On Friday 26 October 2007 01:32:53 Linus Torvalds wrote:
>
> On Fri, 26 Oct 2007, Andi Kleen wrote:
> >
> > No it can't (at least not on x86) as I have explained in the rest of the mail
> > you conveniently snipped.
>
> I "conveniently snipped it" because it was pointless.
>
> "adc" or "cmov" has nothing what-so-ever to do with it. If some routine
> returns 0-vs-1 and gcc then turns "if (routine()) x++" into
> "x+=routine()", what does that have to do with adc or cmov?
That is not what gcc did in that case. I don't think it tracks sets of values
over function calls (or even inside functions) at all.
The generated code was
cmpl $1, %eax ; test res
movl acquires_count, %edx ; load
adcl $0, %edx ; maybe add 1
movl %edx, acquires_count ; store
So it just added the result of a comparison into a variable
by (ab)using carry for this.
In theory such things can be done with CMOV too by redirecting
a store into a dummy variable to cancel it, but gcc doesn't
do that on its own.
> The fact is, these kinds of optimizations are *bogus* and they are
> dangerous.
The conditional add/sub using carry trick is not generally bogus.
It's just bogus for memory addresses not pretty much guaranteed in L1
[aka small stack frame] because there the pipeline benefit is unlikely to
offset the memory costs (and of course poor quality of implementation because of the
missing thread safety).
But for registers it's a fine optimization.
-Andi
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