Doesn't this need the anchor that the compiler links to?
#zero_width_bitfields
R.
On 30/03/2022 11:07, Jakub Jelinek via Gcc-patches wrote:
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
