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

[Ulrich Weigand]

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.

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

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