[llvm] [KnownFPClass] Refine known classes for `atan2(y, x)` when `x` is negative (PR #214629)

[via llvm-commits]

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@@ -5427,12 +5425,25 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
       break;
     }
     case Intrinsic::atan2: {
-      KnownFPClass KnownLHS, KnownRHS;
-      computeKnownFPClass(II->getArgOperand(0), DemandedElts, InterestedClasses,
-                          KnownLHS, Q, Depth + 1);
-      computeKnownFPClass(II->getArgOperand(1), DemandedElts, InterestedClasses,
-                          KnownRHS, Q, Depth + 1);
-      Known = KnownFPClass::atan2(KnownLHS, KnownRHS);
+      FPClassTest InterestedY = InterestedClasses;
+      FPClassTest InterestedX = InterestedClasses;
+
+      // We can rule out zero and subnormal if x cannot have a positive value.
+      if ((InterestedClasses & (fcZero | fcSubnormal)) != fcNone)
+        InterestedX |= fcPositive | fcNegSubnormal;
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ZERICO2005 wrote:

I was unable to construct a test to prove that this is needed. Do you have any pointers on how I can write a test to demonstrate that a fold does not occur without demanding this analysis?

https://github.com/llvm/llvm-project/pull/214629