Re: Semaphore assembly-code bug

[dean gaudet]

On Fri, 29 Oct 2004, Linus Torvalds wrote:

> 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..

it's worse than that in general -- lea typically goes through the AGU 
which has either less throughput or longer latency than the ALUs... 
depending on which x86en.  it's 4 cycles for a lea on p4, vs. 1 for a pop.  
it's 2 cycles for a lea on k8 vs. 1 for a pop.

use pop.

-dean
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