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[CodeGen] Only deduplicate PHIs on critical edges #97064

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Merged
merged 1 commit into from
Jul 3, 2024
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[CodeGen] Only deduplicate PHIs on critical edges #97064

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57 changes: 42 additions & 15 deletions llvm/lib/CodeGen/PHIElimination.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -83,7 +83,8 @@ namespace {
bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);

void LowerPHINode(MachineBasicBlock &MBB,
MachineBasicBlock::iterator LastPHIIt);
MachineBasicBlock::iterator LastPHIIt,
bool AllEdgesCritical);

/// analyzePHINodes - Gather information about the PHI nodes in
/// here. In particular, we want to map the number of uses of a virtual
Expand Down Expand Up @@ -191,7 +192,8 @@ bool PHIElimination::runOnMachineFunction(MachineFunction &MF) {
MRI->leaveSSA();

// Populate VRegPHIUseCount
analyzePHINodes(MF);
if (LV || LIS)
analyzePHINodes(MF);

// Eliminate PHI instructions by inserting copies into predecessor blocks.
for (auto &MBB : MF)
Expand Down Expand Up @@ -239,8 +241,20 @@ bool PHIElimination::EliminatePHINodes(MachineFunction &MF,
MachineBasicBlock::iterator LastPHIIt =
std::prev(MBB.SkipPHIsAndLabels(MBB.begin()));

// If all incoming edges are critical, we try to deduplicate identical PHIs so
// that we generate fewer copies. If at any edge is non-critical, we either
// have less than two predecessors (=> no PHIs) or a predecessor has only us
// as a successor (=> identical PHI node can't occur in different block).
bool AllEdgesCritical = MBB.pred_size() >= 2;
for (MachineBasicBlock *Pred : MBB.predecessors()) {
if (Pred->succ_size() < 2) {
AllEdgesCritical = false;
break;
}
}

while (MBB.front().isPHI())
LowerPHINode(MBB, LastPHIIt);
LowerPHINode(MBB, LastPHIIt, AllEdgesCritical);

return true;
}
Expand All @@ -267,7 +281,8 @@ static bool allPhiOperandsUndefined(const MachineInstr &MPhi,
}
/// LowerPHINode - Lower the PHI node at the top of the specified block.
void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
MachineBasicBlock::iterator LastPHIIt) {
MachineBasicBlock::iterator LastPHIIt,
bool AllEdgesCritical) {
++NumLowered;

MachineBasicBlock::iterator AfterPHIsIt = std::next(LastPHIIt);
Expand All @@ -283,6 +298,7 @@ void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
// Create a new register for the incoming PHI arguments.
MachineFunction &MF = *MBB.getParent();
unsigned IncomingReg = 0;
bool EliminateNow = true; // delay elimination of nodes in LoweredPHIs
bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI?

// Insert a register to register copy at the top of the current block (but
Expand All @@ -297,19 +313,28 @@ void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
TII->get(TargetOpcode::IMPLICIT_DEF), DestReg);
else {
// Can we reuse an earlier PHI node? This only happens for critical edges,
// typically those created by tail duplication.
unsigned &entry = LoweredPHIs[MPhi];
if (entry) {
// typically those created by tail duplication. Typically, an identical PHI
// node can't occur, so avoid hashing/storing such PHIs, which is somewhat
// expensive.
unsigned *Entry = nullptr;
if (AllEdgesCritical)
Entry = &LoweredPHIs[MPhi];
if (Entry && *Entry) {
// An identical PHI node was already lowered. Reuse the incoming register.
IncomingReg = entry;
IncomingReg = *Entry;
reusedIncoming = true;
++NumReused;
LLVM_DEBUG(dbgs() << "Reusing " << printReg(IncomingReg) << " for "
<< *MPhi);
} else {
const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg);
entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
IncomingReg = MF.getRegInfo().createVirtualRegister(RC);
if (Entry) {
EliminateNow = false;
*Entry = IncomingReg;
}
}

// Give the target possiblity to handle special cases fallthrough otherwise
PHICopy = TII->createPHIDestinationCopy(MBB, AfterPHIsIt, MPhi->getDebugLoc(),
IncomingReg, DestReg);
Expand Down Expand Up @@ -445,11 +470,13 @@ void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
}

// Adjust the VRegPHIUseCount map to account for the removal of this PHI node.
for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
if (!MPhi->getOperand(i).isUndef()) {
--VRegPHIUseCount[BBVRegPair(
MPhi->getOperand(i + 1).getMBB()->getNumber(),
MPhi->getOperand(i).getReg())];
if (LV || LIS) {
for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) {
if (!MPhi->getOperand(i).isUndef()) {
--VRegPHIUseCount[BBVRegPair(
MPhi->getOperand(i + 1).getMBB()->getNumber(),
MPhi->getOperand(i).getReg())];
}
}
}

Expand Down Expand Up @@ -646,7 +673,7 @@ void PHIElimination::LowerPHINode(MachineBasicBlock &MBB,
}

