[gcc r14-3159] libstdc++: Do not call log10(0.0) in std::format [PR110860]

[Jonathan Wakely]

https://gcc.gnu.org/g:9e33d71834416b7eddadae2b0f68e85f04cd0c7f

commit r14-3159-g9e33d71834416b7eddadae2b0f68e85f04cd0c7f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Aug 11 18:10:29 2023 +0100

    libstdc++: Do not call log10(0.0) in std::format [PR110860]
    
    Calling log10(0.0) returns -inf which has undefined behaviour when
    converted to an integer. We only need to use log10 for large values
    anyway. If the value is zero then the larger buffer is only needed due
    to a large precision, so we don't need to use log10 to estimate the
    number of digits for the significand.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/110860
            * include/std/format (__formatter_fp::format): Do not call log10
            with zero values.

Diff:
---
 libstdc++-v3/include/std/format | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 2fe430f75f69..23da6b008c5d 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -1490,7 +1490,7 @@ namespace __format
 	      // If the buffer is too small it's probably because of a large
 	      // precision, or a very large value in fixed format.
 	      size_t __guess = 8 + __prec;
-	      if (__fmt == chars_format::fixed) // +ddd.prec
+	      if (__fmt == chars_format::fixed && __v != 0) // +ddd.prec
 		{
 		  if constexpr (is_same_v<_Fp, float>)
 		    __guess += __builtin_log10f(__v < 0.0f ? -__v : __v);