Re: [PATCH] asm-generic/bitops/fls64.h

Alexander van Heukelum
2008-05-13T13:24:23Z

On Tue, 13 May 2008 16:29:04 +0400, "Nickolay Vinogradov"
<nickolay@protei.ru> said:
> Alexander van Heukelum &#1087;&#1080;&#1096;&#1077;&#1090;:
>
> > Hi Nickolay,
> >
> > The change is ok, I guess, but the cast should be a no-op (fls
> > takes an int, which is always 32 bit in linux). What is the problem
> > you are seeing? Does fls64() return a wrong value in some cases? If
> > so, what cpu? Which values?
> >
> > Why would this be a bug on big endian systems only? There is no
> > pointer magic involved, so the compiler should take care of the
> > casts in a correct way.
> >
> > Maybe you see a compiler warning? Which compiler version?
> >
> > (also note that current (development) kernels now have separate
> > versions for 32-bit and 64-bit environments.)
>
> Because fls() is a macro for asm-arm:
>
> #define fls(x) \
> ( __builtin_constant_p(x) ? constant_fls(x) : \
> ({ int __r; asm("clz\t%0, %1" : "=r"(__r) : "r"(x) : "cc");
> 32-__r; }) )
>
> We can fix it right here:
>
> diff --git a/include/asm-arm/bitops.h b/include/asm-arm/bitops.h
> index 5c60bfc..ce3fb6f 100644
> --- a/include/asm-arm/bitops.h
> +++ b/include/asm-arm/bitops.h
> @@ -279,7 +279,7 @@ static inline int constant_fls(int x)
>
> #define fls(x) \
> ( __builtin_constant_p(x) ? constant_fls(x) : \
> - ({ int __r; asm("clz\t%0, %1" : "=r"(__r) : "r"(x) : "cc");
> 32-__r; }) )
> + ({ int __r; asm("clz\t%0, %1" : "=r"(__r) : "r"((__u32)x) :
> "cc"); 32-__r; }) )
> #define ffs(x) ({ unsigned long __t = (x); fls(__t & -__t); })
> #define __ffs(x) (ffs(x) - 1)
> #define ffz(x) __ffs( ~(x) )

Hmm, indeed.

Maybe: ({ int __r, __x = (x); asm("clz\t%0, %1" : "=r"(__r) : "r"(__x) :
"cc"); 32-__r; }) ?

This is a 32-bit machine, right? Doesn't the compiler complain about
feeding a long long into a 32-bit register?

Greetings,
Alexander
--
Alexander van Heukelum
heukelum@fastmail.fm

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