Re: [PATCH] libstdc++: Stop defining _GLIBCXX_ASSERTIONS in floating_to_chars.cc

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 14 Apr 2022 at 20:48, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Assertions were originally enabled in the compiled-in floating-point
> std::to_chars implementation to help shake out any bugs, but they
> apparently impose a significant performance penalty, in particular for
> the hex formatting which is around 25% slower with assertions enabled.
> This seems too high of a cost for unconditionally enabling them.
>
> The newly added calls to __builtin_unreachable work around the compiler
> no longer knowing that the set of valid values of 'fmt' is limited (which
> was previously upheld by an assert).
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

OK, thanks.

>
> libstdc++-v3/ChangeLog:
>
>         * src/c++17/floating_to_chars.cc: Don't define
>         _GLIBCXX_ASSERTIONS.
>         (__floating_to_chars_shortest): Add __builtin_unreachable calls to
>         squelch false-positive -Wmaybe-uninitialized and -Wreturn-type
>         warnings.
>         (__floating_to_chars_precision): Likewise.
> ---
>  libstdc++-v3/src/c++17/floating_to_chars.cc | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
> index 66bd457cbe2..4599d68a39c 100644
> --- a/libstdc++-v3/src/c++17/floating_to_chars.cc
> +++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
> @@ -22,9 +22,6 @@
>  // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>  // <http://www.gnu.org/licenses/>.
>
> -// Activate __glibcxx_assert within this file to shake out any bugs.
> -#define _GLIBCXX_ASSERTIONS 1
> -
>  #include <charconv>
>
>  #include <bit>
> @@ -1114,6 +1111,7 @@ template<typename T>
>        }
>
>      __glibcxx_assert(false);
> +    __builtin_unreachable();
>    }
>
>  template<typename T>
> @@ -1202,6 +1200,8 @@ template<typename T>
>             effective_precision = min(precision, max_eff_scientific_precision);
>             output_specifier = "%.*Lg";
>           }
> +       else
> +         __builtin_unreachable();
>         const int excess_precision = (fmt != chars_format::general
>                                       ? precision - effective_precision : 0);
>
> @@ -1234,6 +1234,8 @@ template<typename T>
>               output_length_upper_bound = sign + strlen("0");
>             output_length_upper_bound += sizeof(radix) + effective_precision;
>           }
> +       else
> +         __builtin_unreachable();
>
>         // Do the sprintf into the local buffer.
>         char buffer[output_length_upper_bound+1];
> @@ -1570,6 +1572,7 @@ template<typename T>
>        }
>
>      __glibcxx_assert(false);
> +    __builtin_unreachable();
>    }
>
>  // Define the overloads for float.
> --
> 2.36.0.rc2.10.g1ac7422e39
>