[libc-commits] [libc] [libc][math][c23] Add f16fmaf C23 math function (PR #95483)

[via libc-commits]

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@@ -251,20 +275,21 @@ template <> LIBC_INLINE double fma<double>(double x, double y, double z) {
 
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overmighty wrote:

GitHub doesn't let me comment on the lines above because they're too far away from changes, but I think this is wrong:

```cpp
  } else {
    // Return +0.0 when there is exact cancellation, i.e., x*y == -z exactly.
    prod_sign = Sign::POS;
  }
```

To quote IEEE Std 754-2019 section "6.3 The sign bit":

> When the sum of two operands with opposite signs (or the difference of two operands with like signs) is
exactly zero, the sign of that sum (or difference) shall be +0 under all rounding-direction attributes except
roundTowardNegative; under that attribute, the sign of an exact zero sum (or difference) shall be −0.
However, under all rounding-direction attributes, when *x* is zero, *x*+*x* and *x*−(−*x*) have the sign of *x*.
>
> When (*a*×*b*)+*c* is exactly zero, the sign of **fusedMultiplyAdd**(*a*, *b*, *c*) shall be determined by the rules
above for a sum of operands. When the exact result of (*a*×*b*)+*c* is non-zero yet the result of
**fusedMultiplyAdd** is zero because of rounding, the zero result takes the sign of the exact result. 

Also see https://godbolt.org/z/KsaKMYKao.

https://github.com/llvm/llvm-project/pull/95483