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[AMDGPU] Remove redundant s_cmp_lg_* sX, 0 #162352

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[AMDGPU] Remove redundant s_cmp_lg_* sX, 0 #162352

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7122185
Pre-commit test for redundant s_cmp sX, 0 removal
LU-JOHN Oct 7, 2025
ba282ce
Delete s_cmp sX, 0 if it is redundant
LU-JOHN Oct 7, 2025
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71 changes: 69 additions & 2 deletions llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10608,6 +10608,73 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
if (SrcReg2 && !getFoldableImm(SrcReg2, *MRI, CmpValue))
return false;

const auto optimizeCmpSelect = [&CmpInstr, SrcReg, CmpValue, MRI,
this]() -> bool {
if (CmpValue != 0)
return false;

MachineInstr *Def = MRI->getUniqueVRegDef(SrcReg);
if (!Def || Def->getParent() != CmpInstr.getParent())
return false;
Comment on lines +10617 to +10618
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Don't see why the parent would matter, but this also looks untested

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Def and CmpInstr need to have the same parent so that the scan will work.


if (!(Def->getOpcode() == AMDGPU::S_LSHL_B32 ||
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Is this just a list of all instructions that define SCC?

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It should be all instructions that set SCC = DST!=0. s_max/s_min are excluded since they set SCC but with a different condition. I need to review the list since I missed some instructions that should be included (e.g. s_absdiff).

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Maybe a switch would be slightly more readable ?

Def->getOpcode() == AMDGPU::S_LSHL_B64 ||
Def->getOpcode() == AMDGPU::S_LSHR_B32 ||
Def->getOpcode() == AMDGPU::S_LSHR_B64 ||
Def->getOpcode() == AMDGPU::S_AND_B32 ||
Def->getOpcode() == AMDGPU::S_AND_B64 ||
Def->getOpcode() == AMDGPU::S_OR_B32 ||
Def->getOpcode() == AMDGPU::S_OR_B64 ||
Def->getOpcode() == AMDGPU::S_XOR_B32 ||
Def->getOpcode() == AMDGPU::S_XOR_B64 ||
Def->getOpcode() == AMDGPU::S_NAND_B32 ||
Def->getOpcode() == AMDGPU::S_NAND_B64 ||
Def->getOpcode() == AMDGPU::S_NOR_B32 ||
Def->getOpcode() == AMDGPU::S_NOR_B64 ||
Def->getOpcode() == AMDGPU::S_XNOR_B32 ||
Def->getOpcode() == AMDGPU::S_XNOR_B64 ||
Def->getOpcode() == AMDGPU::S_ANDN2_B32 ||
Def->getOpcode() == AMDGPU::S_ANDN2_B64 ||
Def->getOpcode() == AMDGPU::S_ORN2_B32 ||
Def->getOpcode() == AMDGPU::S_ORN2_B64 ||
Def->getOpcode() == AMDGPU::S_BFE_I32 ||
Def->getOpcode() == AMDGPU::S_BFE_I64 ||
Def->getOpcode() == AMDGPU::S_BFE_U32 ||
Def->getOpcode() == AMDGPU::S_BFE_U64 ||
Def->getOpcode() == AMDGPU::S_BCNT0_I32_B32 ||
Def->getOpcode() == AMDGPU::S_BCNT0_I32_B64 ||
Def->getOpcode() == AMDGPU::S_BCNT1_I32_B32 ||
Def->getOpcode() == AMDGPU::S_BCNT1_I32_B64 ||
Def->getOpcode() == AMDGPU::S_QUADMASK_B32 ||
Def->getOpcode() == AMDGPU::S_QUADMASK_B64 ||
Def->getOpcode() == AMDGPU::S_NOT_B32 ||
Def->getOpcode() == AMDGPU::S_NOT_B64 ||

((Def->getOpcode() == AMDGPU::S_CSELECT_B32 ||
Def->getOpcode() == AMDGPU::S_CSELECT_B64) &&
Def->getOperand(1).isImm() && Def->getOperand(1).getImm() &&
!Def->getOperand(2).isImm() && !Def->getOperand(2).getImm())))
return false;

for (auto I = std::next(Def->getIterator()), E = CmpInstr.getIterator();
I != E; ++I) {
if (I->modifiesRegister(AMDGPU::SCC, &RI) ||
I->killsRegister(AMDGPU::SCC, &RI))
return false;
}
Comment on lines +10659 to +10664
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I don't think you should need to be scanning around for physreg liveness, I don't see other targets doing that here

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I don't think you should need to be scanning around for physreg liveness, I don't see other targets doing that here

Scanning for register liveness is also done in the other lambda, optimizeCmpAnd, (line 10752) that is used in optimizeCompareInstr. It is necessary to prevent incorrectly deleting a s_cmp if an there is an intervening definition. In PR #161582, tests were added that requires this scanning. PR #161582 was done to add SCC as an implicit destination of s_quadmask*. Without this fix the scanning would not detect a conflict.


