From 260a22de4fa3d4ad3bb0d3ef2cd45d7f03eb3160 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 1 Feb 2024 11:09:29 +0000
Subject: [PATCH] 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.
---
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 = ' ';
--
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