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//===----- PostRAHazardRecognizer.cpp - hazard recognizer -----------------===//
// 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
/// \file
/// This runs the hazard recognizer and emits noops when necessary. This
/// gives targets a way to run the hazard recognizer without running one of
/// the schedulers. Example use cases for this pass would be:
/// - Targets that need the hazard recognizer to be run at -O0.
/// - Targets that want to guarantee that hazards at the beginning of
/// scheduling regions are handled correctly. The post-RA scheduler is
/// a top-down scheduler, but when there are multiple scheduling regions
/// in a basic block, it visits the regions in bottom-up order. This
/// makes it impossible for the scheduler to gauranttee it can correctly
/// handle hazards at the beginning of scheduling regions.
/// This pass traverses all the instructions in a program in top-down order.
/// In contrast to the instruction scheduling passes, this pass never resets
/// the hazard recognizer to ensure it can correctly handles noop hazards at
/// the beginning of blocks.
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/ScheduleHazardRecognizer.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
using namespace llvm;
#define DEBUG_TYPE "post-RA-hazard-rec"
STATISTIC(NumNoops, "Number of noops inserted");
namespace {
class PostRAHazardRecognizer : public MachineFunctionPass {
static char ID;
PostRAHazardRecognizer() : MachineFunctionPass(ID) {}
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &Fn) override;
char PostRAHazardRecognizer::ID = 0;
char &llvm::PostRAHazardRecognizerID = PostRAHazardRecognizer::ID;
"Post RA hazard recognizer", false, false)
bool PostRAHazardRecognizer::runOnMachineFunction(MachineFunction &Fn) {
const TargetInstrInfo *TII = Fn.getSubtarget().getInstrInfo();
std::unique_ptr<ScheduleHazardRecognizer> HazardRec(
// Return if the target has not implemented a hazard recognizer.
if (!HazardRec)
return false;
// Loop over all of the basic blocks
bool Changed = false;
for (auto &MBB : Fn) {
// We do not call HazardRec->reset() here to make sure we are handling noop
// hazards at the start of basic blocks.
for (MachineInstr &MI : MBB) {
// If we need to emit noops prior to this instruction, then do so.
unsigned NumPreNoops = HazardRec->PreEmitNoops(&MI);
TII->insertNoops(MBB, MachineBasicBlock::iterator(MI), NumPreNoops);
NumNoops += NumPreNoops;
if (NumPreNoops)
Changed = true;
if (HazardRec->atIssueLimit()) {
return Changed;