Re: [RFC][PATCH] Faster generic_fls

[Peter Osterlund]

Chuck Ebbert <[e-mail]> writes:

>   GCC is the strangest combination of utterly brilliant and brain-dead
> stupid that I've ever seen... I've seen it do tail merges that took
> my breath away, followed by this:
> 
>   mov <mem1>,eax
>   mov eax,<mem2>
>   mov <mem1>,eax        ; eax already contains mem1 you stupid compiler
>   ret

Not necessarily if mem2 == mem1 + 2. Consider this code:

        #include <string.h>
        int f(char* a, char* b)
        {
            int t;
            memcpy(&t, a, sizeof(int));
            memcpy(b, &t, sizeof(int));
            memcpy(&t, a, sizeof(int));
            return t;
        }

"gcc -O2 -Wall -S test.c -fomit-frame-pointer" correctly generates:

f:
        movl    4(%esp), %ecx
        movl    (%ecx), %eax
        movl    8(%esp), %edx
        movl    %eax, (%edx)
        movl    (%ecx), %eax
        ret

-- 
Peter Osterlund - [e-mail]
http://w1.894.telia.com/~u89404340