llvm/lib/Target/X86/X86MacroFusion.cpp

//===- X86MacroFusion.cpp - X86 Macro Fusion ------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// \file This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the X86 implementation of the DAG scheduling mutation to
// pair instructions back to back.
//
//===----------------------------------------------------------------------===//

#include "X86MacroFusion.h"
#include "X86Subtarget.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetInstrInfo.h"

#define DEBUG_TYPE "misched"

using namespace llvm;

static cl::opt<bool> EnableMacroFusion("x86-misched-fusion", cl::Hidden,
  cl::desc("Enable scheduling for macro fusion."), cl::init(true));

namespace {

/// \brief Verify that the instruction pair, First and Second,
/// should be scheduled back to back.  If either instruction is unspecified,
/// then verify that the other instruction may be part of a pair at all.
static bool shouldScheduleAdjacent(const X86Subtarget &ST,
                                   const MachineInstr *First,
                                   const MachineInstr *Second) {
  // Check if this processor supports macro-fusion. Since this is a minor
  // heuristic, we haven't specifically reserved a feature. hasAVX is a decent
  // proxy for SandyBridge+.
  if (!ST.hasAVX())
    return false;

  enum {
    FuseTest,
    FuseCmp,
    FuseInc
  } FuseKind;

  unsigned FirstOpcode = First
                             ? First->getOpcode()
                             : static_cast<unsigned>(X86::INSTRUCTION_LIST_END);
  unsigned SecondOpcode =
      Second ? Second->getOpcode()
             : static_cast<unsigned>(X86::INSTRUCTION_LIST_END);

  switch (SecondOpcode) {
  default:
    return false;
  case X86::JE_1:
  case X86::JNE_1:
  case X86::JL_1:
  case X86::JLE_1:
  case X86::JG_1:
  case X86::JGE_1:
    FuseKind = FuseInc;
    break;
  case X86::JB_1:
  case X86::JBE_1:
  case X86::JA_1:
  case X86::JAE_1:
    FuseKind = FuseCmp;
    break;
  case X86::JS_1:
  case X86::JNS_1:
  case X86::JP_1:
  case X86::JNP_1:
  case X86::JO_1:
  case X86::JNO_1:
    FuseKind = FuseTest;
    break;
  }

  switch (FirstOpcode) {
  default:
    return false;
  case X86::TEST8rr:
  case X86::TEST16rr:
  case X86::TEST32rr:
  case X86::TEST64rr:
  case X86::TEST8ri:
  case X86::TEST16ri:
  case X86::TEST32ri:
  case X86::TEST32i32:
  case X86::TEST64i32:
  case X86::TEST64ri32:
  case X86::TEST8rm:
  case X86::TEST16rm:
  case X86::TEST32rm:
  case X86::TEST64rm:
  case X86::TEST8ri_NOREX:
  case X86::AND16i16:
  case X86::AND16ri:
  case X86::AND16ri8:
  case X86::AND16rm:
  case X86::AND16rr:
  case X86::AND32i32:
  case X86::AND32ri:
  case X86::AND32ri8:
  case X86::AND32rm:
  case X86::AND32rr:
  case X86::AND64i32:
  case X86::AND64ri32:
  case X86::AND64ri8:
  case X86::AND64rm:
  case X86::AND64rr:
  case X86::AND8i8:
  case X86::AND8ri:
  case X86::AND8rm:
  case X86::AND8rr:
    return true;
  case X86::CMP16i16:
  case X86::CMP16ri:
  case X86::CMP16ri8:
  case X86::CMP16rm:
  case X86::CMP16rr:
  case X86::CMP32i32:
  case X86::CMP32ri:
  case X86::CMP32ri8:
  case X86::CMP32rm:
  case X86::CMP32rr:
  case X86::CMP64i32:
  case X86::CMP64ri32:
  case X86::CMP64ri8:
  case X86::CMP64rm:
  case X86::CMP64rr:
  case X86::CMP8i8:
  case X86::CMP8ri:
  case X86::CMP8rm:
  case X86::CMP8rr:
  case X86::ADD16i16:
  case X86::ADD16ri:
  case X86::ADD16ri8:
  case X86::ADD16ri8_DB:
  case X86::ADD16ri_DB:
  case X86::ADD16rm:
  case X86::ADD16rr:
  case X86::ADD16rr_DB:
  case X86::ADD32i32:
  case X86::ADD32ri:
  case X86::ADD32ri8:
  case X86::ADD32ri8_DB:
  case X86::ADD32ri_DB:
  case X86::ADD32rm:
  case X86::ADD32rr:
  case X86::ADD32rr_DB:
  case X86::ADD64i32:
  case X86::ADD64ri32:
  case X86::ADD64ri32_DB:
  case X86::ADD64ri8:
  case X86::ADD64ri8_DB:
  case X86::ADD64rm:
  case X86::ADD64rr:
  case X86::ADD64rr_DB:
  case X86::ADD8i8:
  case X86::ADD8mi:
  case X86::ADD8mr:
  case X86::ADD8ri:
  case X86::ADD8rm:
  case X86::ADD8rr:
  case X86::SUB16i16:
  case X86::SUB16ri:
  case X86::SUB16ri8:
  case X86::SUB16rm:
  case X86::SUB16rr:
  case X86::SUB32i32:
  case X86::SUB32ri:
  case X86::SUB32ri8:
  case X86::SUB32rm:
  case X86::SUB32rr:
  case X86::SUB64i32:
  case X86::SUB64ri32:
  case X86::SUB64ri8:
  case X86::SUB64rm:
  case X86::SUB64rr:
  case X86::SUB8i8:
  case X86::SUB8ri:
  case X86::SUB8rm:
  case X86::SUB8rr:
    return FuseKind == FuseCmp || FuseKind == FuseInc;
  case X86::INC16r:
  case X86::INC32r:
  case X86::INC64r:
  case X86::INC8r:
  case X86::DEC16r:
  case X86::DEC32r:
  case X86::DEC64r:
  case X86::DEC8r:
    return FuseKind == FuseInc;
  case X86::INSTRUCTION_LIST_END:
    return true;
  }
}

