linux-kernel - Re: find_new_bit bloat from x86 tree...

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Message-ID: <20080428114104.GA28348@mailshack.com>
Date:	Mon, 28 Apr 2008 13:41:05 +0200
From:	Alexander van Heukelum <heukelum@...lshack.com>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	viro@...IV.linux.org.uk, heukelum@...tmail.fm
Subject: Re: find_new_bit bloat from x86 tree...

On Sat, Apr 26, 2008 at 10:07:26PM -0700, David Miller wrote:
> Ingo, what the heck is this?
> 
> commit 64970b68d2b3ed32b964b0b30b1b98518fde388e
> Author: Alexander van Heukelum <heukelum@...lshack.com>
> Date:   Tue Mar 11 16:17:19 2008 +0100
> 
>     x86, generic: optimize find_next_(zero_)bit for small constant-size bitmaps
> 
> Thanks for bloating up the inline expansion of this thing on every
> architecture that doesn't do __ffs() in a simple sequence of a few
> instructions like x86 does.
> 
> Now every call that matches your tests gets this turd inline:
> 
> static inline unsigned long __ffs(unsigned long word)
> {
> 	int num = 0;
> 
> #if BITS_PER_LONG == 64
> 	if ((word & 0xffffffff) == 0) {
> 		num += 32;
> 		word >>= 32;
> 	}
> #endif
> 	if ((word & 0xffff) == 0) {
> 		num += 16;
> 		word >>= 16;
> 	}
> 	if ((word & 0xff) == 0) {
> 		num += 8;
> 		word >>= 8;
> 	}
> 	if ((word & 0xf) == 0) {
> 		num += 4;
> 		word >>= 4;
> 	}
> 	if ((word & 0x3) == 0) {
> 		num += 2;
> 		word >>= 2;
> 	}
> 	if ((word & 0x1) == 0)
> 		num += 1;
> 	return num;
> }
> 
> as well as all of that address formation, bit shifting, and masking.
> 
> Please revert or make this conditional on something architectures can
> opt-in for.

Alternatively, implement __ffs out of line? Like:

static inline unsigned long __ffs(unsigned long word)
{
	return generic___ffs(word);
}

And a generic___ffs implemented in lib/ffs.c?

If __ffs is too big to be inlined it should not be inlined. That
is a generic problem and has nothing to do with this patch, IMHO.

Greetings,
	Alexander

> The version actually applied was posted only on linux-kernel, instead
> of also CC:'ing linux-arch as previous versions had been.  Nobody
> commented on this version other than you Ingo.
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