From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 1 Feb 2024 11:09:29 +0000 (+0000)
Subject: libstdc++: Replace padding bits with bit-fields in __format::_Spec
X-Git-Tag: basepoints/gcc-15~1347
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=260a22de4fa3d4ad3bb0d3ef2cd45d7f03eb3160;p=thirdparty%2Fgcc.git

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 --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 = ' ';