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llvm-project/llvm/lib/Target/AMDGPU/SIOptimizeExecMaskingPreRA.cpp

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//===-- SIOptimizeExecMaskingPreRA.cpp ------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
/// \file
/// This pass performs exec mask handling peephole optimizations which needs
/// to be done before register allocation to reduce register pressure.
///
//===----------------------------------------------------------------------===//
#include "AMDGPU.h"
#include "AMDGPUSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "SIInstrInfo.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/InitializePasses.h"
using namespace llvm;
#define DEBUG_TYPE "si-optimize-exec-masking-pre-ra"
namespace {
class SIOptimizeExecMaskingPreRA : public MachineFunctionPass {
private:
const SIRegisterInfo *TRI;
const SIInstrInfo *TII;
MachineRegisterInfo *MRI;
LiveIntervals *LIS;
unsigned AndOpc;
unsigned Andn2Opc;
unsigned OrSaveExecOpc;
unsigned XorTermrOpc;
MCRegister CondReg;
MCRegister ExecReg;
Register optimizeVcndVcmpPair(MachineBasicBlock &MBB);
bool optimizeElseBranch(MachineBasicBlock &MBB);
public:
static char ID;
SIOptimizeExecMaskingPreRA() : MachineFunctionPass(ID) {
initializeSIOptimizeExecMaskingPreRAPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override {
return "SI optimize exec mask operations pre-RA";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<LiveIntervals>();
AU.setPreservesAll();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
} // End anonymous namespace.
INITIALIZE_PASS_BEGIN(SIOptimizeExecMaskingPreRA, DEBUG_TYPE,
"SI optimize exec mask operations pre-RA", false, false)
INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
INITIALIZE_PASS_END(SIOptimizeExecMaskingPreRA, DEBUG_TYPE,
"SI optimize exec mask operations pre-RA", false, false)
char SIOptimizeExecMaskingPreRA::ID = 0;
char &llvm::SIOptimizeExecMaskingPreRAID = SIOptimizeExecMaskingPreRA::ID;
FunctionPass *llvm::createSIOptimizeExecMaskingPreRAPass() {
return new SIOptimizeExecMaskingPreRA();
}
// See if there is a def between \p AndIdx and \p SelIdx that needs to live
// beyond \p AndIdx.
static bool isDefBetween(const LiveRange &LR, SlotIndex AndIdx,
SlotIndex SelIdx) {
LiveQueryResult AndLRQ = LR.Query(AndIdx);
return (!AndLRQ.isKill() && AndLRQ.valueIn() != LR.Query(SelIdx).valueOut());
}
// FIXME: Why do we bother trying to handle physical registers here?
static bool isDefBetween(const SIRegisterInfo &TRI,
LiveIntervals *LIS, Register Reg,
const MachineInstr &Sel, const MachineInstr &And) {
SlotIndex AndIdx = LIS->getInstructionIndex(And);
SlotIndex SelIdx = LIS->getInstructionIndex(Sel);
if (Reg.isVirtual())
return isDefBetween(LIS->getInterval(Reg), AndIdx, SelIdx);
for (MCRegUnitIterator UI(Reg.asMCReg(), &TRI); UI.isValid(); ++UI) {
if (isDefBetween(LIS->getRegUnit(*UI), AndIdx, SelIdx))
return true;
}
return false;
}
// Optimize sequence
// %sel = V_CNDMASK_B32_e64 0, 1, %cc
// %cmp = V_CMP_NE_U32 1, %1
// $vcc = S_AND_B64 $exec, %cmp
// S_CBRANCH_VCC[N]Z
// =>
// $vcc = S_ANDN2_B64 $exec, %cc
// S_CBRANCH_VCC[N]Z
//
// It is the negation pattern inserted by DAGCombiner::visitBRCOND() in the
// rebuildSetCC(). We start with S_CBRANCH to avoid exhaustive search, but
// only 3 first instructions are really needed. S_AND_B64 with exec is a
// required part of the pattern since V_CNDMASK_B32 writes zeroes for inactive
// lanes.
//
// Returns %cc register on success.
