Re: A Bug in gcc or asm/string.h ?

[Andreas Kies]

On Monday 27 June 2005 21:43, Paolo Ornati wrote:
> PS: I've readded LKML to CC, since I think that this is a problem with the
> ASM template

Yes, you are right, it is not a compiler bug.
My apologies to the GCC team, in case anyone has read it.

[...]

> A little better workaround would be to add "memory" to clobbered registers
> in the asm template:
>
> static inline int strcmp(const char * cs,const char * ct)
> {
> int d0, d1;
> register int __res;
> __asm__ __volatile__(
>         "1:\tlodsb\n\t"
>         "scasb\n\t"
>         "jne 2f\n\t"
>         "testb %%al,%%al\n\t"
>         "jne 1b\n\t"
>         "xorl %%eax,%%eax\n\t"
>         "jmp 3f\n"
>         "2:\tsbbl %%eax,%%eax\n\t"
>         "orb $1,%%al\n"
>         "3:"
>
>         :"=a" (__res), "=&S" (d0), "=&D" (d1)
>         :
>                      :"1" (cs),"2" (ct)
>                      : "memory"); // <--- workaround
>
> return __res;
> }
>
>
> In this way GCC puts everything is cached in register back to memory when
> you call strcmp()... but you can argue that this isn't optimal.

Indeed the compiler has to assume that any memory location has changed.

> I don't know if there is a better way... basically you need to tell GCC to
> NOT cache these values.

There is one, it says that cs and ct address structures with 4 gigabyte size.
This is anyway not 64 bit clean.

static inline int strcmp(const char * cs,const char * ct)
{
        int d0, d1;
        register int __res;
        __asm__ __volatile__(
                        "1:\tlodsb\n\t"
                        "scasb\n\t"
                        "jne 2f\n\t"
                        "testb %%al,%%al\n\t"
                        "jne 1b\n\t"
                        "xorl %%eax,%%eax\n\t"
                        "jmp 3f\n"
                        "2:\tsbbl %%eax,%%eax\n\t"
                        "orb $1,%%al\n"
                        "3:"
                        :"=a" (__res), "=&S" (d0), "=&D" (d1)
                        :"1" (cs),"2" (ct),
                        "m" ( *(struct { char __x[0xfffffff]; } *)cs),
                        "m" ( *(struct { char __x[0xfffffff]; } *)ct));
        return __res;
}


Now, how do i formally submit this as a bug report ?

Andreas.
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