[committed] libstdc++: Fix std::to_chars for IEEE128 long double

[Jonathan Wakely <jwakely@redhat.com>]

Tested x86_64-linux and powerpc64le-linux with both ieee/ibm ABIs.

Pushed to trunk. This should be backported to gcc-11 too.

-- >8 --

The preprocessor check for _GLIBCXX_USE_FLOAT128 is the wrong condition,
because when the compiler is built with --with-long-double-format=ieee
configure determines that __float128 is the same as long double, and so
should not be used. But we do want the std::to_chars overloads for
__float128 in that case, because the floating_to_chars.cc file is built
with -mabi=ibmlongdouble and so the __float128 overloads are actually
the 'long double' ones for -mabi=ieeelongdouble code.

This fixes missing definitions of the __float128 overloads of
std::to_chars for --with-long-double-format=ieee builds. Without this,
there are symbols present in the --with-long-double-abi=ibm build which
are missing from the --with-long-double-abi=ieee build.

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc (FLOAT128_TO_CHARS): Depend on
	LONG_DOUBLE_ALT128_COMPAT instead of USE_FLOAT128.
---
 libstdc++-v3/src/c++17/floating_to_chars.cc | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/src/c++17/floating_to_chars.cc b/libstdc++-v3/src/c++17/floating_to_chars.cc
index 8da2f2385a0..5825e661bf4 100644
--- a/libstdc++-v3/src/c++17/floating_to_chars.cc
+++ b/libstdc++-v3/src/c++17/floating_to_chars.cc
@@ -76,16 +76,12 @@ extern "C" int __sprintfieee128(char*, const char*, ...);
 # define LONG_DOUBLE_KIND LDK_UNSUPPORTED
 #endif
 
-#if defined _GLIBCXX_USE_FLOAT128 && __FLT128_MANT_DIG__ == 113
+// For now we only support __float128 when it's the powerpc64 __ieee128 type.
+#if defined _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __FLT128_MANT_DIG__ == 113
 // Define overloads of std::to_chars for __float128.
 # define FLOAT128_TO_CHARS 1
 #endif
 
-// For now we only support __float128 when it's the powerpc64 __ieee128 type.
-#ifndef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
-# undef FLOAT128_TO_CHARS
-#endif
-
 #ifdef FLOAT128_TO_CHARS
 using F128_type = __float128;
 #else
-- 
2.34.1