[PATCH] libstdc++: Fix ERANGE behavior for fallback FP std::from_chars

[Jonathan Wakely]

On Tue, 1 Nov 2022 at 21:30, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> The fallback implementation of floating-point std::from_chars for e.g.
> float80 just calls the C library's strtod family of functions.  In case
> of overflow of the parsed result, the behavior of these functions is
> rigidly specified:
>
>   If the correct value overflows and default rounding is in effect, plus
>   or minus HUGE_VAL, HUGE_VALF, or HUGE_VALL is returned (according to
>   the return type and sign of the value), and the value of the macro
>   ERANGE is stored in errno.
>
> But in case of underflow, implementations are given more leeway:
>
>   If the result underflows the functions return a value whose magnitude
>   is no greater than the smallest normalized positive number in the
>   return type; whether errno acquires the value ERANGE is
>   implementation-defined.
>
> Thus we can (and do) portably detect overflow, but we can't portably
> detect underflow.  However, glibc (and presumably other high-quality C
> library implementations) will reliably set errno to ERANGE in case of
> underflow too, and it will also return the nearest denormal number to
> the parsed result (including zero in case of true underflow).
>
> Since we can't be perfect here, this patch takes the best effort
> approach of assuming a high quality C library implementation that
> allows us to distinguish between a denormal parsed result and true
> underflow by inspecting the return value.
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?  Dunno

OK for trunk.

> if we should backport this too.  No test because we can't portably
> test this IIUC.

I think it's worth backporting to 11 and 12 because this is a C++17
feature and that's our default mode since GCC 11.
But give it some soak time on trunk first please.


>
> libstdc++-v3/ChangeLog:
>
>         * src/c++17/floating_from_chars.cc (from_chars_impl): In the
>         ERANGE case, also check for a 0 return value before returning
>         result_out_of_range, occurred, otherwise assume it's a denormal
>         number.
> ---
>  libstdc++-v3/src/c++17/floating_from_chars.cc | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/src/c++17/floating_from_chars.cc b/libstdc++-v3/src/c++17/floating_from_chars.cc
> index a25ac5ce3aa..939c751f861 100644
> --- a/libstdc++-v3/src/c++17/floating_from_chars.cc
> +++ b/libstdc++-v3/src/c++17/floating_from_chars.cc
> @@ -637,8 +637,13 @@ namespace
>           {
>             if (__builtin_isinf(tmpval)) // overflow
>               ec = errc::result_out_of_range;
> -           else // underflow (LWG 3081 wants to set value = tmpval here)
> +           else if (tmpval == 0) // underflow (LWG 3081 wants to set value = tmpval here)
>               ec = errc::result_out_of_range;
> +           else // denormal value
> +             {
> +               value = tmpval;
> +               ec = errc();
> +             }
>           }
>         else if (n)
>           {
> --
> 2.38.1.381.gc03801e19c
>