// Really delete the PHI instruction now, if it is not in the LoweredPHIs map.
if (reusedIncoming || !IncomingReg) {
if (EliminateNow) {
if (LIS)
LIS->RemoveMachineInstrFromMaps(*MPhi);
MF.deleteMachineInstr(MPhi);
Expand Down
4 changes: 2 additions & 2 deletions llvm/test/CodeGen/AMDGPU/branch-folding-implicit-def-subreg.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -804,6 +804,7 @@ define amdgpu_kernel void @f1(ptr addrspace(1) %arg, ptr addrspace(1) %arg1, i64
; GFX90A-NEXT: renamable $vgpr15 = V_ALIGNBIT_B32_e64 $vgpr15, $vgpr14, 1, implicit $exec
; GFX90A-NEXT: renamable $sgpr48_sgpr49 = S_XOR_B64 $exec, -1, implicit-def dead $scc
; GFX90A-NEXT: renamable $sgpr60_sgpr61 = S_OR_B64 renamable $sgpr28_sgpr29, $exec, implicit-def dead $scc
; GFX90A-NEXT: renamable $vgpr10 = COPY renamable $vgpr14, implicit $exec
; GFX90A-NEXT: S_BRANCH %bb.61
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: bb.58:
Expand Down Expand Up @@ -886,11 +887,10 @@ define amdgpu_kernel void @f1(ptr addrspace(1) %arg, ptr addrspace(1) %arg1, i64
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: bb.61.Flow31:
; GFX90A-NEXT: successors: %bb.62(0x80000000)
; GFX90A-NEXT: liveins: $sgpr12, $sgpr13, $sgpr14, $sgpr15, $vgpr15, $vgpr17, $vgpr18, $vgpr30, $vgpr31, $vgpr52, $vgpr53, $sgpr4_sgpr5, $sgpr6_sgpr7:0x000000000000000F, $sgpr8_sgpr9, $sgpr10_sgpr11, $sgpr16_sgpr17, $sgpr24_sgpr25, $sgpr26_sgpr27, $sgpr28_sgpr29, $sgpr30_sgpr31, $sgpr34_sgpr35, $sgpr36_sgpr37, $sgpr38_sgpr39, $sgpr40_sgpr41, $sgpr42_sgpr43, $sgpr44_sgpr45, $sgpr46_sgpr47, $sgpr48_sgpr49, $sgpr50_sgpr51, $sgpr58_sgpr59, $sgpr60_sgpr61, $sgpr16_sgpr17_sgpr18_sgpr19:0x00000000000000F0, $sgpr20_sgpr21_sgpr22_sgpr23:0x000000000000003C, $vgpr0_vgpr1:0x000000000000000F, $vgpr2_vgpr3:0x000000000000000F, $vgpr4_vgpr5:0x000000000000000F, $vgpr6_vgpr7:0x000000000000000F, $vgpr8_vgpr9:0x000000000000000F, $vgpr10_vgpr11:0x000000000000000C, $vgpr12_vgpr13:0x000000000000000F, $vgpr14_vgpr15:0x0000000000000003, $vgpr16_vgpr17:0x0000000000000003, $vgpr40_vgpr41:0x000000000000000F, $vgpr42_vgpr43:0x000000000000000F, $vgpr44_vgpr45:0x000000000000000F, $vgpr46_vgpr47:0x000000000000000F, $vgpr56_vgpr57:0x000000000000000F, $vgpr58_vgpr59:0x0000000000000003, $vgpr60_vgpr61:0x000000000000000F, $vgpr62_vgpr63:0x000000000000000F, $sgpr0_sgpr1_sgpr2_sgpr3
; GFX90A-NEXT: liveins: $sgpr12, $sgpr13, $sgpr14, $sgpr15, $vgpr15, $vgpr17, $vgpr18, $vgpr30, $vgpr31, $vgpr52, $vgpr53, $sgpr4_sgpr5, $sgpr6_sgpr7:0x000000000000000F, $sgpr8_sgpr9, $sgpr10_sgpr11, $sgpr16_sgpr17, $sgpr24_sgpr25, $sgpr26_sgpr27, $sgpr28_sgpr29, $sgpr30_sgpr31, $sgpr34_sgpr35, $sgpr36_sgpr37, $sgpr38_sgpr39, $sgpr40_sgpr41, $sgpr42_sgpr43, $sgpr44_sgpr45, $sgpr46_sgpr47, $sgpr48_sgpr49, $sgpr50_sgpr51, $sgpr58_sgpr59, $sgpr60_sgpr61, $sgpr16_sgpr17_sgpr18_sgpr19:0x00000000000000F0, $sgpr20_sgpr21_sgpr22_sgpr23:0x000000000000003C, $vgpr0_vgpr1:0x000000000000000F, $vgpr2_vgpr3:0x000000000000000F, $vgpr4_vgpr5:0x000000000000000F, $vgpr6_vgpr7:0x000000000000000F, $vgpr8_vgpr9:0x000000000000000F, $vgpr10_vgpr11:0x000000000000000F, $vgpr12_vgpr13:0x000000000000000F, $vgpr14_vgpr15:0x0000000000000003, $vgpr16_vgpr17:0x0000000000000003, $vgpr40_vgpr41:0x000000000000000F, $vgpr42_vgpr43:0x000000000000000F, $vgpr44_vgpr45:0x000000000000000F, $vgpr46_vgpr47:0x000000000000000F, $vgpr56_vgpr57:0x000000000000000F, $vgpr58_vgpr59:0x0000000000000003, $vgpr60_vgpr61:0x000000000000000F, $vgpr62_vgpr63:0x000000000000000F, $sgpr0_sgpr1_sgpr2_sgpr3
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: $exec = S_OR_B64 $exec, killed renamable $sgpr50_sgpr51, implicit-def $scc
; GFX90A-NEXT: renamable $sgpr50_sgpr51 = S_MOV_B64 0
; GFX90A-NEXT: renamable $vgpr10 = COPY renamable $vgpr14, implicit $exec
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: bb.62.Flow30:
; GFX90A-NEXT: successors: %bb.56(0x80000000)
Expand Down
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