduced allocas and nothing else are in the entry block.
#include "llvm/Transforms/Scalar/Reg2Mem.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PassManager.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include <list>
using namespace llvm;
#define DEBUG_TYPE "reg2mem"
STATISTIC(NumRegsDemoted, "Number of registers demoted");
STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
static bool valueEscapes(const Instruction &Inst) {
if (!Inst.getType()->isSized())
return false;
const BasicBlock *BB = Inst.getParent();
for (const User *U : Inst.users()) {
const Instruction *UI = cast<Instruction>(U);
if (UI->getParent() != BB || isa<PHINode>(UI))
return true;
return false;
static bool runPass(Function &F) {
// Insert all new allocas into entry block.
BasicBlock *BBEntry = &F.getEntryBlock();
assert(pred_empty(BBEntry) &&
"Entry block to function must not have predecessors!");
// Find first non-alloca instruction and create insertion point. This is
// safe if block is well-formed: it always have terminator, otherwise
// we'll get and assertion.
BasicBlock::iterator I = BBEntry->begin();
while (isa<AllocaInst>(I)) ++I;
CastInst *AllocaInsertionPoint = new BitCastInst(
Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
// Find the escaped instructions. But don't create stack slots for
// allocas in entry block.
std::list<Instruction*> WorkList;
for (Instruction &I : instructions(F))
if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
// Demote escaped instructions
NumRegsDemoted += WorkList.size();
for (Instruction *I : WorkList)
DemoteRegToStack(*I, false, AllocaInsertionPoint);
// Find all phi's
for (BasicBlock &BB : F)
for (auto &Phi : BB.phis())
// Demote phi nodes
NumPhisDemoted += WorkList.size();
for (Instruction *I : WorkList)
DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
return true;
PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
auto *LI = &AM.getResult<LoopAnalysis>(F);
unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
bool Changed = runPass(F);
if (N == 0 && !Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
namespace {
struct RegToMemLegacy : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
RegToMemLegacy() : FunctionPass(ID) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnFunction(Function &F) override {
if (F.isDeclaration() || skipFunction(F))
return false;
return runPass(F);
} // namespace
char RegToMemLegacy::ID = 0;
INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem",
"Demote all values to stack slots", false, false)
INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem",
"Demote all values to stack slots", false, false)
// createDemoteRegisterToMemory - Provide an entry point to create this pass.
char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID;
FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
return new RegToMemLegacy();