Date: Sun, 7 Jun 2015 16:54:53 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Erich Dollansky <erichsfreebsdlist@alogt.com> Cc: Hans Petter Selasky <hps@selasky.org>, "freebsd-hackers@freebsd.org" <freebsd-hackers@FreeBSD.org> Subject: Re: allow ffs & co. a binary search Message-ID: <20150607135453.GH2499@kib.kiev.ua> In-Reply-To: <20150607195245.62dc191f@B85M-HD3-0.alogt.com> References: <20150607081315.7c0f09fb@B85M-HD3-0.alogt.com> <5573EA5E.40806@selasky.org> <20150607195245.62dc191f@B85M-HD3-0.alogt.com>
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On Sun, Jun 07, 2015 at 07:52:45PM +0800, Erich Dollansky wrote: > What I saw is that all CPUs except ARM uses the software version [of ffs]. Without quantifiers, this statement is not true. i386 libc function ffs(3) uses bsfl instruction to do the job. Compilers know about ffs(3) and friends as well, so e.g. gcc 5.1.0 generates the following code for the given fragment: return (ffs(x) + 1); is translated to 0: 0f bc c7 bsf %edi,%eax 3: ba ff ff ff ff mov $0xffffffff,%edx 8: 0f 44 c2 cmove %edx,%eax b: 83 c0 02 add $0x2,%eax (arg in %edi, result in %eax). I wrote a patch for amd64 libc long time ago to convert ffs/fls etc to use of the bitstring instruction, but Bruce Evans argued that this would be excessive. Your patch is excessive for the similar reasons. My guess is that significantly clever compiler would recognize a pattern used by native ffs implementation and automatically use bitstring instructions. E.g., this already happens with popcnt and recent gcc/clang, I am just lazy to verify ffs.
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