[ValueTracking] Fix assertion failure when computeKnownFPClass returns fcNone

[dtcxzyw]

Fixes #92084 (comment).

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Yingwei Zheng (dtcxzyw)

Changes

Fixes #92084 (comment).

---

Full diff: https://github.com/llvm/llvm-project/pull/92355.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ValueTracking.cpp (+2-1)
• (modified) llvm/test/Transforms/InstCombine/known-bits.ll (+40)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c8c527a2d4d2f..087cfcc792ee4 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1126,7 +1126,8 @@ static void computeKnownBitsFromOperator(const Operator *I,
       KnownFPClass Result = computeKnownFPClass(V, fcAllFlags, Depth + 1, Q);
       FPClassTest FPClasses = Result.KnownFPClasses;
 
-      if (Result.isKnownNever(fcNormal | fcSubnormal | fcNan)) {
+      if (FPClasses != fcNone &&
+          Result.isKnownNever(fcNormal | fcSubnormal | fcNan)) {
         Known.Zero.setAllBits();
         Known.One.setAllBits();
 
diff --git a/llvm/test/Transforms/InstCombine/known-bits.ll b/llvm/test/Transforms/InstCombine/known-bits.ll
index 7a38020500517..cb404c0b7f889 100644
--- a/llvm/test/Transforms/InstCombine/known-bits.ll
+++ b/llvm/test/Transforms/InstCombine/known-bits.ll
@@ -1658,5 +1658,45 @@ define i32 @test_qnan_only(float nofpclass(snan sub norm zero inf) %x) {
   ret i32 %and
 }
 
+; Make sure that we don't crash when the use of x is unreachable.
+define i64 @pr92084(double %x) {
+; CHECK-LABEL: @pr92084(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp uno double [[X:%.*]], 0.000000e+00
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN1:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then1:
+; CHECK-NEXT:    br i1 [[CMP]], label [[IF_ELSE]], label [[IF_THEN2:%.*]]
+; CHECK:       if.then2:
+; CHECK-NEXT:    [[CAST:%.*]] = bitcast double [[X]] to i64
+; CHECK-NEXT:    [[AND:%.*]] = and i64 [[CAST]], 1
+; CHECK-NEXT:    ret i64 [[AND]]
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i64 0
+;
+  %cmp = fcmp uno double %x, 0.000000e+00
+  br i1 %cmp, label %if.then1, label %if.else
+
+if.then1:
+  br i1 %cmp, label %if.else, label %if.then2
+
+if.then2:
+  %cast = bitcast double %x to i64
+  %and = and i64 %cast, 1
+  ret i64 %and
+
+if.else:
+  ret i64 0
+}
+
+define i32 @test_none(float nofpclass(all) %x) {
+; CHECK-LABEL: @test_none(
+; CHECK-NEXT:    [[Y:%.*]] = bitcast float [[X:%.*]] to i32
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[Y]], 4194304
+; CHECK-NEXT:    ret i32 [[AND]]
+;
+  %y = bitcast float %x to i32
+  %and = and i32 %y, 4194304
+  ret i32 %and
+}
+
 declare void @use(i1)
 declare void @sink(i8)

[arsenm]

Probably should do this as a separate PR, this probably has the need-to-consistently-be-zero-for-undef problem

[dtcxzyw]

I will leave it as a todo.

[arsenm]

It's probably OK as long as we handle undef correctly inside computeKnownFPClass, the contradictory paths implies UB happened somewhere