/// \brief Post-process the DAG to create cluster edges between instructions
/// that may be fused by the processor into a single operation.
class X86MacroFusion : public ScheduleDAGMutation {
public:
  X86MacroFusion() {}

  void apply(ScheduleDAGInstrs *DAGInstrs) override;
};

void X86MacroFusion::apply(ScheduleDAGInstrs *DAGInstrs) {
  ScheduleDAGMI *DAG = static_cast<ScheduleDAGMI*>(DAGInstrs);
  const X86Subtarget &ST = DAG->MF.getSubtarget<X86Subtarget>();

  // For now, assume targets can only fuse with the branch.
  SUnit &ExitSU = DAG->ExitSU;
  MachineInstr *Branch = ExitSU.getInstr();
  if (!shouldScheduleAdjacent(ST, nullptr, Branch))
    return;

  for (SDep &PredDep : ExitSU.Preds) {
    if (PredDep.isWeak())
      continue;
    SUnit &SU = *PredDep.getSUnit();
    MachineInstr &Pred = *SU.getInstr();
    if (!shouldScheduleAdjacent(ST, &Pred, Branch))
      continue;

    // Create a single weak edge from SU to ExitSU. The only effect is to cause
    // bottom-up scheduling to heavily prioritize the clustered SU.  There is no
    // need to copy predecessor edges from ExitSU to SU, since top-down
    // scheduling cannot prioritize ExitSU anyway. To defer top-down scheduling
    // of SU, we could create an artificial edge from the deepest root, but it
    // hasn't been needed yet.
    bool Success = DAG->addEdge(&ExitSU, SDep(&SU, SDep::Cluster));
    (void)Success;
    assert(Success && "No DAG nodes should be reachable from ExitSU");

    // Adjust latency of data deps between the nodes.
    for (SDep &PredDep : ExitSU.Preds)
      if (PredDep.getSUnit() == &SU)
        PredDep.setLatency(0);
    for (SDep &SuccDep : SU.Succs)
      if (SuccDep.getSUnit() == &ExitSU)
        SuccDep.setLatency(0);

    DEBUG(dbgs() << "Macro fuse ";
          SU.print(dbgs(), DAG);
          dbgs() << " - ExitSU" << '\n');

    break;
  }
}

} // end namespace

namespace llvm {

std::unique_ptr<ScheduleDAGMutation>
createX86MacroFusionDAGMutation () {
  return EnableMacroFusion ? make_unique<X86MacroFusion>() : nullptr;
}

} // end namespace llvm