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

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

Never mind, just spotted it.

:)



On 30/03/2022 13:10, Richard Earnshaw wrote:
> 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
>>