From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Cree Subject: Re: Bad Code generated from __{get,put}_unaligned functions Date: Sun, 09 Aug 2009 10:27:48 +1200 Message-ID: <4A7DFBE4.6060101@orcon.net.nz> References: <20090806183941.GA6854@minet.uni-jena.de> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20090806183941.GA6854@minet.uni-jena.de> Sender: linux-alpha-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Falk Hueffner Cc: Matt Turner , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org Falk Hueffner wrote: > On Thu, Aug 06, 2009 at 01:56:12PM -0400, Matt Turner wrote: > >> I was researching different ways of writing unaligned load/store >> macros, so I checked how the kernel did it -- the most general way >> possible. See include/linux/unaligned.h. As such, very bad code is >> generated, for example on alpha with BWX, we can implement all these >> functions with a single instruction, whereas we get stuff like this >> generated from the generic functions. >> >> __get_unaligned_le32: >> .frame $30,0,$26,0 >> .prologue 0 >> ldbu $0,1($16) >> ldbu $1,2($16) >> ldbu $2,3($16) >> ldbu $3,0($16) >> sll $1,16,$1 >> sll $0,8,$0 >> bis $0,$1,$0 >> sll $2,24,$2 >> bis $0,$3,$0 >> bis $0,$2,$0 >> addl $31,$0,$0 >> ret $31,($26),1 >> >> 4 load byte instructions, shift, shift, or, shift, or, or, sign extend >> -- or ldl_u instruction. The code is more than doubly-bad for le64. >> >> Do we use the generic functions for a reason I don't see? It appears >> that it would be easy enough to add architecture-specific unaligned >> get/put functions in arch/*/include/asm/unaligned.h >> > > There should be no need for architecture specific code for Alpha. GCC > can generate the optimal code sequence for reads from unaligned struct > members as in linux/unaligned/packed_struct.h, and this code should be > used. So you should try to find out why it isn't. > Indeed, the above code looks like what one would expect from the routines in linux/unaligned/le_byteshift.h rather than packed_struct.h Michael.