[gcc r12-6665] libstdc++: Use GCC's predefined macro for endianness [PR104080]

[Jonathan Wakely]

https://gcc.gnu.org/g:8f6b62e0f0c99e421d07bf1847259744db22924b

commit r12-6665-g8f6b62e0f0c99e421d07bf1847259744db22924b
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jan 18 10:01:23 2022 +0000

    libstdc++: Use GCC's predefined macro for endianness [PR104080]
    
    Instead of hardcoded preprocessor conditionals with explicit target
    checks, just rely on the fact that __BYTE_ORDER__ is always defined by
    GCC.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/104080
            * src/c++17/fast_float/LOCAL_PATCHES: Update.
            * src/c++17/fast_float/fast_float.h (FASTFLOAT_IS_BIG_ENDIAN):
            Define in terms of __BYTE_ORDER__.

Diff:
---
 libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES | 1 +
 libstdc++-v3/src/c++17/fast_float/fast_float.h  | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
index 447c7ed2cdb..5bb42933398 100644
--- a/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
+++ b/libstdc++-v3/src/c++17/fast_float/LOCAL_PATCHES
@@ -1,3 +1,4 @@
 r12-6647
 r12-6648
 r12-6664
+r12-6665
diff --git a/libstdc++-v3/src/c++17/fast_float/fast_float.h b/libstdc++-v3/src/c++17/fast_float/fast_float.h
index ee129309ba3..31fb88b8aba 100644
--- a/libstdc++-v3/src/c++17/fast_float/fast_float.h
+++ b/libstdc++-v3/src/c++17/fast_float/fast_float.h
@@ -128,7 +128,9 @@ from_chars_result from_chars_advanced(const char *first, const char *last,
 #define FASTFLOAT_VISUAL_STUDIO 1
 #endif
 
-#ifdef _WIN32
+#ifdef __BYTE_ORDER__
+#define FASTFLOAT_IS_BIG_ENDIAN (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
+#elif defined _WIN32
 #define FASTFLOAT_IS_BIG_ENDIAN 0
 #else
 #if defined(__APPLE__) || defined(__FreeBSD__)