llvm-project/llvm/lib/Transforms/Scalar/Reg2Mem.cpp

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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 https://llvm.org/LICENSE.txt 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/ADT/Statistic.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.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");
namespace {
struct RegToMem : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
RegToMem() : FunctionPass(ID) {
initializeRegToMemPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequiredID(BreakCriticalEdgesID);
AU.addPreservedID(BreakCriticalEdgesID);
}
[C++11] Add range based accessors for the Use-Def chain of a Value. This requires a number of steps. 1) Move value_use_iterator into the Value class as an implementation detail 2) Change it to actually be a *Use* iterator rather than a *User* iterator. 3) Add an adaptor which is a User iterator that always looks through the Use to the User. 4) Wrap these in Value::use_iterator and Value::user_iterator typedefs. 5) Add the range adaptors as Value::uses() and Value::users(). 6) Update *all* of the callers to correctly distinguish between whether they wanted a use_iterator (and to explicitly dig out the User when needed), or a user_iterator which makes the Use itself totally opaque. Because #6 requires churning essentially everything that walked the Use-Def chains, I went ahead and added all of the range adaptors and switched them to range-based loops where appropriate. Also because the renaming requires at least churning every line of code, it didn't make any sense to split these up into multiple commits -- all of which would touch all of the same lies of code. The result is still not quite optimal. The Value::use_iterator is a nice regular iterator, but Value::user_iterator is an iterator over User*s rather than over the User objects themselves. As a consequence, it fits a bit awkwardly into the range-based world and it has the weird extra-dereferencing 'operator->' that so many of our iterators have. I think this could be fixed by providing something which transforms a range of T&s into a range of T*s, but that *can* be separated into another patch, and it isn't yet 100% clear whether this is the right move. However, this change gets us most of the benefit and cleans up a substantial amount of code around Use and User. =] llvm-svn: 203364
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bool valueEscapes(const Instruction *Inst) const {
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;
}
bool runOnFunction(Function &F) override;
};
}
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char RegToMem::ID = 0;
INITIALIZE_PASS_BEGIN(RegToMem, "reg2mem", "Demote all values to stack slots",
false, false)
INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
INITIALIZE_PASS_END(RegToMem, "reg2mem", "Demote all values to stack slots",
false, false)
bool RegToMem::runOnFunction(Function &F) {
if (F.isDeclaration() || skipFunction(F))
return false;
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// Insert all new allocas into entry block.
BasicBlock *BBEntry = &F.getEntryBlock();
assert(pred_empty(BBEntry) &&
"Entry block to function must not have predecessors!");
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// 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;
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CastInst *AllocaInsertionPoint = new BitCastInst(
Constant::getNullValue(Type::getInt32Ty(F.getContext())),
Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
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// Find the escaped instructions. But don't create stack slots for
// allocas in entry block.
std::list<Instruction*> WorkList;
for (BasicBlock &ibb : F)
for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie;
++iib) {
if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
valueEscapes(&*iib)) {
WorkList.push_front(&*iib);
}
}
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// Demote escaped instructions
NumRegsDemoted += WorkList.size();
for (Instruction *ilb : WorkList)
DemoteRegToStack(*ilb, false, AllocaInsertionPoint);
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WorkList.clear();
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// Find all phi's
for (BasicBlock &ibb : F)
for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie;
++iib)
if (isa<PHINode>(iib))
WorkList.push_front(&*iib);
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// Demote phi nodes
NumPhisDemoted += WorkList.size();
for (Instruction *ilb : WorkList)
DemotePHIToStack(cast<PHINode>(ilb), AllocaInsertionPoint);
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return true;
}
// createDemoteRegisterToMemory - Provide an entry point to create this pass.
char &llvm::DemoteRegisterToMemoryID = RegToMem::ID;
FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
return new RegToMem();
}