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//===------------ BPFIRPeephole.cpp - IR Peephole Transformation ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// IR level peephole optimization, specifically removing @llvm.stacksave() and
// @llvm.stackrestore().
//
//===----------------------------------------------------------------------===//
#include "BPF.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#define DEBUG_TYPE "bpf-ir-peephole"
using namespace llvm;
namespace {
static bool BPFIRPeepholeImpl(Function &F) {
LLVM_DEBUG(dbgs() << "******** BPF IR Peephole ********\n");
bool Changed = false;
Instruction *ToErase = nullptr;
for (auto &BB : F) {
for (auto &I : BB) {
// The following code pattern is handled:
// %3 = call i8* @llvm.stacksave()
// store i8* %3, i8** %saved_stack, align 8
// ...
// %4 = load i8*, i8** %saved_stack, align 8
// call void @llvm.stackrestore(i8* %4)
// ...
// The goal is to remove the above four instructions,
// so we won't have instructions with r11 (stack pointer)
// if eventually there is no variable length stack allocation.
// InstrCombine also tries to remove the above instructions,
// if it is proven safe (constant alloca etc.), but depending
// on code pattern, it may still miss some.
//
// With unconditionally removing these instructions, if alloca is
// constant, we are okay then. Otherwise, SelectionDag will complain
// since BPF does not support dynamic allocation yet.
if (ToErase) {
ToErase->eraseFromParent();
ToErase = nullptr;
}
if (auto *II = dyn_cast<IntrinsicInst>(&I)) {
if (II->getIntrinsicID() != Intrinsic::stacksave)
continue;
if (!II->hasOneUser())
continue;
auto *Inst = cast<Instruction>(*II->user_begin());
LLVM_DEBUG(dbgs() << "Remove:"; I.dump());
LLVM_DEBUG(dbgs() << "Remove:"; Inst->dump(); dbgs() << '\n');
Changed = true;
Inst->eraseFromParent();
ToErase = &I;
continue;
}
if (auto *LD = dyn_cast<LoadInst>(&I)) {
if (!LD->hasOneUser())
continue;
auto *II = dyn_cast<IntrinsicInst>(*LD->user_begin());
if (!II)
continue;
if (II->getIntrinsicID() != Intrinsic::stackrestore)
continue;
LLVM_DEBUG(dbgs() << "Remove:"; I.dump());
LLVM_DEBUG(dbgs() << "Remove:"; II->dump(); dbgs() << '\n');
Changed = true;
II->eraseFromParent();
ToErase = &I;
}
}
}
return Changed;
}
} // End anonymous namespace
PreservedAnalyses BPFIRPeepholePass::run(Function &F,
FunctionAnalysisManager &AM) {
return BPFIRPeepholeImpl(F) ? PreservedAnalyses::none()
: PreservedAnalyses::all();
}
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