if (!(Def->getOpcode() == AMDGPU::S_CSELECT_B32 ||
Def->getOpcode() == AMDGPU::S_CSELECT_B64)) {
Comment on lines +10666 to +10667
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demorgan this condition

MachineOperand *SccDef =
Def->findRegisterDefOperand(AMDGPU::SCC, /*TRI=*/nullptr);
assert(SccDef && "Def instruction must define SCC");
SccDef->setIsDead(false);
}

CmpInstr.eraseFromParent();
return true;
};

const auto optimizeCmpAnd = [&CmpInstr, SrcReg, CmpValue, MRI,
this](int64_t ExpectedValue, unsigned SrcSize,
bool IsReversible, bool IsSigned) -> bool {
Expand Down Expand Up @@ -10735,15 +10802,15 @@ bool SIInstrInfo::optimizeCompareInstr(MachineInstr &CmpInstr, Register SrcReg,
case AMDGPU::S_CMP_LG_I32:
case AMDGPU::S_CMPK_LG_U32:
case AMDGPU::S_CMPK_LG_I32:
return optimizeCmpAnd(0, 32, true, false);
return optimizeCmpAnd(0, 32, true, false) || optimizeCmpSelect();
case AMDGPU::S_CMP_GT_U32:
case AMDGPU::S_CMPK_GT_U32:
return optimizeCmpAnd(0, 32, false, false);
case AMDGPU::S_CMP_GT_I32:
case AMDGPU::S_CMPK_GT_I32:
return optimizeCmpAnd(0, 32, false, true);
case AMDGPU::S_CMP_LG_U64:
return optimizeCmpAnd(0, 64, true, false);
return optimizeCmpAnd(0, 64, true, false) || optimizeCmpSelect();
}

return false;
Expand Down
2 changes: 0 additions & 2 deletions llvm/test/CodeGen/AMDGPU/GlobalISel/llvm.amdgcn.ballot.i32.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -140,7 +140,6 @@ define amdgpu_cs i32 @branch_divergent_ballot_eq_zero_non_compare(i32 %v) {
; CHECK-NEXT: v_and_b32_e32 v0, 1, v0
; CHECK-NEXT: v_cmp_ne_u32_e32 vcc_lo, 0, v0
; CHECK-NEXT: s_and_b32 s0, vcc_lo, exec_lo
; CHECK-NEXT: s_cmp_lg_u32 s0, 0
; CHECK-NEXT: s_cbranch_scc0 .LBB9_2
; CHECK-NEXT: ; %bb.1: ; %false
; CHECK-NEXT: s_mov_b32 s0, 33
Expand Down Expand Up @@ -345,7 +344,6 @@ define amdgpu_cs i32 @branch_divergent_ballot_eq_zero_and(i32 %v1, i32 %v2) {
; CHECK-NEXT: v_cmp_gt_u32_e32 vcc_lo, 12, v0
; CHECK-NEXT: v_cmp_lt_u32_e64 s0, 34, v1
; CHECK-NEXT: s_and_b32 s0, vcc_lo, s0
; CHECK-NEXT: s_cmp_lg_u32 s0, 0
; CHECK-NEXT: s_cbranch_scc0 .LBB17_2
; CHECK-NEXT: ; %bb.1: ; %false
; CHECK-NEXT: s_mov_b32 s0, 33
Expand Down
2 changes: 0 additions & 2 deletions llvm/test/CodeGen/AMDGPU/GlobalISel/llvm.amdgcn.ballot.i64.ll
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Expand Up @@ -143,7 +143,6 @@ define amdgpu_cs i32 @branch_divergent_ballot_eq_zero_non_compare(i32 %v) {
; CHECK-NEXT: v_and_b32_e32 v0, 1, v0
; CHECK-NEXT: v_cmp_ne_u32_e32 vcc, 0, v0
; CHECK-NEXT: s_and_b64 s[0:1], vcc, exec
; CHECK-NEXT: s_cmp_lg_u64 s[0:1], 0
; CHECK-NEXT: s_cbranch_scc0 .LBB9_2
; CHECK-NEXT: ; %bb.1: ; %false
; CHECK-NEXT: s_mov_b32 s0, 33
Expand Down Expand Up @@ -348,7 +347,6 @@ define amdgpu_cs i32 @branch_divergent_ballot_eq_zero_and(i32 %v1, i32 %v2) {
; CHECK-NEXT: v_cmp_gt_u32_e32 vcc, 12, v0
; CHECK-NEXT: v_cmp_lt_u32_e64 s[0:1], 34, v1
; CHECK-NEXT: s_and_b64 s[0:1], vcc, s[0:1]
; CHECK-NEXT: s_cmp_lg_u64 s[0:1], 0
; CHECK-NEXT: s_cbranch_scc0 .LBB17_2
; CHECK-NEXT: ; %bb.1: ; %false
; CHECK-NEXT: s_mov_b32 s0, 33
Expand Down
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