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DAG: Handle AssertNoFPClass in computeKnownBits (#167289)

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It's possible to determine the sign bit if the value is known
one of the positive/negative classes and not-nan.
This commit is contained in:
Matt Arsenault 2025-11-10 08:56:08 -08:00 committed by GitHub
parent a3d00e1ae4
commit fa98fcd02e
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GPG Key ID: B5690EEEBB952194
2 changed files with 19 additions and 2 deletions
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19
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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@ -4149,6 +4149,25 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
Known.One.clearLowBits(LogOfAlign);
break;
}
case ISD::AssertNoFPClass: {
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
FPClassTest NoFPClass =
static_cast<FPClassTest>(Op.getConstantOperandVal(1));
const FPClassTest NegativeTestMask = fcNan | fcNegative;
if ((NoFPClass & NegativeTestMask) == NegativeTestMask) {
// Cannot be negative.
Known.makeNonNegative();
}
const FPClassTest PositiveTestMask = fcNan | fcPositive;
if ((NoFPClass & PositiveTestMask) == PositiveTestMask) {
// Cannot be positive.
Known.makeNegative();
}
break;
}
case ISD::FGETSIGN:
// All bits are zero except the low bit.
Known.Zero.setBitsFrom(1);

2
llvm/test/CodeGen/AMDGPU/compute-known-bits-nofpclass.ll
View File

@ -5,7 +5,6 @@ define i32 @known_positive(float nofpclass(nan ninf nzero nsub nnorm) %signbit.z
; CHECK-LABEL: known_positive:
; CHECK: ; %bb.0:
; CHECK-NEXT: s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
; CHECK-NEXT: v_and_b32_e32 v0, 0x7fffffff, v0
; CHECK-NEXT: s_setpc_b64 s[30:31]
%cast = bitcast float %signbit.zero to i32
%and = and i32 %cast, 2147483647
@ -27,7 +26,6 @@ define i32 @known_negative(float nofpclass(nan pinf pzero psub pnorm) %signbit.o
; CHECK-LABEL: known_negative:
; CHECK: ; %bb.0:
; CHECK-NEXT: s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
; CHECK-NEXT: v_or_b32_e32 v0, 0x80000000, v0
; CHECK-NEXT: s_setpc_b64 s[30:31]
%cast = bitcast float %signbit.one to i32
%or = or i32 %cast, -2147483648