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

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Message-Id: <20080513091111.bd9437c1.akpm@linux-foundation.org>
Date:	Tue, 13 May 2008 09:11:11 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andreas Schwab <schwab@...e.de>
Cc:	Russell King <rmk+lkml@....linux.org.uk>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Nickolay Vinogradov <nickolay@...tei.ru>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] asm-generic/bitops/fls64.h

On Tue, 13 May 2008 16:35:25 +0200 Andreas Schwab <schwab@...e.de> wrote:

> 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.
> 

Yes, and that's what other architectures do.

It's not pretty, but I do think the arm implementation should zap the
upper 32 bits as other architectures do, and as one would expect from
the usual C type conversion rules.

Converting it to a C implementation would be a suitable means...
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