Re: volatile and R/M/W operations

[Robert Dewar <dewar at adacore dot com>]

Samuel Tardieu wrote:

> For this pattern (isolated setting of one bit in the middle of a byte at
> a random memory location), this is the best code on this platform AFAIK.
>
> As an evidence, if you mark neither variable as volatile GCC generates
> with -O3 -fomit-frame-pointer:
>
> f:
>         orb     $16, x
>         orb     $32, y
>         ret
>
> And I sure expect that GCC didn't choose to generate worst code when
> I *removed* the volatile constraint :)

OK, sounds reasonable, but then I don't understand the logic behind
avoiding this instruction sequence for the volatile case, this is
two accesses at the bus level so what's the difference? I think
on earlier pentiums these instructions were supposed to be avoided
but of course this may have changed.