Re: [wwwdocs] Document zero width bit-field passing ABI changes in gcc-12/changes.html [PR104796]

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

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
>