On Fri, Aug 17, 2007 at 09:13:35PM -0700, Linus Torvalds wrote:
>
>
> On Sat, 18 Aug 2007, Satyam Sharma wrote:
> >
> > No code does (or would do, or should do):
> >
> > x.counter++;
> >
> > on an "atomic_t x;" anyway.
>
> That's just an example of a general problem.
>
> No, you don't use "x.counter++". But you *do* use
>
> if (atomic_read(&x) <= 1)
>
> and loading into a register is stupid and pointless, when you could just
> do it as a regular memory-operand to the cmp instruction.
>
> And as far as the compiler is concerned, the problem is the 100% same:
> combining operations with the volatile memop.
>
> The fact is, a compiler that thinks that
>
> movl mem,reg
> cmpl $val,reg
>
> is any better than
>
> cmpl $val,mem
>
> is just not a very good compiler. But when talking about "volatile",
> that's exactly what ytou always get (and always have gotten - this is
> not a regression, and I doubt gcc is alone in this).
One of the gcc guys claimed that he thought that the two-instruction
sequence would be faster on some x86 machines. I pointed out that
there might be a concern about code size. I chose not to point out
that people might also care about the other x86 machines. ;-)
Thanx, Paul
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