On Wed, May 31, 2006 at 05:39:17PM +0000, Angus Leeming wrote:
> Lars Gullik Bjønnes <[EMAIL PROTECTED]> writes:
> > | > Apart from it can be shortened by two lines:
> > | > for (size_t i = cur.depth(); i--; )
> > |
> > | Now you're just being evil, André At least be explicit about the exit
> > | condition. I *think* that your code is equivalent to:
> > |
> > | for (size_t i = cur.depth(); i > 0; i--)
> >
> > but then we want '--i' as well.
>
> As a matter of style, yes, probably. However, for PODs there's no efficiency
> penalty to i-- vis-à-vis --i.
To add a bit more verbosity:
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 gcc -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 the optimal version ('baz').
The only unfortunate ascpect here is that it won't work with unsigned
counters as C++'s.
Amdre'
