[libc] [llvm] [APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. (PR #143959)

[Matt Arsenault via llvm-commits]

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@@ -6155,6 +6158,29 @@ float APFloat::convertToFloat() const {
   return Temp.getIEEE().convertToFloat();
 }
 
+static constexpr int getFEnvRoundingMode(llvm::RoundingMode rm) {
+  switch (rm) {
+  case APFloat::rmTowardPositive:
+    return FE_UPWARD;
+  case APFloat::rmTowardNegative:
+    return FE_DOWNWARD;
+  case APFloat::rmTowardZero:
+    return FE_TOWARDZERO;
+  default:
+    // TODO: fix rmNearestTiesToAway for platform without FE_TONEARESTFROMZERO.
+    return FE_TONEAREST;
+  };
+}
+
+APFloat exp(const APFloat &X, RoundingMode rounding_mode) {
+  if (&X.getSemantics() == &APFloat::IEEEsingle()) {
+    float result = LIBC_NAMESPACE::shared::expf(
+        X.convertToFloat(), getFEnvRoundingMode(rounding_mode));
+    return APFloat(result);
+  }
+  assert(false && "Unsupported floating-point semantics");
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arsenm wrote:

```suggestion
  llvm_unreachable("unsupported floating-point semantics");
```

https://github.com/llvm/llvm-project/pull/143959