linux-kernel - Re: [PATCH] x86: merge the simple bitops and move them to bitops.h

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Message-Id: <1205532078.32744.1242485965@webmail.messagingengine.com>
Date:	Fri, 14 Mar 2008 23:01:18 +0100
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Alexander van Heukelum" <heukelum@...lshack.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: merge the simple bitops and move them to bitops.h

On Fri, 14 Mar 2008 22:42:05 +0100, "Andi Kleen" <andi@...stfloor.org>
said:
> On Fri, Mar 14, 2008 at 10:33:29PM +0100, Alexander van Heukelum wrote:
> > static inline int fls64(__u64 x)
> > {
> >         __u32 h = x >> 32;
> >         if (h)
> >                 return fls(h) + 32;
> >         return fls(x);
> > }
> > 
> > I just wanted to move the 64-bit version to that header, with some
> > ifdefs to select the right one.
> 
> That's still far more than the single 64bit instruction fls64 uses

I agree that it should end up using bsr. It would look like this in
the end, I guess. Might be familiar.

#if BITS_PER_LONG == 32
static inline int fls64(__u64 x)
{
        __u32 h = x >> 32;
        if (h)
                return __fls(h) + 33;
        return fls(x);
}
#else
static inline int fls64(__u64 x)
{
        if (x == 0)
                return 0;
        return __fls(x) + 1;
}
#endif

> > In fact I just found out that it only had an effect for 64 bit
> > machines. Still, setting it unconditionally feels wrong.
> 
> I don't think your feeling is correct.

This is the only reason that this define exists. With another
name it would be fine. HWEIGHT_USE_MULTIPLIER?

> > > > x86_64 has a mysterious inline function set_bit_string, which is
> > > > only used by pci-calgary_64.c and pci-gart_64.c. Not sure what to
> > > > do with it.
> > > 
> > > It's generic and could really live in linux/bitops.h
> > 
> > It could. But it is a trivial (slow?) implementation. Probably fine
> 
> It is this way because the callers in 95+% of all cases only
> set a single bit. For that case it is not slow.

And my feeling is that this is exactly the reason why this is
not a good version for a generic implementation in bitops.h. But
I don't care much.

Greetings,
    Alexander

> -Andi
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

-- 
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