suggestion to improve optimization

suggestion to improve optimization

[Matthias Klose]

[this suggestions was submitted on the Debian BTS; please CC
 67206@bugs.debian.org on replies.]

Same behaviour in 20000720 CVS.

Re: suggestion to improve optimization

[Herbert Xu]

Here's another example where register pressure causes gcc to overlook some
simple optimisations.  For the attached program, gcc 2.95.2 generates

foo:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        movl    n, %eax
        subl    $28, %esp
        movl    %eax, -16(%ebp)
        movl    b, %eax
        movl    8(%ebp), %ebx
        movl    %eax, a
        jmp     .L141
        .p2align 2
.L143:
        movl    $.LC0+1, %edx
        cmpl    %edx, %edx
        movl    -16(%ebp), %edi
        je      .L214

...

.L141:
        movb    (%ebx), %al
        testb   %al, %al
        jne     .L143
        movl    -16(%ebp), %eax
        leal    -12(%ebp), %esp
        popl    %ebx
        popl    %esi
        popl    %edi
        popl    %ebp
        ret

Now n could have been moved to %edi in the first place to eliminate the
second reading from -16(%ebp).  This is what happens when some of the code
at the end of the loop is removed which reduces register pressure.
-- 
Debian GNU/Linux 2.2 is out! ( http://www.debian.org/ )
Email:  Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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