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

[Eric Ross via llvm-commits]

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@@ -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.
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ZERICO2005 wrote:

I have added a note in `KnownFPClass.cpp` stating that it assumes +0.1 and +10.0  are both positive normal values (Since `exp10(-1.0) == 0.1` and `exp10(+1.0) == 10.0`). Should we also have this note in `KnownFPClass.h`?

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