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Date: Sun, 09 Aug 2009 10:27:48 +1200
From: Michael Cree <mcree@...on.net.nz>
To: Falk Hueffner <falk@...ian.org>
CC: Matt Turner <mattst88@...il.com>, linux-kernel@...r.kernel.org,
linux-alpha@...r.kernel.org
Subject: Re: Bad Code generated from __{get,put}_unaligned functions
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.
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