[gcc r12-8194] libstdc++: Stop defining _GLIBCXX_ASSERTIONS in floating_to_chars.cc

[gcc r12-8194] libstdc++: Stop defining _GLIBCXX_ASSERTIONS in floating_to_chars.cc

[Patrick Palka]

https://gcc.gnu.org/g:cd3964ebd3e94ed0df4ecaadb7fd34e991cec753

commit r12-8194-gcd3964ebd3e94ed0df4ecaadb7fd34e991cec753
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 19 10:20:04 2022 -0400

    libstdc++: Stop defining _GLIBCXX_ASSERTIONS in floating_to_chars.cc
    
    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.

Diff:
---
 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.