Re: Implement C11 _Atomic

[Hans-Peter Nilsson <hp@bitrange.com>]

On Tue, 5 Nov 2013, Joseph S. Myers wrote:

Thanks for doing this!  However, without examples I have trouble
reading out the bits I need as a target maintainer, and I can't
read out the answers from the patch, so pardon a few questions.

> This patch, relative to trunk and based on work done on the C11-atomic
> branch, adds support for C11 _Atomic.  It is intended to include all
> the required language support.
>
> It does not include the <stdatomic.h> header; there's a version on the
> branch, but it needs further review against the standard and test
> coverage adding to the testsuite before I can propose it for mainline.
>
> Support for atomic types having bigger alignment than the
> corresponding non-atomic types is limited: it includes the code to
> increase the alignment of types whose size is exactly 1, 2, 4, 8 or 16
> to that of the corresponding integer type [*], but not anything for
> target-specific alignment increases.

Target-maintainer perspective here: do I read that correctly,
that by default adding _Atomic raises the alignment of that type
to the "natural" one, for all targets?

To wit,

> There's code for target-specific
> alignment on the branch (and I intend to merge trunk back to the
> branch once this patch is on trunk, so it's easy to tell what the
> changes still left on the branch are), should any target maintainers
> wish to have such alignment.

...is that part needed for alignment that is only
target-specific and other-than-natural?  For example, 8-byte
aligment where required for atomic 4-byte types?

Or is that part also required for
anything-other-than-ordinary-C-type alignment for the target;
say, natural 4-byte alignment of 4-byte-types for targets where
alignment is otherwise "packed"; where only 1-byte alignment of
the basic type is ABI-mandated?

brgds, H-P