[RFC] Do not consider volatile asms as optimization barriers #1

[Richard Sandiford]

Eric Botcazou <ebotcazou@adacore.com> writes:
>> non-local labels should block most optimizations by the fact they
>> are a receiver of control flow and thus should have an abnormal
>> edge coming into them.  If that's not the case (no abnormal edge)
>> then that's the bug to fix.
>
> It's (of course) more complicated, you need to look at HP's fix and testcase 
> to see why we need a full optimization barrier.  See also the prologue and 
> epilogue of many architectures which also need a blockage when they are 
> establishing or destroying the frame.

But the prologue/epilogue case often doesn't need to be a full blockage.
We could move a load-immediate instruction -- or even an accesss to known-
global memory -- before the allocation or after the deallocation.  This can
actually be important on architectures that use load-multiple to restore the
return register and want the prefetcher to see the target address as early
as possible.

So I think the prologue and epilogue is one case where we really do want
to spell out what's clobbered by the allocation and deallocation.

Thanks,
Richard