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Re: [PATCH] Introducing SAD (Sum of Absolute Differences) operation to GCC vectorizer.


Hi James

Sorry for the late reply.


On Fri, Nov 8, 2013 at 2:55 AM, James Greenhalgh
<james.greenhalgh@arm.com> wrote:
>> On Tue, Nov 5, 2013 at 9:58 AM, Cong Hou <congh@google.com> wrote:
>> > Thank you for your detailed explanation.
>> >
>> > Once GCC detects a reduction operation, it will automatically
>> > accumulate all elements in the vector after the loop. In the loop the
>> > reduction variable is always a vector whose elements are reductions of
>> > corresponding values from other vectors. Therefore in your case the
>> > only instruction you need to generate is:
>> >
>> >     VABAL   ops[3], ops[1], ops[2]
>> >
>> > It is OK if you accumulate the elements into one in the vector inside
>> > of the loop (if one instruction can do this), but you have to make
>> > sure other elements in the vector should remain zero so that the final
>> > result is correct.
>> >
>> > If you are confused about the documentation, check the one for
>> > udot_prod (just above usad in md.texi), as it has very similar
>> > behavior as usad. Actually I copied the text from there and did some
>> > changes. As those two instruction patterns are both for vectorization,
>> > their behavior should not be difficult to explain.
>> >
>> > If you have more questions or think that the documentation is still
>> > improper please let me know.
>
> Hi Cong,
>
> Thanks for your reply.
>
> I've looked at Dorit's original patch adding WIDEN_SUM_EXPR and
> DOT_PROD_EXPR and I see that the same ambiguity exists for
> DOT_PROD_EXPR. Can you please add a note in your tree.def
> that SAD_EXPR, like DOT_PROD_EXPR can be expanded as either:
>
>   tmp = WIDEN_MINUS_EXPR (arg1, arg2)
>   tmp2 = ABS_EXPR (tmp)
>   arg3 = PLUS_EXPR (tmp2, arg3)
>
> or:
>
>   tmp = WIDEN_MINUS_EXPR (arg1, arg2)
>   tmp2 = ABS_EXPR (tmp)
>   arg3 = WIDEN_SUM_EXPR (tmp2, arg3)
>
> Where WIDEN_MINUS_EXPR is a signed MINUS_EXPR, returning a
> a value of the same (widened) type as arg3.
>


I have added it, although we currently don't have WIDEN_MINUS_EXPR (I
mentioned it in tree.def).


> Also, while looking for the history of DOT_PROD_EXPR I spotted this
> patch:
>
>   [autovect] [patch] detect mult-hi and sad patterns
>   http://gcc.gnu.org/ml/gcc-patches/2005-10/msg01394.html
>
> I wonder what the reason was for that patch to be dropped?
>

It has been 8 years.. I have no idea why this patch is not accepted
finally. There is even no reply in that thread. But I believe the SAD
pattern is very important to be recognized. ARM also provides
instructions for it.


Thank you for your comment again!


thanks,
Cong



> Thanks,
> James
>

Attachment: patch-sad.txt
Description: Text document


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