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

[Jonathan Wakely via Gcc-patches]

On Thu, 14 Apr 2022 at 20:48, Patrick Palka via Libstdc++
 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
>  // .
>
> -// Activate __glibcxx_assert within this file to shake out any bugs.
> -#define _GLIBCXX_ASSERTIONS 1
> -
>  #include 
>
>  #include 
> @@ -1114,6 +,7 @@ template
>}
>
>  __glibcxx_assert(false);
> +__builtin_unreachable();
>}
>
>  template
> @@ -1202,6 +1200,8 @@ template
> 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
>   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
>}
>
>  __glibcxx_assert(false);
> +__builtin_unreachable();
>}
>
>  // Define the overloads for float.
> --
> 2.36.0.rc2.10.g1ac7422e39
>

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

[Patrick Palka via Gcc-patches]

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?

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
 // .
 
-// Activate __glibcxx_assert within this file to shake out any bugs.
-#define _GLIBCXX_ASSERTIONS 1
-
 #include 
 
 #include 
@@ -1114,6 +,7 @@ template
   }
 
 __glibcxx_assert(false);
+__builtin_unreachable();
   }
 
 template
@@ -1202,6 +1200,8 @@ template
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
  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
   }
 
 __glibcxx_assert(false);
+__builtin_unreachable();
   }
 
 // Define the overloads for float.
-- 
2.36.0.rc2.10.g1ac7422e39