Register
SIOptimizeExecMaskingPreRA::optimizeVcndVcmpPair(MachineBasicBlock &MBB) {
auto I = llvm::find_if(MBB.terminators(), [](const MachineInstr &MI) {
unsigned Opc = MI.getOpcode();
return Opc == AMDGPU::S_CBRANCH_VCCZ ||
Opc == AMDGPU::S_CBRANCH_VCCNZ; });
if (I == MBB.terminators().end())
return Register();
auto *And =
TRI->findReachingDef(CondReg, AMDGPU::NoSubRegister, *I, *MRI, LIS);
if (!And || And->getOpcode() != AndOpc ||
!And->getOperand(1).isReg() || !And->getOperand(2).isReg())
return Register();
MachineOperand *AndCC = &And->getOperand(1);
Register CmpReg = AndCC->getReg();
unsigned CmpSubReg = AndCC->getSubReg();
if (CmpReg == Register(ExecReg)) {
AndCC = &And->getOperand(2);
CmpReg = AndCC->getReg();
CmpSubReg = AndCC->getSubReg();
} else if (And->getOperand(2).getReg() != Register(ExecReg)) {
return Register();
}
auto *Cmp = TRI->findReachingDef(CmpReg, CmpSubReg, *And, *MRI, LIS);
if (!Cmp || !(Cmp->getOpcode() == AMDGPU::V_CMP_NE_U32_e32 ||
Cmp->getOpcode() == AMDGPU::V_CMP_NE_U32_e64) ||
Cmp->getParent() != And->getParent())
return Register();
MachineOperand *Op1 = TII->getNamedOperand(*Cmp, AMDGPU::OpName::src0);
MachineOperand *Op2 = TII->getNamedOperand(*Cmp, AMDGPU::OpName::src1);
if (Op1->isImm() && Op2->isReg())
std::swap(Op1, Op2);
if (!Op1->isReg() || !Op2->isImm() || Op2->getImm() != 1)
return Register();
Register SelReg = Op1->getReg();
auto *Sel = TRI->findReachingDef(SelReg, Op1->getSubReg(), *Cmp, *MRI, LIS);
if (!Sel || Sel->getOpcode() != AMDGPU::V_CNDMASK_B32_e64)
return Register();
if (TII->hasModifiersSet(*Sel, AMDGPU::OpName::src0_modifiers) ||
TII->hasModifiersSet(*Sel, AMDGPU::OpName::src1_modifiers))
return Register();
Op1 = TII->getNamedOperand(*Sel, AMDGPU::OpName::src0);
Op2 = TII->getNamedOperand(*Sel, AMDGPU::OpName::src1);
MachineOperand *CC = TII->getNamedOperand(*Sel, AMDGPU::OpName::src2);
if (!Op1->isImm() || !Op2->isImm() || !CC->isReg() ||
Op1->getImm() != 0 || Op2->getImm() != 1)
return Register();
Register CCReg = CC->getReg();
// If there was a def between the select and the and, we would need to move it
// to fold this.
if (isDefBetween(*TRI, LIS, CCReg, *Sel, *And))
return Register();
LLVM_DEBUG(dbgs() << "Folding sequence:\n\t" << *Sel << '\t' << *Cmp << '\t'
<< *And);
LIS->RemoveMachineInstrFromMaps(*And);
MachineInstr *Andn2 =
BuildMI(MBB, *And, And->getDebugLoc(), TII->get(Andn2Opc),
And->getOperand(0).getReg())
.addReg(ExecReg)
.addReg(CCReg, getUndefRegState(CC->isUndef()), CC->getSubReg());
MachineOperand &AndSCC = And->getOperand(3);
assert(AndSCC.getReg() == AMDGPU::SCC);
MachineOperand &Andn2SCC = Andn2->getOperand(3);
assert(Andn2SCC.getReg() == AMDGPU::SCC);
Andn2SCC.setIsDead(AndSCC.isDead());
And->eraseFromParent();
LIS->InsertMachineInstrInMaps(*Andn2);
LLVM_DEBUG(dbgs() << "=>\n\t" << *Andn2 << '\n');
// Try to remove compare. Cmp value should not used in between of cmp
// and s_and_b64 if VCC or just unused if any other register.
