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* gcc-wwwdocs branch master updated. 3be1a28f58d6063258407b0751e8fb55df4749c8
@ 2022-03-30 11:16 Jakub Jelinek
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From: Jakub Jelinek @ 2022-03-30 11:16 UTC (permalink / raw)
To: gcc-cvs-wwwdocs
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- Log -----------------------------------------------------------------
commit 3be1a28f58d6063258407b0751e8fb55df4749c8
Author: Jakub Jelinek <jakub@redhat.com>
Date: Wed Mar 30 13:16:18 2022 +0200
Document zero width bitfield ABI changes.
diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html
index 689feeba..f82f1a97 100644
--- a/htdocs/gcc-12/changes.html
+++ b/htdocs/gcc-12/changes.html
@@ -28,6 +28,31 @@ a work-in-progress.</p>
<!-- .................................................................. -->
<h2>Caveats</h2>
<ul>
+ <li>
+ An <a name="zero_width_bitfields">ABI</a> incompatibility between C and
+ C++ when passing or returning by value certain aggregates containing zero
+ width bit-fields has been discovered on various targets.
+ As mentioned in <a href="https://gcc.gnu.org/PR102024">PR102024</a>,
+ since the <a href="https://gcc.gnu.org/PR42217">PR42217</a> fix in
+ GCC 4.5 the C++ front-end has been removing zero width bit-fields
+ from the internal representation of the aggregates after the layout of those
+ aggregates, but the C front-end kept them, so passing e.g.
+ <code>struct S { float a; int : 0; float b; }</code> or
+ <code>struct T { float c; int : 0; }</code> by value could differ
+ between C and C++. Starting with GCC 12 the C++ front-end no longer
+ removes those bit-fields from the internal representation and
+ per clarified psABI some targets have been changed, so that they
+ either ignore those bit-fields in the argument passing by value
+ decisions in both C and C++, or they always take them into account.
+ x86-64, ARM and AArch64 will always ignore them (so there is
+ a C ABI incompatibility between GCC 11 and earlier with GCC 12 or
+ later), PowerPC64 ELFv2 and S/390 always take them into account
+ (so there is a C++ ABI incompatibility, GCC 4.4 and earlier compatible
+ with GCC 12 or later, incompatible with GCC 4.5 through GCC 11).
+ RISC-V has changed the handling of these already starting with GCC 10.
+ GCC 12 on the above targets will report such incompatibilities as
+ warnings or other diagnostics unless <code>-Wno-psabi</code> is used.
+ </li>
<li>
<strong>C:</strong>
Computed gotos require a pointer type now.
-----------------------------------------------------------------------
Summary of changes:
htdocs/gcc-12/changes.html | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
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2022-03-30 11:16 gcc-wwwdocs branch master updated. 3be1a28f58d6063258407b0751e8fb55df4749c8 Jakub Jelinek
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