RFC: add "cmpxchg weak" to LLVM IR

[Chandler Carruth]

On Thu, Jun 12, 2014 at 5:50 PM, Chandler Carruth <chandlerc at google.com>
wrote:

> On Thu, Jun 12, 2014 at 5:22 PM, Tim Northover <t.p.northover at gmail.com>
> wrote:
>
>> 1. All cmpxchg instructions now return { iN, i1 } where the first
>> value is what we got before (the loaded result), the second == 1 if an
>> exchange took place.
>>
>
> Having to do extractvalue to get stuff out of these kinds of aggregates
> is... somewhat horrible... have you thought of any alternatives?


Having thought about this a bit, I don't see anything better.

If we lived in a world w/o the select instruction, there might be a better
design for this (and for the overflow observing arithmetic intrinsics).

It is a bit sad, especially because I can't see a good reason to make the
weak variant an intrinsic and force this through the call path...


going back to the meat of the proposal...
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