volatile access optimization (C++ / x86_64)

[Andrew Haley]

On 27/12/14 18:04, Matt Godbolt wrote:
> On Sat, Dec 27, 2014 at 11:57 AM, Andrew Haley <aph@redhat.com> wrote:

>> if you don't need an atomic access, why do you care that it uses a
>> read-modify-write instruction instead of three instructions?  Is it
>> faster?  Have you measured it?  Is it so much faster that it's
>> critical for your application?
> 
> Good point. No; I've yet to measure it but I will. I'll be honest: my
> instinct is that really it won't make a measurable difference. From a
> microarchitectural point of view it devolves to almost exactly the
> same set of micro-operations (barring the duplicate memory address
> calculation). It does encode to a longer instruction stream (15 bytes
> vs 7 bytes), so there's an argument it puts more pressure than needed
> on the i-cache. But honestly, it's more from an aesthetic point of
> view I prefer the increment.

Aha!  I get it now.

I know that it's not really necessary to know a questioner's
motivation for a question like this: it makes no difference to the
answer.  But you really had me mystified because I was assuming that
there had to be a practical reason for all this.

> Also, it's always nice to understand why particular optimisations
> aren't performed by the compiler from a correctness point of view! :)

Sure.  Well, in this case you now know: it's a bug!  But one that it's
fairly hard to care deeply about, although it might get fixed now.

Andrew.