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SubjectRe: nasm over gas?
Date
On Sunday 07 September 2003 21:49, Eric W. Biederman wrote:
> insecure <insecure@mail.od.ua> writes:
> > On Friday 05 September 2003 15:59, Michael Frank wrote:
> > > Just got another reply to this thread which helps to explain what I
> > > meant by "better coders in 98+% of applications"
> > >
> > > On Friday 05 September 2003 19:42, Jörn Engel wrote:
> > > > How big is the .text of the asm and c variant? If the text of yours
> > > > is much bigger, you just traded 2fish performance for general
> > > > performance. Everything else will suffer from cache misses. Forget
> > > > your microbenchmark, your variant will make the machine slower.
> >
> > A random example form one small unrelated program (gcc 3.2):
> >
> > main:
> > pushl %ebp
> > pushl %edi
> > pushl %esi
> > pushl %ebx
> > subl $32, %esp
> > xorl %ebp, %ebp
> > cmpl $1, 52(%esp)
> > movl $0, 20(%esp)
> > movl $1000000, %edi <----
> > movl $1000000, 16(%esp) <----
> > movl $0, 12(%esp)
> > movl $.LC27, 8(%esp)
> > je .L274
> > movl $1, %esi
> > cmpl 52(%esp), %esi
> > jge .L272
> >
> > No sane human will do that.
> >
> >
> > main:
> > pushl %ebp
> > pushl %edi
> > pushl %esi
> > pushl %ebx
> > subl $32, %esp
> > xorl %ebp, %ebp
> > cmpl $1, 52(%esp)
> > movl $0, 20(%esp)
> > movl $1000000, %edi
> > movl %edi, 16(%esp) <-- save 4 bytes
> > movl %ebp, 12(%esp) <-- save 4 bytes
> > movl $.LC27, 8(%esp)
> > je .L274
> > movl $1, %esi
> > cmpl 52(%esp), %esi
> > jge .L272
> >
> > 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.

That instruction is in main() initialization sequence. I.e.
it is executed once per program invocation.
Summary: we lost 8 bytes for no gain. There's not even a speed gain -
we lost 8 bytes of _icache_, that will bite us somewhere else.

> > 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.

I had an impression people think gcc generates code which
is 'mostly good' even compared to handwritted code.
That is not true (yet).
--
vda
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