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//===- MacroFusion.cpp - Macro Fusion -------------------------------------===//
// 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 file contains the implementation of the DAG scheduling mutation
/// to pair instructions back to back.
#include "llvm/CodeGen/MacroFusion.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/ScheduleDAG.h"
#include "llvm/CodeGen/ScheduleDAGInstrs.h"
#include "llvm/CodeGen/ScheduleDAGMutation.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#define DEBUG_TYPE "machine-scheduler"
STATISTIC(NumFused, "Number of instr pairs fused");
using namespace llvm;
static cl::opt<bool> EnableMacroFusion("misched-fusion", cl::Hidden,
cl::desc("Enable scheduling for macro fusion."), cl::init(true));
static bool isHazard(const SDep &Dep) {
return Dep.getKind() == SDep::Anti || Dep.getKind() == SDep::Output;
static SUnit *getPredClusterSU(const SUnit &SU) {
for (const SDep &SI : SU.Preds)
if (SI.isCluster())
return SI.getSUnit();
return nullptr;
bool llvm::hasLessThanNumFused(const SUnit &SU, unsigned FuseLimit) {
unsigned Num = 1;
const SUnit *CurrentSU = &SU;
while ((CurrentSU = getPredClusterSU(*CurrentSU)) && Num < FuseLimit) Num ++;
return Num < FuseLimit;
bool llvm::fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
SUnit &SecondSU) {
// Check that neither instr is already paired with another along the edge
// between them.
for (SDep &SI : FirstSU.Succs)
if (SI.isCluster())
return false;
for (SDep &SI : SecondSU.Preds)
if (SI.isCluster())
return false;
// Though the reachability checks above could be made more generic,
// perhaps as part of ScheduleDAGInstrs::addEdge(), since such edges are valid,
// the extra computation cost makes it less interesting in general cases.
// Create a single weak edge between the adjacent instrs. The only effect is
// to cause bottom-up scheduling to heavily prioritize the clustered instrs.
if (!DAG.addEdge(&SecondSU, SDep(&FirstSU, SDep::Cluster)))
return false;
// TODO - If we want to chain more than two instructions, we need to create
// artifical edges to make dependencies from the FirstSU also dependent
// on other chained instructions, and other chained instructions also
// dependent on the dependencies of the SecondSU, to prevent them from being
// scheduled into these chained instructions.
assert(hasLessThanNumFused(FirstSU, 2) &&
"Currently we only support chaining together two instructions");
// Adjust the latency between both instrs.
for (SDep &SI : FirstSU.Succs)
if (SI.getSUnit() == &SecondSU)
for (SDep &SI : SecondSU.Preds)
if (SI.getSUnit() == &FirstSU)
dbgs() << "Macro fuse: "; DAG.dumpNodeName(FirstSU); dbgs() << " - ";
DAG.dumpNodeName(SecondSU); dbgs() << " / ";
dbgs() << DAG.TII->getName(FirstSU.getInstr()->getOpcode()) << " - "
<< DAG.TII->getName(SecondSU.getInstr()->getOpcode()) << '\n';);
// Make data dependencies from the FirstSU also dependent on the SecondSU to
// prevent them from being scheduled between the FirstSU and the SecondSU.
if (&SecondSU != &DAG.ExitSU)
for (const SDep &SI : FirstSU.Succs) {
SUnit *SU = SI.getSUnit();
if (SI.isWeak() || isHazard(SI) ||
SU == &DAG.ExitSU || SU == &SecondSU || SU->isPred(&SecondSU))
LLVM_DEBUG(dbgs() << " Bind "; DAG.dumpNodeName(SecondSU);
dbgs() << " - "; DAG.dumpNodeName(*SU); dbgs() << '\n';);
DAG.addEdge(SU, SDep(&SecondSU, SDep::Artificial));
// Make the FirstSU also dependent on the dependencies of the SecondSU to
// prevent them from being scheduled between the FirstSU and the SecondSU.
