From: Jakub Jelinek <jakub@redhat.com>
To: gcc-patches@gcc.gnu.org
Subject: [wwwdocs] Document zero width bit-field passing ABI changes in gcc-12/changes.html [PR104796]
Date: Wed, 30 Mar 2022 12:07:55 +0200 [thread overview]
Message-ID: <YkQr+9Z/MTSkeuJ4@tucnak> (raw)
Hi!
This patch documents the PR102024 ABI changes.
The x86-64, ARM and AArch64 backends refer to this in their -Wpsabi
diagnostics.
Ok for wwwdocs?
diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html
index 689feeba..dc0e4074 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 with 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.
Jakub
next reply other threads:[~2022-03-30 10:08 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-30 10:07 Jakub Jelinek [this message]
2022-03-30 11:00 ` Richard Biener
2022-03-30 11:30 ` Jakub Jelinek
2022-10-22 16:47 ` Gerald Pfeifer
2022-03-30 12:10 ` Richard Earnshaw
2022-03-30 12:13 ` Richard Earnshaw
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