[llvm] [ValueTracking] Fix incorrect inferrence about the signbit of sqrt (PR #92510)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

According to IEEE Std 754-2019, `sqrt` returns nan when the input is negative (except for -0). In this case, we cannot make assumptions about sign bit of the result.
BTW, alive2 seems to treat it as poison :( See https://github.com/AliveToolkit/alive2/issues/1037

Fixes https://github.com/llvm/llvm-project/issues/92217


---
Full diff: https://github.com/llvm/llvm-project/pull/92510.diff


3 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+1-4) 
- (modified) llvm/test/Transforms/InstCombine/known-bits.ll (+15) 
- (modified) llvm/unittests/Analysis/ValueTrackingTest.cpp (+1-1) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e8c5f9b3dc25d..0f754523ba351 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4940,11 +4940,8 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
       // subnormal input could produce a negative zero output.
       const Function *F = II->getFunction();
       if (Q.IIQ.hasNoSignedZeros(II) ||
-          (F && KnownSrc.isKnownNeverLogicalNegZero(*F, II->getType()))) {
+          (F && KnownSrc.isKnownNeverLogicalNegZero(*F, II->getType())))
         Known.knownNot(fcNegZero);
-        if (KnownSrc.isKnownNeverNaN())
-          Known.signBitMustBeZero();
-      }
 
       break;
     }
diff --git a/llvm/test/Transforms/InstCombine/known-bits.ll b/llvm/test/Transforms/InstCombine/known-bits.ll
index 82cd24027e4e1..b4e79282f7012 100644
--- a/llvm/test/Transforms/InstCombine/known-bits.ll
+++ b/llvm/test/Transforms/InstCombine/known-bits.ll
@@ -1698,6 +1698,21 @@ define i32 @test_none(float nofpclass(all) %x) {
   ret i32 %and
 }
 
+; We cannot make assumptions about the sign of result of sqrt
+; when the input is a negative value (except for -0).
+define i1 @pr92217() {
+; CHECK-LABEL: @pr92217(
+; CHECK-NEXT:    [[X:%.*]] = call float @llvm.sqrt.f32(float 0xC6DEBE9E60000000)
+; CHECK-NEXT:    [[Y:%.*]] = bitcast float [[X]] to i32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[Y]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %x = call float @llvm.sqrt.f32(float 0xC6DEBE9E60000000)
+  %y = bitcast float %x to i32
+  %cmp = icmp slt i32 %y, 0
+  ret i1 %cmp
+}
+
 define i8 @test_icmp_add(i8 %n, i8 %n2, i8 %other) {
 ; CHECK-LABEL: @test_icmp_add(
 ; CHECK-NEXT:  entry:
diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index 8738af91b652b..a30db468c7729 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -2005,7 +2005,7 @@ TEST_F(ComputeKnownFPClassTest, SqrtNszSignBit) {
         computeKnownFPClass(A4, M->getDataLayout(), fcAllFlags, 0, nullptr,
                             nullptr, nullptr, nullptr, /*UseInstrInfo=*/true);
     EXPECT_EQ(fcPositive | fcQNan, UseInstrInfoNSZNoNan.KnownFPClasses);
-    EXPECT_EQ(false, UseInstrInfoNSZNoNan.SignBit);
+    EXPECT_EQ(std::nullopt, UseInstrInfoNSZNoNan.SignBit);
 
     KnownFPClass NoUseInstrInfoNSZNoNan =
         computeKnownFPClass(A4, M->getDataLayout(), fcAllFlags, 0, nullptr,

``````````

</details>


https://github.com/llvm/llvm-project/pull/92510