On Friday 09 February 2007 10:15, David Miller wrote:
> From: Eric Dumazet <[EMAIL PROTECTED]>
> Date: Fri, 9 Feb 2007 10:06:24 +0100
>
> > Yes, but a decent C compiler for such targets should not use a
> > multiply instruction to perform a (idx * 12) operation... :)
>
> Good point.
>
> Actually, I could never get GCC to avoid a divide on sparc64 for
> certain kinds of pointer arithmetic when the elements were not
> a power of two. It probably has something to do with signedness.
>
> I think I narrowed is down to the fact that you can't legally replace
> a signed divide with shift/add/subtract. But I could be remembering
> things wrong.
Thats strange, because pointer arithmetic is unsigned...
I dont know when gcc started to use reciprocal division, maybe your gcc was
very old ?
$ cat div.c
struct s1 { int pad[3]; };
unsigned long diffptr(struct s1 *a, struct s1 *b)
{
return a - b;
}
If compiled on i386 , gcc-4.1.1 :
$ gcc -O2 -fomit-frame-pointer -S div.c
diffptr:
movl 4(%esp), %eax
subl 8(%esp), %eax
sarl $2, %eax
imull $-1431655765, %eax, %eax
ret
If compiled on x86_64 , gcc-4.1.1:
diffptr:
subq %rsi, %rdi
movabsq $-6148914691236517205, %rax
sarq $2, %rdi
imulq %rax, %rdi
movq %rdi, %rax
ret
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