linux-kernel - Bad Code generated from __{get,put}

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Message-ID: <b4198de60908061056m2b27402cjd25f94b6bbed0707@mail.gmail.com>
Date:	Thu, 6 Aug 2009 13:56:12 -0400
From:	Matt Turner <mattst88@...il.com>
To:	linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org
Subject: Bad Code generated from __{get,put}_unaligned functions

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

CC me on replies please.

Matt Turner
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