Re: [Sbcl-devel] interaction with untagged data

[Vitaly M. <v.m...@gm...>]

At Wed, 3 Dec 2008 16:31:39 -0500, Paul Khuong wrote:

Got further question, now regarding to values passing between VOPs (?)

> (defun test (x)
>    (declare (type (simple-array (unsigned-byte 8) 1) x)
>             (optimize speed (safety 0)))
>    (let ((length (sb-c::widenify (length x)))
>          (ix     (sb-c::widenify 0))
>          (sum    (sb-c::widenify 0)))
>      (declare (type (unsigned-byte 64) length ix sum))
>      (loop while (< ix length)
>            do (incf sum (aref x ix))
>               (incf ix (sb-c::widenify 1))
>            finally (return sum))))
> 
> ; 045174DF:       488B4AF9         MOV RCX, [RDX-7]           ; no-arg- 
> parsing entry point
> ;      4E3:       48C1F903         SAR RCX, 3
> ;      4E7:       488BD9           MOV RBX, RCX
> ;      4EA:       31C0             XOR EAX, EAX
> ;      4EC:       31C9             XOR ECX, ECX
> ;      4EE:       EB14             JMP L1

==========>
> ;      4F0: L0:   488BF0           MOV RSI, RAX
> ;      4F3:       480FB6743201     MOVZX RSI, BYTE PTR [RDX+RSI+1]
<==========

> ;      4F9:       4801F1           ADD RCX, RSI
> ;      4FC:       BE01000000       MOV ESI, 1
> ;      501:       4801F0           ADD RAX, RSI
> ;      504: L1:   4839D8           CMP RAX, RBX
> ;      507:       72E7             JB L0

Selected fragment in IR2 representation is:

    0: WORD-MOVE IX!30[RCX] => t34[RAX]
    1: WORD-MOVE t34[RAX] => t35[RAX]
    2: DATA-VECTOR-REF/SIMPLE-ARRAY-UNSIGNED-BYTE-8 X!11[RSI] t35[RAX]
                                                    {0}
                                                    => t36[RAX]

The second `move' was eliminated by conditional load (:load-if (not
(location= ...))), but why do we need to pass already loaded value in
another register? Is it some kind of transformation? I can only
imagine, it is done so, because proper register can be changed in VOP
by side effect (that's not true for data-vector-ref).

--
wbr, Vitaly