Mailinglist Archive: opensuse-programming (27 mails)

[Per Jessen <per@xxxxxxxxxxxx>]

Thomas Hertweck wrote:

> On 13/01/10 07:37, Per Jessen wrote:
> > [...]
> > No myth at all.  Perhaps you haven't met many very good assembler
> > programmers?  It's not really a topic I have a desire to debate (I
> > know from experience that I am right).
>
> I am (at least with one leg) in the HPC software development business
> and I say your point of view is somewhat biased. 

Yeah, that I can't deny.

> Nowadays there's hardly any reason for writing assembler code
> directly. There are, of course, a few occasions where you are right
> and hand-optimized assembler code will outperform code generated by
> any compiler. But that's not a statement you can apply globally. 

I think it is - what you may have is a situation where the difference is
not measurable or not significant, or not worth it when weighed up
against the other factors (maintainability, productivity).

> Of course if you write bad C/Fortran/C++ or whatever code and you hope
> for the compiler to come up with some magic to create an ultra-fast
> assembler out of it, then you're wrong. 

Certainly, that would not be a fair comparison. 

> I would like to see the C code you used as baseline reference and
> information about the compiler and access to your hand-optimized code
> to actually have a look at it myself. Without showing evidence, you
> just make empty statements. 

Well, it's up to people who believe otherwise to prove me wrong :-)

All I did were some plain wall-time measurements, I didn't use the
C-code performance as a baseline, I just took a quick look at the
generated code, and knew I could do better.

AFAIC, the main argument is that if the compiler uses 100 instructions
to implement an algorithm, and I use 90, that's 10 instructions saved
(all instructions being equal).  I've been writing assembler code for
more than 20 years, and I'm quite certain I can do better. 


/Per Jessen, Zürich

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