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libstdc++: Stop defining _GLIBCXX_ASSERTIONS in floating_to_chars.cc

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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, most notably for
the hex formatting which is around 25% slower with assertions enabled.
This seems too high 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).

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc (_GLIBCXX_ASSERTIONS): Don't
	define.
	(__floating_to_chars_shortest): Add __builtin_unreachable calls to
	squelch false-positive -Wmaybe-uninitialized and -Wreturn-type
	warnings.
	(__floating_to_chars_precision): Likewise.
This commit is contained in:
Patrick Palka 2022-04-19 10:20:04 -04:00
parent fdb3f82fb3
commit cd3964ebd3
1 changed files with 6 additions and 3 deletions
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9
libstdc++-v3/src/c++17/floating_to_chars.cc
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@ -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.