[EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> Say for a minute, that f(n) returned a constant and IPCP could replace
> the f(n) with a constant, say 5.
>
> For example:
>
> for (int i = 5; i--; )
>
>
> On x86, gcc would generate for your example:
>
> movl $4, %ebx
> L1:
> decl %ebx
> .... ; loop body
> cmpl $-1, %ebx
> jne L1
Funny. Last time I looked there was no cmpl in this case.
Indeed, 3.3.1 produces for
int main()
{
int i;
int s = 0;
for (i = 5; --i; )
s += i;
return s;
}
the following output (for -O3)
.file "1.c"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
pushl %ebp
movl %esp, %ebp
movl $4, %edx
pushl %eax
pushl %eax
andl $-16, %esp
xorl %eax, %eax
.p2align 4,,7
.L6:
addl %edx, %eax
decl %edx
jne .L6
leave
ret
.size main, .-main
.ident "GCC: (GNU) 3.3.1 (SuSE Linux)"
So, no 'cmpl', and the loop body is just an add, a dec, and a jump.
As tight as it can get (without unrolling)
What version are you using?
Aehm, wait. Did you use '--i' or 'i--'? You need the latter (both to be
correct and to achieve the savings)
> where mine would be:
> xorl %ebx, %ebx
> L1:
> incl %ebx
> .... ; loop body
> cmpl $5, %ebx
> jne L1
>
> 'Technically', my code would be faster because zeroing out ebx is
> cheaper than a move. :)
>
> Anyways, my point is that I avoid doing 'micro' optimizations because
> it does not always make the code faster, and it usually makes the code
> slightly harder to read.
I tend not to do micro optimizations myself as well - unless I think I
know what am doing.
Andre'
PS:
for (i = n; i--; )
is about 25% less typing than
for (i = 0; i != n; ++i)
and feels as 'idiomatic' as the latter after a while.
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