Re: Disable 64-bit atomics for MIPS n32 [committed]

[Torvald Riegel <triegel at redhat dot com>]

On Wed, 2015-01-28 at 18:45 +0000, Joseph Myers wrote:
> This patch disables use of 64-bit atomics for MIPS n32 to fix the
> problems with unaligned semaphores.

That works.  I would probably preferred to just do this in the semaphore
bits (ie, checking for __HAVE_64B_ATOMICS and _LP64, but both have the
same effect for 2.21.

> Before 64-bit atomics are used for anything for which such alignment
> issues do not arise, and before the addition of any new ILP32 ports
> with 64-bit semaphores for which the ABI can be set to have the
> greater alignment (AARCH64?), a better approach will need to be
> established that allows architectures to declare their 64-bit atomics
> availability accurately, without doing so causing inappropriate use of
> such atomics on unaligned semaphores.

I guess I'll have to add another flag for whether 64B atomics need to be
naturally aligned or not.  I'm not yet sure how to express this best, so
suggestions are welcome.  If we can say (for now) that we only support
ILP32 and LP64, this might clarify the options we have -- I suspect that
in most cases where 64B atomics will be useful, this will be for legacy
code.

OTOH, I'm not sure whether non-naturally-aligned 64B atomics actually
really help us anywhere.  So we could as well just require them to be
naturally aligned, and use _LP64 to check whether alignment for legacy
code is sufficient.

IOW, I believe I messed this up at the semaphore side, not in the
atomics :)