Re: nasm over gas?

[Ihar 'Philips' Filipau]

Eric W. Biederman wrote:
> insecure <insecure@xxxxxxxxxx> writes:
> >        movl    $0, 20(%esp)
> >        movl    $1000000, %edi      <----
> >        movl    $1000000, 16(%esp)  <----
> >        movl    $0, 12(%esp)
> >
> > No sane human will do that.
> > main:
> >        movl    $1000000, %edi
> >        movl    %edi, 16(%esp)	<-- save 4 bytes
> >        movl    %ebp, 12(%esp)  <-- save 4 bytes
> >        movl    $.LC27, 8(%esp)
> >
> > And this is only from a cursory examination.
>
> Actually it is no as simple as that.  With the instruction that uses
> %edi following immediately after the instruction that populates it you cannot
> execute those two instructions in parallel.  So the code may be slower.  The
> exact rules depend on the architecture of the cpu.

  It will depend on arch CPU only in case if you have unlimited i$ size.
  Servers with 8MB of cache - yes it is faster.
  Celeron with 128k of cache - +4bytes == higher probability of i$ miss 
== lower performance.

> > What gives you an impression that anyone is going to rewrite linux in asm?
> > I _only_ saying that compiler-generated asm is not 'good'. It's mediocre.
> > Nothing more. I am not asm zealot.
>
>
> I think I would agree with that statement most compiler-generated assembly
> code is mediocre in general.  At the same time I would add most human
> generated assembly is poor, and a pain to maintain.
>
> If you concentrate on those handful of places where you need to
> optimize that is reasonable.  Beyond that there simply are not the
> developer resources to do good assembly.  And things like algorithmic
> transformations in assembly are an absolute nightmare.  Where they are
> quite simple in C.
>
> And if the average generated code quality bothers you enough with C
> the compiler can be fixed, or another compiler can be written that
> does a better job, and the benefit applies to a lot more code.

  e.g. C-- project: something like C, where you can operate with 
registers just like another variables. Under DOS was producing .com 
files witout any overhead: program with only 'int main() { return 0; }' 
was optimized to one byte 'ret' ;-) But sure it was not complete C 
implementation.

  Sure I would prefere to have nasm used for kernel asm parts - but 
obviously gas already became standard.

P.S. Add having good macroprocessor for assembler is a must: CPP is 
terribly stupid by design. I beleive gas has no preprocessor comparable 
to masm's one? I bet they are using C's cpp. This is degradation: macros 
is the major feature of any translator I was working with. They can save 
you a lot of time and make code much more cleaner/readable/mantainable. 
CPP is just too dumb for asm...
Good old times, when people were responsible to _every_ byte of their 
programmes... Yeh... Memory/programmers are cheap nowadays...

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