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[X86] Do not allow FixupSetCC to relax constraints 

The build bots caught two additional pre-existing problems exposed by the test change part of my change https://reviews.llvm.org/D91339, when expensive checks are enabled. https://reviews.llvm.org/D91924 fixes one of them, this fixes the other.

FixupSetCC will change code in the form of

  %setcc = SETCCr ...
  %ext1 = MOVZX32rr8 %setcc

to

  %zero = MOV32r0
  %setcc = SETCCr ...
  %ext2 = INSERT_SUBREG %zero, %setcc, %subreg.sub_8bit

and replace uses of %ext1 with %ext2.

The register class for %ext2 did not take into account any constraints on %ext1, which may have been required by its uses. This change ensures that the original constraints are honoured, by instead of creating a new %ext2 register, reusing %ext1 and further constraining it as needed. This requires a slight reorganisation to account for the fact that it is possible for the constraining to fail, in which case no changes should be made.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D91933
This commit is contained in:
Harald van Dijk 2020-11-28 17:46:56 +00:00
parent c6b62efb91
commit 47e2fafbf3
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GPG Key ID: 7D71BB318A5BD56D
1 changed files with 10 additions and 7 deletions
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17
llvm/lib/Target/X86/X86FixupSetCC.cpp
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@ -97,28 +97,31 @@ bool X86FixupSetCCPass::runOnMachineFunction(MachineFunction &MF) {
if (FlagsDefMI->readsRegister(X86::EFLAGS)) if (FlagsDefMI->readsRegister(X86::EFLAGS))
continue; continue;
++NumSubstZexts;
Changed = true;
// On 32-bit, we need to be careful to force an ABCD register. // On 32-bit, we need to be careful to force an ABCD register.
const TargetRegisterClass *RC = MF.getSubtarget<X86Subtarget>().is64Bit() const TargetRegisterClass *RC = MF.getSubtarget<X86Subtarget>().is64Bit()
? &X86::GR32RegClass ? &X86::GR32RegClass
: &X86::GR32_ABCDRegClass; : &X86::GR32_ABCDRegClass;
Register ZeroReg = MRI->createVirtualRegister(RC); if (!MRI->constrainRegClass(ZExt->getOperand(0).getReg(), RC)) {
Register InsertReg = MRI->createVirtualRegister(RC); // If we cannot constrain the register, we would need an additional copy
// and are better off keeping the MOVZX32rr8 we have now.
continue;
}
++NumSubstZexts;
Changed = true;
// Initialize a register with 0. This must go before the eflags def // Initialize a register with 0. This must go before the eflags def
Register ZeroReg = MRI->createVirtualRegister(RC);
BuildMI(MBB, FlagsDefMI, MI.getDebugLoc(), TII->get(X86::MOV32r0), BuildMI(MBB, FlagsDefMI, MI.getDebugLoc(), TII->get(X86::MOV32r0),
ZeroReg); ZeroReg);
// X86 setcc only takes an output GR8, so fake a GR32 input by inserting // X86 setcc only takes an output GR8, so fake a GR32 input by inserting
// the setcc result into the low byte of the zeroed register. // the setcc result into the low byte of the zeroed register.
BuildMI(*ZExt->getParent(), ZExt, ZExt->getDebugLoc(), BuildMI(*ZExt->getParent(), ZExt, ZExt->getDebugLoc(),
TII->get(X86::INSERT_SUBREG), InsertReg) TII->get(X86::INSERT_SUBREG), ZExt->getOperand(0).getReg())
.addReg(ZeroReg) .addReg(ZeroReg)
.addReg(MI.getOperand(0).getReg()) .addReg(MI.getOperand(0).getReg())
.addImm(X86::sub_8bit); .addImm(X86::sub_8bit);
MRI->replaceRegWith(ZExt->getOperand(0).getReg(), InsertReg);
ToErase.push_back(ZExt); ToErase.push_back(ZExt);
} }
} }