[EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
> Change your more recent test case to i--, then look at the assembly.
> There will be a cmpl $-1
Urm, yes. You are right. Problem between desk and chair.
Last try (promised...)
int foo(int n) int bar(int n) int baz(int n)
{ { {
int s = 0; int s = 0; int s = 0;
int i; int i; int i;
for (i = n; i--; ) for (i = 0; i != n; ++i) for (i = n; --i >= 0; )
s += i; s += i; s += i;
return s; return s; return s;
} } }
gives with -O3:
foo: bar: baz:
pushl %ebp pushl %ebp pushl %ebp
xorl %eax, %eax xorl %eax, %eax xorl %eax, %eax
movl %esp, %ebp movl %esp, %ebp movl %esp, %ebp
movl 8(%ebp), %edx xorl %edx, %edx movl 8(%ebp), %edx
decl %edx movl 8(%ebp), %ecx decl %edx
cmpl $-1, %edx cmpl %ecx, %eax js .L60
je .L8 je .L19 .p2align 4,,7
.p2align 4,,7 .p2align 4,,7 .L61:
.L9: .L20: addl %edx, %eax
addl %edx, %eax addl %edx, %eax decl %edx
decl %edx incl %edx jns .L61
cmpl $-1, %edx cmpl %ecx, %edx .L60:
jne .L9 jne .L20 popl %ebp
.L8: .L19: ret
popl %ebp popl %ebp
ret ret
So it looks as though the 'canonical' solution ('bar') uses one
more register (ecx) than my original micro-optimized version ('foo').
Of course, I am not sure anymore what the difference of the cmpl
against a register or a constant will translate to in parctice.
In any case, using 'baz' it seems indeed to be possible to skip the
'cmpl' entirely (_and_ save the register). The only unfortunate ascpect
here is that it won't work with unsigned counters as C++'s
std::container::size_types.
Andre'
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