volatile access optimization (C++ / x86_64)

[H.J. Lu]

On Sat, Dec 27, 2014 at 9:45 AM, Andrew Haley <aph@redhat.com> wrote:
> On 27/12/14 16:02, Paul_Koning@Dell.com wrote:
>>
>> In the case of volatile variables, the external interface in
>> question is the one at the point where that address is implemented —
>> a memory cell, or memory mapped I/O device on a bus.  So the
>> required behavior is that load and store operations (read and write
>> transactions at that interface) occur as written.
>
> I believe this is incorrect.  For accesses to reach memory in program
> order on most architectures would require volatile memory references
> to emit memory barriers, and the C committee decided not to require
> that.
>
>> If a processor has add instructions that support memory references
>> (as in x86 and vax, but not mips), such an instruction will perform
>> a read cycle followed by a write cycle.  So as seen at the critical
>> interface, the behavior is the same as if you were to do an explicit
>> load, register add, store sequence.  Therefore the use of a single
>> add-to-memory is a valid implementation.
>
> I agree.
>

Can we add a target hook so that combine will allow a single
add-to-memory instruction for volatile memory reference on
architectures like x86?


-- 
H.J.