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Re: [PATCH 4.9][AArch64] Backport 211892: PR/60825 Make float64x1_t in arm_neon.h a proper vector type


On 8 July 2014 13:31, Jakub Jelinek <jakub@redhat.com> wrote:

> Many people are using different patch levels interchangeably and expect
> them to be compatible.  If X1 vectors are relatively rare, the better,
> fewer people will be affected when switching from 4.9.x to 4.10.x.
>
> But IMHO changing ABI between patchlevels, at least when the ABI is not
> inconsistent (we had in the past cases where e.g. caller and callee didn't
> agree where to pass some argument even when using the same compiler, then
> obviously no working code can use those and it can be fixed), is a very bad
> idea.  My strong preference is to maintain ABI compatibility between
> patchlevel versions.
>
> Just add a -Wps-abi warning to 4.10 when you encounter this (look at what
> e.g. x86_64 does).  You can also add a warning to 4.9.1 that next GCC
> version will change ABI for passing those.
>
>         Jakub

Alan, Ramana and I just discussed this further f2f the outcome of which was:

*) Alan will look into modifying the 4.9 patches to issue a -Wpsabi
gated warning that the ABI/mangling will change in 4.10 instead of
making the break now.  Initial thoughts were that he would need to
take most of the patch in order to detect the scenario and issue the
warning.

*) Alan will run up a  trunk patch to issue a diagnostic based on
-Wpsabi to warn that mangling / ABI has changed.

*) We recommend that linaro not back port the trunk mangling/ABI
changes to linaro-4.9.

Cheers
/Marcus


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