[gcc r11-7790] libstdc++: Disable "ALT128" long double support for Clang

[gcc r11-7790] libstdc++: Disable "ALT128" long double support for Clang

[Jonathan Wakely]

https://gcc.gnu.org/g:baef0cffb58be7f5d9aeac6313ea9d8becc017b1

commit r11-7790-gbaef0cffb58be7f5d9aeac6313ea9d8becc017b1
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Mar 23 13:08:32 2021 +0000

    libstdc++: Disable "ALT128" long double support for Clang
    
    Clang does not currently support the __ibm128 type [1] and only supports
    the __ieee128 type in the unreleased 12.0.0 version [2]. That means it
    is not possible to provide support for -mabi=ieeelongdouble with Clang
    in an ABI compatible way (as we do for GCC by defining new facets and
    other types in the __gnu_cxx_ldbl128 namespace).
    
    By preventing the definition of _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT when
    compiling with Clang, all uses of __ibm128 and __ieee128 types will be
    disabled. This can be revisited in future when Clang supports the types
    (and provides a way to detect that support using the preprocessor).
    
    [1] https://reviews.llvm.org/D93377
    [2] https://reviews.llvm.org/D97846
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/c++config (_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT):
            Do not define when compiling with Clang.

Diff:
---
 libstdc++-v3/include/bits/c++config | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index b57ff339990..72ec91949de 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -425,9 +425,12 @@ _GLIBCXX_END_NAMESPACE_VERSION
 // GLIBCXX_ABI Deprecated
 // Define if compatibility should be provided for -mlong-double-64.
 #undef _GLIBCXX_LONG_DOUBLE_COMPAT
+
 // Define if compatibility should be provided for alternative 128-bit long
-// double formats.
+// double formats. Not possible for Clang until __ibm128 is supported.
+#ifndef __clang__
 #undef _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT
+#endif
 
 // Inline namespaces for long double 128 modes.
 #if defined _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT \