Re: Semaphore assembly-code bug

[Jeff Garzik]

Linus Torvalds wrote:
> 	popl %eax
> 	popl %ecx
>
> should one cycle on a Pentium. I pretty much _guarantee_ that
>
> 	lea 4(%esp),%esp
> 	popl %ecx
>
> takes longer, since they have a data dependency on %esp that is hard to 
> break (the P4 trace-cache _may_ be able to break it, but the only CPU that 
> I think is likely to break it is actually the Transmeta CPU's, which did 
> that kind of thing by default and _will_ parallelise the two, and even 
> combine the stack offsetting into one single micro-op).


One of my favorite "optimizing for Pentium" docs is

	http://www.agner.org/assem/pentopt.pdf
		from
	http://www.agner.org/assem/

which is current through newer P4's AFAICS.

It notes on the P4 specifically that LEA is split into additions and 
shifts.  Not sure what it does on the P3, but I bet it generates more 
uops in addition to the data dependency.

	Jeff


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