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| //===-- MLxExpansionPass.cpp - Expand MLx instrs to avoid hazards ---------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Expand VFP / NEON floating point MLA / MLS instructions (each to a pair of
// multiple and add / sub instructions) when special VMLx hazards are detected.
//
//===----------------------------------------------------------------------===//
#include "ARM.h"
#include "ARMBaseInstrInfo.h"
#include "ARMSubtarget.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "mlx-expansion"
static cl::opt<bool>
ForceExapnd("expand-all-fp-mlx", cl::init(false), cl::Hidden);
static cl::opt<unsigned>
ExpandLimit("expand-limit", cl::init(~0U), cl::Hidden);
STATISTIC(NumExpand, "Number of fp MLA / MLS instructions expanded");
namespace {
struct MLxExpansion : public MachineFunctionPass {
static char ID;
MLxExpansion() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &Fn) override;
StringRef getPassName() const override {
return "ARM MLA / MLS expansion pass";
}
private:
const ARMBaseInstrInfo *TII;
const TargetRegisterInfo *TRI;
MachineRegisterInfo *MRI;
bool isLikeA9;
bool isSwift;
unsigned MIIdx;
MachineInstr* LastMIs[4];
SmallPtrSet<MachineInstr*, 4> IgnoreStall;
void clearStack();
void pushStack(MachineInstr *MI);
MachineInstr *getAccDefMI(MachineInstr *MI) const;
unsigned getDefReg(MachineInstr *MI) const;
bool hasLoopHazard(MachineInstr *MI) const;
bool hasRAWHazard(unsigned Reg, MachineInstr *MI) const;
bool FindMLxHazard(MachineInstr *MI);
void ExpandFPMLxInstruction(MachineBasicBlock &MBB, MachineInstr *MI,
unsigned MulOpc, unsigned AddSubOpc,
bool NegAcc, bool HasLane);
bool ExpandFPMLxInstructions(MachineBasicBlock &MBB);
};
char MLxExpansion::ID = 0;
}
void MLxExpansion::clearStack() {
std::fill(LastMIs, LastMIs + 4, nullptr);
MIIdx = 0;
}
void MLxExpansion::pushStack(MachineInstr *MI) {
LastMIs[MIIdx] = MI;
if (++MIIdx == 4)
MIIdx = 0;
}
MachineInstr *MLxExpansion::getAccDefMI(MachineInstr *MI) const {
// Look past COPY and INSERT_SUBREG instructions to find the
// real definition MI. This is important for _sfp instructions.
Register Reg = MI->getOperand(1).getReg();
if (Register::isPhysicalRegister(Reg))
return nullptr;
MachineBasicBlock *MBB = MI->getParent();
MachineInstr *DefMI = MRI->getVRegDef(Reg);
while (true) {
if (DefMI->getParent() != MBB)
break;
if (DefMI->isCopyLike()) {
Reg = DefMI->getOperand(1).getReg();
if (Register::isVirtualRegister(Reg)) {
DefMI = MRI->getVRegDef(Reg);
continue;
}
} else if (DefMI->isInsertSubreg()) {
Reg = DefMI->getOperand(2).getReg();
if (Register::isVirtualRegister(Reg)) {
DefMI = MRI->getVRegDef(Reg);
continue;
}
}
break;
}
return DefMI;
}
unsigned MLxExpansion::getDefReg(MachineInstr *MI) const {
Register Reg = MI->getOperand(0).getReg();
if (Register::isPhysicalRegister(Reg) || !MRI->hasOneNonDBGUse(Reg))
return Reg;
MachineBasicBlock *MBB = MI->getParent();
MachineInstr *UseMI = &*MRI->use_instr_nodbg_begin(Reg);
if (UseMI->getParent() != MBB)
return Reg;
while (UseMI->isCopy() || UseMI->isInsertSubreg()) {
Reg = UseMI->getOperand(0).getReg();
if (Register::isPhysicalRegister(Reg) || !MRI->hasOneNonDBGUse(Reg))
return Reg;
UseMI = &*MRI->use_instr_nodbg_begin(Reg);
if (UseMI->getParent() != MBB)
return Reg;
}
return Reg;
}
/// hasLoopHazard - Check whether an MLx instruction is chained to itself across
/// a single-MBB loop.
bool MLxExpansion::hasLoopHazard(MachineInstr *MI) const {
Register Reg = MI->getOperand(1).getReg();
if (Register::isPhysicalRegister(Reg))
return false;
MachineBasicBlock *MBB = MI->getParent();
MachineInstr *DefMI = MRI->getVRegDef(Reg);
while (true) {
outer_continue:
if (DefMI->getParent() != MBB)
break;
if (DefMI->isPHI()) {
for (unsigned i = 1, e = DefMI->getNumOperands(); i < e; i += 2) {
if (DefMI->getOperand(i + 1).getMBB() == MBB) {
Register SrcReg = DefMI->getOperand(i).getReg();
if (Register::isVirtualRegister(SrcReg)) {
DefMI = MRI->getVRegDef(SrcReg);
goto outer_continue;
}
}
}
} else if (DefMI->isCopyLike()) {
Reg = DefMI->getOperand(1).getReg();
if (Register::isVirtualRegister(Reg)) {
DefMI = MRI->getVRegDef(Reg);
continue;
}
} else if (DefMI->isInsertSubreg()) {
Reg = DefMI->getOperand(2).getReg();
if (Register::isVirtualRegister(Reg)) {
DefMI = MRI->getVRegDef(Reg);
continue;
}
}
break;
}
return DefMI == MI;
}
bool MLxExpansion::hasRAWHazard(unsigned Reg, MachineInstr *MI) const {
// FIXME: Detect integer instructions properly.
const MCInstrDesc &MCID = MI->getDesc();
unsigned Domain = MCID.TSFlags & ARMII::DomainMask;
if (MI->mayStore())
return false;
unsigned Opcode = MCID.getOpcode();
if (Opcode == ARM::VMOVRS || Opcode == ARM::VMOVRRD)
return false;
if ((Domain & ARMII::DomainVFP) || (Domain & ARMII::DomainNEON))
return MI->readsRegister(Reg, TRI);
return false;
}
static bool isFpMulInstruction(unsigned Opcode) {
switch (Opcode) {
case ARM::VMULS:
case ARM::VMULfd:
case ARM::VMULfq:
case ARM::VMULD:
case ARM::VMULslfd:
case ARM::VMULslfq:
return true;
default:
return false;
}
}
bool MLxExpansion::FindMLxHazard(MachineInstr *MI) {
if (NumExpand >= ExpandLimit)
return false;
if (ForceExapnd)
return true;
MachineInstr *DefMI = getAccDefMI(MI);
if (TII->isFpMLxInstruction(DefMI->getOpcode())) {
// r0 = vmla
// r3 = vmla r0, r1, r2
// takes 16 - 17 cycles
//
// r0 = vmla
// r4 = vmul r1, r2
// r3 = vadd r0, r4
// takes about 14 - 15 cycles even with vmul stalling for 4 cycles.
IgnoreStall.insert(DefMI);
return true;
}
// On Swift, we mostly care about hazards from multiplication instructions
// writing the accumulator and the pipelining of loop iterations by out-of-
// order execution.
if (isSwift)
return isFpMulInstruction(DefMI->getOpcode()) || hasLoopHazard(MI);
if (IgnoreStall.count(MI))
return false;
// If a VMLA.F is followed by an VADD.F or VMUL.F with no RAW hazard, the
// VADD.F or VMUL.F will stall 4 cycles before issue. The 4 cycle stall
// preserves the in-order retirement of the instructions.
// Look at the next few instructions, if *most* of them can cause hazards,
// then the scheduler can't *fix* this, we'd better break up the VMLA.
unsigned Limit1 = isLikeA9 ? 1 : 4;
unsigned Limit2 = isLikeA9 ? 1 : 4;
for (unsigned i = 1; i <= 4; ++i) {
int Idx = ((int)MIIdx - i + 4) % 4;
MachineInstr *NextMI = LastMIs[Idx];
if (!NextMI)
continue;
if (TII->canCauseFpMLxStall(NextMI->getOpcode())) {
if (i <= Limit1)
return true;
}
// Look for VMLx RAW hazard.
if (i <= Limit2 && hasRAWHazard(getDefReg(MI), NextMI))
return true;
}
return false;
}
/// ExpandFPMLxInstructions - Expand a MLA / MLS instruction into a pair
/// of MUL + ADD / SUB instructions.
