[PATCH][RFC] Implement target-specific __attribute__((aligned)) value

[Richard Smith]

This generally looks fine.

On Mon, Mar 30, 2015 at 11:20 AM, Ulrich Weigand <Ulrich.Weigand at de.ibm.com>
wrote:

>
>
> Hello,
>
> the GCC construct __attribute__((aligned)) is defined to set alignment
> to "the default alignment for the target architecture" according to
> the GCC documentation:
>
>   The default alignment is sufficient for all scalar types, but may not be
>   enough for all vector types on a target that supports vector operations.
>   The default alignment is fixed for a particular target ABI.
>

I don't think this means to define a general term "default alignment";
instead, I think this is just shorthand for "default alignment for
__attribute__((aligned))". I'd prefer if you used a bit more of a specific
name for this field. Something like DefaultAlignForAttributeAligned would
be suitably clear.

clang currently hard-codes an alignment of 16 bytes for that construct,
> which is correct on some platforms (including X86), but wrong on others
> (including SystemZ).  Since this value is ABI-relevant, it is important
> to get correct for compatibility purposes.
>
> The following patch adds a new TargetInfo member "DefaultAlign" that
> targets can set to the appropriate default __attribute__((aligned)) value.
>
> Note that I'm deliberately *not* using the existing "SuitableAlign"
> value, which is used to set the pre-defined macro __BIGGEST_ALIGNMENT__,
> since those two values may not be the same on all platforms.  In fact,
> on X86, __attribute__((aligned)) always uses 16-byte alignment, while
> __BIGGEST_ALIGNMENT__ may be larger if AVX-2 or AVX-512 are supported.
> (This is actually not yet correctly implemented in clang either.)
>
> The patch provides a value for DefaultAlign only for SystemZ, and leaves
> the default for all other targets at 16, which means no visible change
> in behavior on all other targets.  (The value is still wrong for some
> other targets, but I'd prefer to leave it to the target maintainers for
> those platforms to fix and test.)
>
> (See attached file: clang-align-attribute)
>
>
> Mit freundlichen Gruessen / Best Regards
>
> Ulrich Weigand
>
> --
>   Dr. Ulrich Weigand | Phone: +49-7031/16-3727
>   STSM, GNU/Linux compilers and toolchain
>   IBM Deutschland Research & Development GmbH
>   Vorsitzende des Aufsichtsrats: Martina Koederitz | Geschäftsführung: Dirk
> Wittkopp
>   Sitz der Gesellschaft: Böblingen | Registergericht: Amtsgericht
> Stuttgart, HRB 243294
> _______________________________________________
> cfe-commits mailing list
> cfe-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20150420/1d09c760/attachment.html>