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Date: Wed, 30 Apr 2003 18:07:13 -0700 (PDT)
From: Linus Torvalds <torvalds@transmeta.com>
To: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>
cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, <dphillips@sistina.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH] Faster generic_fls
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On 1 May 2003, Falk Hueffner wrote:
> 
> There appears to be hardware support for fls, too. This is what gcc
> generates for

Yeah, except if you want best code generation you should probably use

	static inline int fls(int x)
	{
		int bit;
		/* Only well-defined for non-zero */
		asm("bsrl %1,%0":"=r" (bit):"rm" (x));
		return x ? bit : 32;
	}

which should generate (assuming you give "-march=xxxx" to tell gcc to use 
cmov):

		movl    $32, %eax
		bsrl %edx,%ecx
		testl   %edx, %edx
		cmovne  %ecx, %eax

which looks pretty optimal.

		Linus