if ((CmpReg.isVirtual() && MRI->use_nodbg_empty(CmpReg)) ||
(CmpReg == Register(CondReg) &&
std::none_of(std::next(Cmp->getIterator()), Andn2->getIterator(),
[&](const MachineInstr &MI) {
return MI.readsRegister(CondReg, TRI);
}))) {
LLVM_DEBUG(dbgs() << "Erasing: " << *Cmp << '\n');
LIS->RemoveMachineInstrFromMaps(*Cmp);
Cmp->eraseFromParent();
// Try to remove v_cndmask_b32.
if (SelReg.isVirtual() && MRI->use_nodbg_empty(SelReg)) {
LLVM_DEBUG(dbgs() << "Erasing: " << *Sel << '\n');
LIS->RemoveMachineInstrFromMaps(*Sel);
Sel->eraseFromParent();
}
}
return CCReg;
}
// Optimize sequence
// %dst = S_OR_SAVEEXEC %src
// ... instructions not modifying exec ...
// %tmp = S_AND $exec, %dst
// $exec = S_XOR_term $exec, %tmp
// =>
// %dst = S_OR_SAVEEXEC %src
// ... instructions not modifying exec ...
// $exec = S_XOR_term $exec, %dst
//
// Clean up potentially unnecessary code added for safety during
// control flow lowering.
//
// Return whether any changes were made to MBB.
bool SIOptimizeExecMaskingPreRA::optimizeElseBranch(MachineBasicBlock &MBB) {
if (MBB.empty())
return false;
// Check this is an else block.
auto First = MBB.begin();
MachineInstr &SaveExecMI = *First;
if (SaveExecMI.getOpcode() != OrSaveExecOpc)
return false;
auto I = llvm::find_if(MBB.terminators(), [this](const MachineInstr &MI) {
return MI.getOpcode() == XorTermrOpc;
});
if (I == MBB.terminators().end())
return false;
MachineInstr &XorTermMI = *I;
if (XorTermMI.getOperand(1).getReg() != Register(ExecReg))
return false;
Register SavedExecReg = SaveExecMI.getOperand(0).getReg();
Register DstReg = XorTermMI.getOperand(2).getReg();
// Find potentially unnecessary S_AND
MachineInstr *AndExecMI = nullptr;
I--;
while (I != First && !AndExecMI) {
if (I->getOpcode() == AndOpc && I->getOperand(0).getReg() == DstReg &&
I->getOperand(1).getReg() == Register(ExecReg))
AndExecMI = &*I;
I--;
}
if (!AndExecMI)
return false;
// Check for exec modifying instructions.
// Note: exec defs do not create live ranges beyond the
// instruction so isDefBetween cannot be used.
// Instead just check that the def segments are adjacent.
SlotIndex StartIdx = LIS->getInstructionIndex(SaveExecMI);
SlotIndex EndIdx = LIS->getInstructionIndex(*AndExecMI);
for (MCRegUnitIterator UI(ExecReg, TRI); UI.isValid(); ++UI) {
LiveRange &RegUnit = LIS->getRegUnit(*UI);
if (RegUnit.find(StartIdx) != std::prev(RegUnit.find(EndIdx)))
return false;
}
// Remove unnecessary S_AND
LIS->removeInterval(SavedExecReg);
LIS->removeInterval(DstReg);
SaveExecMI.getOperand(0).setReg(DstReg);
LIS->RemoveMachineInstrFromMaps(*AndExecMI);
AndExecMI->eraseFromParent();
LIS->createAndComputeVirtRegInterval(DstReg);
return true;
}
bool SIOptimizeExecMaskingPreRA::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(MF.getFunction()))
return false;
const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
TRI = ST.getRegisterInfo();
TII = ST.getInstrInfo();
MRI = &MF.getRegInfo();
LIS = &getAnalysis<LiveIntervals>();
const bool Wave32 = ST.isWave32();
AndOpc = Wave32 ? AMDGPU::S_AND_B32 : AMDGPU::S_AND_B64;
Andn2Opc = Wave32 ? AMDGPU::S_ANDN2_B32 : AMDGPU::S_ANDN2_B64;
OrSaveExecOpc =
Wave32 ? AMDGPU::S_OR_SAVEEXEC_B32 : AMDGPU::S_OR_SAVEEXEC_B64;
XorTermrOpc = Wave32 ? AMDGPU::S_XOR_B32_term : AMDGPU::S_XOR_B64_term;
CondReg = MCRegister::from(Wave32 ? AMDGPU::VCC_LO : AMDGPU::VCC);
ExecReg = MCRegister::from(Wave32 ? AMDGPU::EXEC_LO : AMDGPU::EXEC);
DenseSet<Register> RecalcRegs({AMDGPU::EXEC_LO, AMDGPU::EXEC_HI});
bool Changed = false;
for (MachineBasicBlock &MBB : MF) {
if (optimizeElseBranch(MBB)) {
RecalcRegs.insert(AMDGPU::SCC);
Changed = true;
}
if (Register Reg = optimizeVcndVcmpPair(MBB)) {
RecalcRegs.insert(Reg);
RecalcRegs.insert(AMDGPU::VCC_LO);
RecalcRegs.insert(AMDGPU::VCC_HI);
RecalcRegs.insert(AMDGPU::SCC);
Changed = true;
}
// Try to remove unneeded instructions before s_endpgm.
