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* [PATCH] libstdc++: Fix preprocessor conditions for std::from_chars [PR109921]
@ 2023-07-19 20:43 Jonathan Wakely
  0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2023-07-19 20:43 UTC (permalink / raw)
  To: libstdc++, gcc-patches; +Cc: Patrick Palka

I'm testing this patch for gcc-13 as a better fix for PR109921, without
the breakage that r14-1431-g7037e7b6e4ac41 caused on trunk.

If testing goes well I'll push this before the 13.2 release candidate.

-- >8 --

We use the from_chars_strtod function with __strtof128 to read a
_Float128 value, but from_chars_strtod is not defined unless uselocale
is available. This can lead to compilation failures for some targets,
because we try to define the _Float128 overload in terms of a
non-existing from_chars_strtod function.

Only try to use __strtof128 if uselocale is available and therefore we
can use the from_chars_strtod function template.

This is a simpler change than r14-1431-g7037e7b6e4ac41 on trunk, because
that caused unwanted ABI regressions (PR libstdc++/110077).


	PR libstdc++/109921
	* src/c++17/ (USE_STRTOF128_FOR_FROM_CHARS):
	Only define when USE_STRTOD_FOR_FROM_CHARS is also defined.
	(USE_STRTOD_FOR_FROM_CHARS): Do not undefine when long double is
	(from_chars(const char*, const char*, double&, chars_format)):
	Check __LDBL_MANT_DIG__ == __DBL_MANT_DIG__ here.
 libstdc++-v3/src/c++17/ | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/src/c++17/ b/libstdc++-v3/src/c++17/
index 78b9d92cdc0..b3061bdca01 100644
--- a/libstdc++-v3/src/c++17/
+++ b/libstdc++-v3/src/c++17/
@@ -64,7 +64,7 @@
 // strtold for __ieee128
 extern "C" __ieee128 __strtoieee128(const char*, char**);
 #elif __FLT128_MANT_DIG__ == 113 && __LDBL_MANT_DIG__ != 113 \
-      && defined(__GLIBC_PREREQ)
+      && defined(__GLIBC_PREREQ) && defined(USE_STRTOD_FOR_FROM_CHARS)
 extern "C" _Float128 __strtof128(const char*, char**)
   __asm ("strtof128")
@@ -77,10 +77,6 @@ extern "C" _Float128 __strtof128(const char*, char**)
     && __SIZE_WIDTH__ >= 32
 # define USE_LIB_FAST_FLOAT 1
-# if __LDBL_MANT_DIG__ == __DBL_MANT_DIG__
-// No need to use strtold.
-# endif
@@ -1211,7 +1207,7 @@ from_chars_result
 from_chars(const char* first, const char* last, long double& value,
 	   chars_format fmt) noexcept
   // Either long double is the same as double, or we can't use strtold.
   // In the latter case, this might give an incorrect result (e.g. values
   // out of range of double give an error, even if they fit in long double).
@@ -1280,6 +1276,7 @@ _ZSt10from_charsPKcS0_RDF128_St12chars_format(const char* first,
 					      chars_format fmt) noexcept
 __attribute__((alias ("_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format")));
 #elif defined(USE_STRTOF128_FOR_FROM_CHARS)
+// Overload for _Float128 is not defined inline in <charconv>, define it here.
 from_chars(const char* first, const char* last, _Float128& value,
 	   chars_format fmt) noexcept

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