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

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Date:	Tue, 13 May 2008 16:29:04 +0400
From:	Nickolay Vinogradov <nickolay@...tei.ru>
To:	Alexander van Heukelum <heukelum@...tmail.fm>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic/bitops/fls64.h

Alexander van Heukelum пишет:

> 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) )


-- 
Nickolay Vinogradov
Protei Research and Development Center
St.Petersburg, 194044, Russia
Tel.: +7 812 449 47 27
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