if (&FirstSU != &DAG.EntrySU) {
for (const SDep &SI : SecondSU.Preds) {
SUnit *SU = SI.getSUnit();
if (SI.isWeak() || isHazard(SI) || &FirstSU == SU || FirstSU.isSucc(SU))
LLVM_DEBUG(dbgs() << " Bind "; DAG.dumpNodeName(*SU); dbgs() << " - ";
DAG.dumpNodeName(FirstSU); dbgs() << '\n';);
DAG.addEdge(&FirstSU, SDep(SU, SDep::Artificial));
// ExitSU comes last by design, which acts like an implicit dependency
// between ExitSU and any bottom root in the graph. We should transfer
// this to FirstSU as well.
if (&SecondSU == &DAG.ExitSU) {
for (SUnit &SU : DAG.SUnits) {
if (SU.Succs.empty())
DAG.addEdge(&FirstSU, SDep(&SU, SDep::Artificial));
return true;
namespace {
/// Post-process the DAG to create cluster edges between instrs that may
/// be fused by the processor into a single operation.
class MacroFusion : public ScheduleDAGMutation {
std::vector<MacroFusionPredTy> Predicates;
bool FuseBlock;
bool scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU);
MacroFusion(ArrayRef<MacroFusionPredTy> Predicates, bool FuseBlock)
: Predicates(Predicates.begin(), Predicates.end()), FuseBlock(FuseBlock) {
void apply(ScheduleDAGInstrs *DAGInstrs) override;
bool shouldScheduleAdjacent(const TargetInstrInfo &TII,
const TargetSubtargetInfo &STI,
const MachineInstr *FirstMI,
const MachineInstr &SecondMI);
} // end anonymous namespace
bool MacroFusion::shouldScheduleAdjacent(const TargetInstrInfo &TII,
const TargetSubtargetInfo &STI,
const MachineInstr *FirstMI,
const MachineInstr &SecondMI) {
return llvm::any_of(Predicates, [&](MacroFusionPredTy Predicate) {
return Predicate(TII, STI, FirstMI, SecondMI);
void MacroFusion::apply(ScheduleDAGInstrs *DAG) {
if (FuseBlock)
// For each of the SUnits in the scheduling block, try to fuse the instr in
// it with one in its predecessors.
for (SUnit &ISU : DAG->SUnits)
scheduleAdjacentImpl(*DAG, ISU);
if (DAG->ExitSU.getInstr())
// Try to fuse the instr in the ExitSU with one in its predecessors.
scheduleAdjacentImpl(*DAG, DAG->ExitSU);
/// Implement the fusion of instr pairs in the scheduling DAG,
/// anchored at the instr in AnchorSU..
bool MacroFusion::scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU) {
const MachineInstr &AnchorMI = *AnchorSU.getInstr();
const TargetInstrInfo &TII = *DAG.TII;
const TargetSubtargetInfo &ST = DAG.MF.getSubtarget();
// Check if the anchor instr may be fused.
if (!shouldScheduleAdjacent(TII, ST, nullptr, AnchorMI))
return false;
// Explorer for fusion candidates among the dependencies of the anchor instr.
for (SDep &Dep : AnchorSU.Preds) {
// Ignore dependencies other than data or strong ordering.
if (Dep.isWeak() || isHazard(Dep))
SUnit &DepSU = *Dep.getSUnit();
if (DepSU.isBoundaryNode())
// Only chain two instructions together at most.
const MachineInstr *DepMI = DepSU.getInstr();
if (!hasLessThanNumFused(DepSU, 2) ||
!shouldScheduleAdjacent(TII, ST, DepMI, AnchorMI))
if (fuseInstructionPair(DAG, DepSU, AnchorSU))
return true;
return false;
llvm::createMacroFusionDAGMutation(ArrayRef<MacroFusionPredTy> Predicates,
bool BranchOnly) {
if (EnableMacroFusion)
return std::make_unique<MacroFusion>(Predicates, !BranchOnly);
return nullptr;