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SubjectRe: Semaphore assembly-code bug


On Fri, 29 Oct 2004, linux-os wrote:
> > with the following:
> >
> > leal 4(%esp),%esp
>
> Probably so because I'm pretty certain that the 'pop' (a memory
> access) is not going to be faster than a simple register operation.

Bzzt, wrong answer.

It's not "simple register operation". It's really about the fact that
modern CPU's are smarter - yet dumber - then you think. They do things
like speculate the value of %esp in order to avoid having to calculate it,
and it's entirely possible that "pop" is much faster, simply because I
guarantee you that a CPU will speculate %esp correctly across a "pop", but
the same is not necessarily true for "lea %esp".

Somebody should check what the Pentium M does. It might just notice that
"lea 4(%esp),%esp" is the same as "add 4 to esp", but it's entirely
possible that lea will confuse its stack engine logic and cause
stack-related address generation stalls..

Linus
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