Re: [RFA wwwdocs] Re: [PATCH v2] Target-specific limits on vector alignment

[Richard Earnshaw <rearnsha at arm dot com>]

On 10/08/12 16:18, Ulrich Weigand wrote:
> Richard Earnshaw wrote:
>> On 10/08/12 14:44, Ulrich Weigand wrote:
>>> Would the following htdocs patch be OK with you?   Feel free to suggest
>>> a more appropriate wording  ...
>>
>> I think we need to make it clear that this also fixes a bug in the
>> compiler that could lead to a run-time error.  Otherwise, people will be
>> asking why an abi-breaking change was made mid-cycle.
> 
> Something along the following lines?
> 
> <p>The default alignment of vector types on ARM EABI targets has been changed
> to 8 bytes (for types larger than 8 bytes in size).  This change was necessary
> in order to fix a bug that could lead to generation of wrong code in certain
> cases.  It also brings the ABI as implemented by GCC back in compliance with
> the AAPCS.  This is an ABI change that affects e.g. layout of structures having
> a member of vector type.  Code using such types may be incompatible with binary
> objects built with older versions of GCC.</p>
> 
> Bye,
> Ulrich
> 

How about:

<p>On ARM, a bug has been fixed in GCC's implementation of the AAPCS
rules for the layout of vectors that could lead to wrong code being
generated.  Vectors larger than 8 bytes in size are now by default
aligned to an 8-byte boundary.  This is an ABI change: code that makes
explicit use of vector types may be incompatible with binary objects
built with older versions of GCC.  Auto-vectorized code is not affected
by this change.</p>

R.