Re: i486 base architecture

["William M. Quarles" <quarlewm jmu edu>]

Arjan van de Ven wrote:

> On Tue, 2004-11-30 at 01:33 -0500, William M. Quarles wrote:
>
> > Arjan van de Ven wrote:
> >
> >
> > > On Sun, 2004-11-28 at 19:46 -0500, William M. Quarles wrote:
> > >
> > >
> > >
> > > > I would, but are there any free ways of doing benchmarks?  Not to 
> > > > mention I'm not really much of a programmer, so I don't know what 
> > > > oprofile/gprof are.
> > >
> > > for what it's worth... cmov isn't faster on newer (pM/pIV/amd64 level)
> > > CPUs than the open coded conditional jump anymore....
> > > so there no longer really is a reason to use cmov-only code.
> >
> > More terminology that I am not aware of... cmov? 
>
>
> cmov is a conditional move instruction on x86. Basically a C code
> construct like this
>
> if (some_condition == 5)
>    A = B;
>
> normally gets translated into (pseudo asm)
>
>   compare some_condition, 5
>   jump_if_not_equal label;
>   move B into A
> label:
>   ... the rest of the program
>
> the "jump_if_not_equal" instruction is a conditional instruction, which
> means that the cpu cannot look ahead and decide what the next
> instruction is, until the actual compare is finished. With the current
> deeply pipelined cpus that is sort of a problem (the solution is that
> the cpu makes a guess what it'll be based on past decisions for this
> line of code, and if wrong, it backtracks).
>
> Now with cmov, the code looks like
>
>   compare some_condition, 5
>   move_if_equal B into A
>   ... the rest of the program
>
> and in theory there is no question about which instructions will be
> executed when, so the "cost" of having an empty pipeline until the
> decision is known wouldn't be there. And that's mostly true for PPro/PII
> level CPUS.

Thanks, that cleared thins up a lot.

As someone who was still using a Pentium II a year ago, and is now only 
using a Pentium III computer (which would still have this benefit since 
the only difference between those and the PIII is an additional yet in 
this case irrelevant instruction set), I'd really like to see that 
performance difference.  Now were you saying that this cmov is part of 
i486 or i686?  I'd like to try rebuilding everything to see if it makes 
a difference.

Would I have to worry about any trademark problems?

Is there a script that Red Hat uses to automate the build process?  Does 
it use SRPMS or do the sources and specs have to be "preinstalled?"

> However, newer ones (both AMD and Intel) operate in such a way that the
> advantage of this no longer is an advantage, they need to know the
> result anyway in effect (and also make a guess about the "if" result)

Is the advantage still there, or is it just no longer a significant 
advantage?  Or is it a cost?  Unless it's a cost, I don't see what's 
wrong with putting in the advantage, unless it's a lot more work on your 
part.  Is it more work?

> > I know that I'm a 
> > novice about development, you don't have to further proove it to me.
>
> I absolutely don't mean it in that way.

I guess that I should have put a smiley face next to that one. :-)

----
Peace,
William