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

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Date:	Tue, 13 May 2008 16:35:25 +0200
From:	Andreas Schwab <schwab@...e.de>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Alexander van Heukelum <heukelum@...tmail.fm>,
	Nickolay Vinogradov <nickolay@...tei.ru>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] asm-generic/bitops/fls64.h

Russell King <rmk+lkml@....linux.org.uk> writes:

> On Tue, May 13, 2008 at 03:24:13PM +0200, Alexander van Heukelum wrote:
>> On Tue, 13 May 2008 16:29:04 +0400, "Nickolay Vinogradov"
>> <nickolay@...tei.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:
>
> No.  "fls" is for finding the last set bit in an _int_.  It is not
> supposed to have random crap passed to it, such as types longer than
> sizeof(int).

If you write fls as an (inline) function then the argument is implicitly
converted.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@...e.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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