[gcc r14-8792] libstdc++: Replace padding bits with bit-fields in __format::_Spec

[Jonathan Wakely]

https://gcc.gnu.org/g:260a22de4fa3d4ad3bb0d3ef2cd45d7f03eb3160

commit r14-8792-g260a22de4fa3d4ad3bb0d3ef2cd45d7f03eb3160
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 1 11:09:29 2024 +0000

    libstdc++: Replace padding bits with bit-fields in __format::_Spec
    
    This ensures that the unused bits will be zero-initialized reliably, and
    so can be used later by assigning them values in formatter
    specializations. For example, formatters for std::chrono will need to
    use an extra bit for a boolean flag to optimize the conversions between
    locale encodings and UTF-8.
    
    Adding the 16-bit _M_reserved2 bit-field results in an increased size
    for targets that use 1- or 2-byte alignment for all integral types, e.g.
    cris-elf or m68k.  Placing that member before the _M_width member
    adjusts the layout for all targets, but keeps all the bit-fields
    together. We can't make that change once C++20 support is ABI stable and
    non-experimental, so do it now before GCC 14 is released. The _M_fill
    data member already change from char to char32_t in
    r14-6991-g37a4c5c23a270c so _Spec is already incompatible with gcc-13
    anyway.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/format (__format::_Spec::_M_reserved): Define new
            bit-field members to reserve padding bits for future extensions.

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

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 0eca8b58bfa8..961441e355b3 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -406,6 +406,8 @@ namespace __format
       _WidthPrec _M_width_kind : 2;
       _WidthPrec _M_prec_kind : 2;
       _Pres_type _M_type : 4;
+      unsigned   _M_reserved : 1;
+      unsigned   _M_reserved2 : 16;
       unsigned short _M_width;
       unsigned short _M_prec;
       char32_t _M_fill = ' ';