if (MBB.succ_empty()) {
if (MBB.empty())
continue;
// Skip this if the endpgm has any implicit uses, otherwise we would need
// to be careful to update / remove them.
// S_ENDPGM always has a single imm operand that is not used other than to
// end up in the encoding
MachineInstr &Term = MBB.back();
if (Term.getOpcode() != AMDGPU::S_ENDPGM || Term.getNumOperands() != 1)
continue;
SmallVector<MachineBasicBlock*, 4> Blocks({&MBB});
while (!Blocks.empty()) {
auto CurBB = Blocks.pop_back_val();
auto I = CurBB->rbegin(), E = CurBB->rend();
if (I != E) {
if (I->isUnconditionalBranch() || I->getOpcode() == AMDGPU::S_ENDPGM)
++I;
else if (I->isBranch())
continue;
}
while (I != E) {
if (I->isDebugInstr()) {
I = std::next(I);
continue;
}
if (I->mayStore() || I->isBarrier() || I->isCall() ||
I->hasUnmodeledSideEffects() || I->hasOrderedMemoryRef())
break;
LLVM_DEBUG(dbgs()
<< "Removing no effect instruction: " << *I << '\n');
for (auto &Op : I->operands()) {
if (Op.isReg())
RecalcRegs.insert(Op.getReg());
}
auto Next = std::next(I);
LIS->RemoveMachineInstrFromMaps(*I);
I->eraseFromParent();
I = Next;
Changed = true;
}
if (I != E)
continue;
// Try to ascend predecessors.
for (auto *Pred : CurBB->predecessors()) {
if (Pred->succ_size() == 1)
Blocks.push_back(Pred);
}
}
continue;
}
// If the only user of a logical operation is move to exec, fold it now
// to prevent forming of saveexec. I.e:
//
// %0:sreg_64 = COPY $exec
// %1:sreg_64 = S_AND_B64 %0:sreg_64, %2:sreg_64
// =>
// %1 = S_AND_B64 $exec, %2:sreg_64
unsigned ScanThreshold = 10;
for (auto I = MBB.rbegin(), E = MBB.rend(); I != E
&& ScanThreshold--; ++I) {
// Continue scanning if this is not a full exec copy
if (!(I->isFullCopy() && I->getOperand(1).getReg() == Register(ExecReg)))
continue;
Register SavedExec = I->getOperand(0).getReg();
if (SavedExec.isVirtual() && MRI->hasOneNonDBGUse(SavedExec) &&
MRI->use_instr_nodbg_begin(SavedExec)->getParent() ==
I->getParent()) {
LLVM_DEBUG(dbgs() << "Redundant EXEC COPY: " << *I << '\n');
LIS->RemoveMachineInstrFromMaps(*I);
I->eraseFromParent();
MRI->replaceRegWith(SavedExec, ExecReg);
LIS->removeInterval(SavedExec);
Changed = true;
}
break;
}
}
if (Changed) {
for (auto Reg : RecalcRegs) {
if (Reg.isVirtual()) {
LIS->removeInterval(Reg);
if (!MRI->reg_empty(Reg))
LIS->createAndComputeVirtRegInterval(Reg);
} else {
LIS->removeAllRegUnitsForPhysReg(Reg);
}
}
}
return Changed;
}
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