[llvm] [KnownFPClass] Improve known class deductions for exp/exp2/exp10 (PR #217526)

[Eric Ross via llvm-commits]

================
@@ -520,16 +520,19 @@ KnownFPClass KnownFPClass::exp(const KnownFPClass &KnownSrc) {
 
   Known.propagateNonNaN(KnownSrc);
 
-  if (KnownSrc.cannotBeOrderedLessThanZero()) {
-    // If the source is positive this cannot underflow.
+  // Only a negative normal or negative infinity can produce positive zero.
+  // A negative subnormal input is too small to produce positive zero.
----------------
ZERICO2005 wrote:

Done. Documented `KnownFPClass::exp` assumptions in `KnownFPClass.h`.

https://github.com/llvm/llvm-project/pull/217526