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[X86] Avoid useless DomTree in flags copy lowering #97628

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Merged
merged 3 commits into from
Jul 4, 2024
Merged

[X86] Avoid useless DomTree in flags copy lowering #97628

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b47f5cf
[X86] Avoid useless DomTree in flags copy lowering
aengelke Jul 3, 2024
e2e7f63
Fix test and use llvm::none_of
aengelke Jul 4, 2024
80e883a
Always return true in slow path
aengelke Jul 4, 2024
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23 changes: 21 additions & 2 deletions llvm/lib/Target/X86/X86FlagsCopyLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -128,7 +128,7 @@ FunctionPass *llvm::createX86FlagsCopyLoweringPass() {
char X86FlagsCopyLoweringPass::ID = 0;

void X86FlagsCopyLoweringPass::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineDominatorTreeWrapperPass>();
AU.addUsedIfAvailable<MachineDominatorTreeWrapperPass>();
MachineFunctionPass::getAnalysisUsage(AU);
}

Expand Down Expand Up @@ -258,13 +258,32 @@ bool X86FlagsCopyLoweringPass::runOnMachineFunction(MachineFunction &MF) {
MRI = &MF.getRegInfo();
TII = Subtarget->getInstrInfo();
TRI = Subtarget->getRegisterInfo();
MDT = &getAnalysis<MachineDominatorTreeWrapperPass>().getDomTree();
PromoteRC = &X86::GR8RegClass;

if (MF.empty())
// Nothing to do for a degenerate empty function...
return false;

if (none_of(MRI->def_instructions(X86::EFLAGS), [](const MachineInstr &MI) {
return MI.getOpcode() == TargetOpcode::COPY;
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Why don't check MI.getOperand(0).getReg() == X86::EFLAGS as well?

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I don't think that's necessary (or beneficial): if there's a def of eflags from a copy instruction, then the first operand must be eflags.

}))
return false;

// We change the code, so we don't preserve the dominator tree anyway. If we
// got a valid MDT from the pass manager, use that, otherwise construct one
// now. This is an optimization that avoids unnecessary MDT construction for
// functions that have no flag copies.

auto MDTWrapper = getAnalysisIfAvailable<MachineDominatorTreeWrapperPass>();
std::unique_ptr<MachineDominatorTree> OwnedMDT;
if (MDTWrapper) {
MDT = &MDTWrapper->getDomTree();
} else {
OwnedMDT = std::make_unique<MachineDominatorTree>();
OwnedMDT->getBase().recalculate(MF);
MDT = OwnedMDT.get();
}

// Collect the copies in RPO so that when there are chains where a copy is in
// turn copied again we visit the first one first. This ensures we can find
// viable locations for testing the original EFLAGS that dominate all the
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1 change: 0 additions & 1 deletion llvm/test/CodeGen/X86/O0-pipeline.ll
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Expand Up @@ -44,7 +44,6 @@
; CHECK-NEXT: Finalize ISel and expand pseudo-instructions
; CHECK-NEXT: Local Stack Slot Allocation
; CHECK-NEXT: X86 speculative load hardening
; CHECK-NEXT: MachineDominator Tree Construction
; CHECK-NEXT: X86 EFLAGS copy lowering
; CHECK-NEXT: X86 DynAlloca Expander
; CHECK-NEXT: Fast Tile Register Preconfigure
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1 change: 0 additions & 1 deletion llvm/test/CodeGen/X86/opt-pipeline.ll
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Expand Up @@ -125,7 +125,6 @@
; CHECK-NEXT: X86 Optimize Call Frame
; CHECK-NEXT: X86 Avoid Store Forwarding Block
; CHECK-NEXT: X86 speculative load hardening
; CHECK-NEXT: MachineDominator Tree Construction
; CHECK-NEXT: X86 EFLAGS copy lowering
; CHECK-NEXT: X86 DynAlloca Expander
; CHECK-NEXT: MachineDominator Tree Construction
Expand Down
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