[ValueTracking] Bail out on x86_fp80 when computing fpclass with knownbits #130477
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@llvm/pr-subscribers-llvm-transforms @llvm/pr-subscribers-llvm-analysis Author: Yingwei Zheng (dtcxzyw) ChangesIn #97762, we assume the minimum possible value of X is NaN implies X is NaN. But it doesn't hold for x86_fp80 format. If the knownbits of X are Closes #130408. Full diff: https://github.com/llvm/llvm-project/pull/130477.diff 2 Files Affected:
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 08dc34f89130a..2920738445d5b 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -6208,13 +6208,14 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
else if (Bits.isNegative())
Known.signBitMustBeOne();
- if (Ty->isIEEE()) {
+ if (Ty->isIEEELikeFPTy()) {
// IEEE floats are NaN when all bits of the exponent plus at least one of
// the fraction bits are 1. This means:
// - If we assume unknown bits are 0 and the value is NaN, it will
// always be NaN
// - If we assume unknown bits are 1 and the value is not NaN, it can
// never be NaN
+ // Note: They do not hold for x86_fp80 format.
if (APFloat(Ty->getFltSemantics(), Bits.One).isNaN())
Known.KnownFPClasses = fcNan;
else if (!APFloat(Ty->getFltSemantics(), ~Bits.Zero).isNaN())
diff --git a/llvm/test/Transforms/InstSimplify/fcmp.ll b/llvm/test/Transforms/InstSimplify/fcmp.ll
index 64132f5fb7db7..0c2be5210a741 100644
--- a/llvm/test/Transforms/InstSimplify/fcmp.ll
+++ b/llvm/test/Transforms/InstSimplify/fcmp.ll
@@ -16,3 +16,20 @@ define i1 @poison2(float %x) {
%v = fcmp ueq float %x, poison
ret i1 %v
}
+
+define i1 @pr130408(x86_fp80 %x) {
+; CHECK-LABEL: @pr130408(
+; CHECK-NEXT: [[BITS:%.*]] = bitcast x86_fp80 [[X:%.*]] to i80
+; CHECK-NEXT: [[MASKED:%.*]] = and i80 [[BITS]], -604444463063240877801473
+; CHECK-NEXT: [[OR:%.*]] = or i80 [[MASKED]], 302194561415509874573312
+; CHECK-NEXT: [[FP:%.*]] = bitcast i80 [[OR]] to x86_fp80
+; CHECK-NEXT: [[RES:%.*]] = fcmp uno x86_fp80 [[FP]], 0xK00000000000000000000
+; CHECK-NEXT: ret i1 [[RES]]
+;
+ %bits = bitcast x86_fp80 %x to i80
+ %masked = and i80 %bits, -604444463063240877801473
+ %or = or i80 %masked, 302194561415509874573312
+ %fp = bitcast i80 %or to x86_fp80
+ %res = fcmp uno x86_fp80 %fp, 0xK00000000000000000000
+ ret i1 %res
+}
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arsenm
approved these changes
Mar 9, 2025
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Probably should remove the isIEEE from APFloat. It has repeatedly caused confusion and it only points out an irrelevant APFloat implementation detail |
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/cherry-pick 029e102 |
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/pull-request #130489 |
swift-ci
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Mar 11, 2025
…nbits (llvm#130477) In llvm#97762, we assume the minimum possible value of X is NaN implies X is NaN. But it doesn't hold for x86_fp80 format. If the knownbits of X are `?'011111111111110'????????????????????????????????????????????????????????????????`, the minimum possible value of X is NaN/unnormal. However, it can be a normal value. Closes llvm#130408. (cherry picked from commit 029e102)
dtcxzyw
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Apr 14, 2025
Address comment #130477 (comment).
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In #97762, we assume the minimum possible value of X is NaN implies X is NaN. But it doesn't hold for x86_fp80 format. If the knownbits of X are
?'011111111111110'????????????????????????????????????????????????????????????????, the minimum possible value of X is NaN/unnormal. However, it can be a normal value.Closes #130408.