Re: Bad Code generated from __{get,put}_unaligned functions

[Michael Cree <mcree@xxxxxxxxxxxx>]

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.

--
To unsubscribe from this list: send the line "unsubscribe linux-alpha" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html