[libc-commits] [libc] [libc][math] Update getpayload and fmul with NaN inputs. (PR #99812)

[via libc-commits]

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@@ -50,19 +50,19 @@ mul(InType x, InType y) {
         raise_except_if_required(FE_INVALID);
 
       if (x_bits.is_quiet_nan()) {
-        InStorageType x_payload = static_cast<InStorageType>(getpayload(x));
-        if ((x_payload & ~(OutFPBits::FRACTION_MASK >> 1)) == 0)
-          return OutFPBits::quiet_nan(x_bits.sign(),
-                                      static_cast<OutStorageType>(x_payload))
-              .get_val();
+        InStorageType x_payload = x_bits.get_mantissa();
+        x_payload >>= (InFPBits::FRACTION_LEN - OutFPBits::FRACTION_LEN);
+        return OutFPBits::quiet_nan(x_bits.sign(),
+                                    static_cast<OutStorageType>(x_payload))
+            .get_val();
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overmighty wrote:

IEEE Std 754-2019, section 6.2.3 "NaN propagation":

> An operation that propagates a NaN operand to its result and has a single NaN as an input should produce a
NaN with the payload of the input NaN if representable in the destination format.
>
> [...]
>
> Conversion of a quiet NaN to a floating-point format of the same or a different radix that does not allow
the payload to be preserved shall return a quiet NaN that should provide some language-defined diagnostic
information.

But I'm not sure if conversion covers rounding the result to a narrower type in this case, and if the second paragraph is complementary to the first one (an operation that propagates a NaN operand should return a quiet NaN with language-defined diagnostic information if the payload of the input NaN is not representable in the destination format) or if it's just a different case (an operation that propagates a NaN operand can return a NaN with whatever payload if the payload of the input NaN is not representable in the destination format).

Maybe conversion just means conversion operators/functions. glibc seems to just truncate the lower bits of the mantissa: https://godbolt.org/z/aP7Yhccje.

https://github.com/llvm/llvm-project/pull/99812