void
MLxExpansion::ExpandFPMLxInstruction(MachineBasicBlock &MBB, MachineInstr *MI,
unsigned MulOpc, unsigned AddSubOpc,
bool NegAcc, bool HasLane) {
Register DstReg = MI->getOperand(0).getReg();
bool DstDead = MI->getOperand(0).isDead();
Register AccReg = MI->getOperand(1).getReg();
Register Src1Reg = MI->getOperand(2).getReg();
Register Src2Reg = MI->getOperand(3).getReg();
bool Src1Kill = MI->getOperand(2).isKill();
bool Src2Kill = MI->getOperand(3).isKill();
unsigned LaneImm = HasLane ? MI->getOperand(4).getImm() : 0;
unsigned NextOp = HasLane ? 5 : 4;
ARMCC::CondCodes Pred = (ARMCC::CondCodes)MI->getOperand(NextOp).getImm();
Register PredReg = MI->getOperand(++NextOp).getReg();
const MCInstrDesc &MCID1 = TII->get(MulOpc);
const MCInstrDesc &MCID2 = TII->get(AddSubOpc);
const MachineFunction &MF = *MI->getParent()->getParent();
Register TmpReg =
MRI->createVirtualRegister(TII->getRegClass(MCID1, 0, TRI, MF));
MachineInstrBuilder MIB = BuildMI(MBB, MI, MI->getDebugLoc(), MCID1, TmpReg)
.addReg(Src1Reg, getKillRegState(Src1Kill))
.addReg(Src2Reg, getKillRegState(Src2Kill));
if (HasLane)
MIB.addImm(LaneImm);
MIB.addImm(Pred).addReg(PredReg);
MIB = BuildMI(MBB, MI, MI->getDebugLoc(), MCID2)
.addReg(DstReg, getDefRegState(true) | getDeadRegState(DstDead));
if (NegAcc) {
bool AccKill = MRI->hasOneNonDBGUse(AccReg);
MIB.addReg(TmpReg, getKillRegState(true))
.addReg(AccReg, getKillRegState(AccKill));
} else {
MIB.addReg(AccReg).addReg(TmpReg, getKillRegState(true));
}
MIB.addImm(Pred).addReg(PredReg);
LLVM_DEBUG({
dbgs() << "Expanding: " << *MI;
dbgs() << " to:\n";
MachineBasicBlock::iterator MII = MI;
MII = std::prev(MII);
MachineInstr &MI2 = *MII;
MII = std::prev(MII);
MachineInstr &MI1 = *MII;
dbgs() << " " << MI1;
dbgs() << " " << MI2;
});
MI->eraseFromParent();
++NumExpand;
}
bool MLxExpansion::ExpandFPMLxInstructions(MachineBasicBlock &MBB) {
bool Changed = false;
clearStack();
IgnoreStall.clear();
unsigned Skip = 0;
MachineBasicBlock::reverse_iterator MII = MBB.rbegin(), E = MBB.rend();
while (MII != E) {
MachineInstr *MI = &*MII++;
if (MI->isPosition() || MI->isImplicitDef() || MI->isCopy())
continue;
const MCInstrDesc &MCID = MI->getDesc();
if (MI->isBarrier()) {
clearStack();
Skip = 0;
continue;
}
unsigned Domain = MCID.TSFlags & ARMII::DomainMask;
if (Domain == ARMII::DomainGeneral) {
if (++Skip == 2)
// Assume dual issues of non-VFP / NEON instructions.
pushStack(nullptr);
} else {
Skip = 0;
unsigned MulOpc, AddSubOpc;
bool NegAcc, HasLane;
if (!TII->isFpMLxInstruction(MCID.getOpcode(),
MulOpc, AddSubOpc, NegAcc, HasLane) ||
!FindMLxHazard(MI))
pushStack(MI);
else {
ExpandFPMLxInstruction(MBB, MI, MulOpc, AddSubOpc, NegAcc, HasLane);
Changed = true;
}
}
}
return Changed;
}
bool MLxExpansion::runOnMachineFunction(MachineFunction &Fn) {
if (skipFunction(Fn.getFunction()))
return false;
TII = static_cast<const ARMBaseInstrInfo *>(Fn.getSubtarget().getInstrInfo());
TRI = Fn.getSubtarget().getRegisterInfo();
MRI = &Fn.getRegInfo();
const ARMSubtarget *STI = &Fn.getSubtarget<ARMSubtarget>();
if (!STI->expandMLx())
return false;
isLikeA9 = STI->isLikeA9() || STI->isSwift();
isSwift = STI->isSwift();
bool Modified = false;
for (MachineBasicBlock &MBB : Fn)
Modified |= ExpandFPMLxInstructions(MBB);
return Modified;
}
FunctionPass *llvm::createMLxExpansionPass() {
return new MLxExpansion();
}
|