From: Marc Glisse <marc.glisse@inria.fr>
To: Vidya Praveen <vidyapraveen@arm.com>
Cc: gcc@gcc.gnu.org, rguenther@suse.de, ook@ucw.cz
Subject: Re: [RFC] Vectorization of indexed elements
Date: Mon, 09 Sep 2013 18:02:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.10.1309091949090.3565@laptop-mg.saclay.inria.fr> (raw)
In-Reply-To: <20130909172533.GA25330@e103625-lin.cambridge.arm.com>
On Mon, 9 Sep 2013, Vidya Praveen wrote:
> Hello,
>
> This post details some thoughts on an enhancement to the vectorizer that
> could take advantage of the SIMD instructions that allows indexed element
> as an operand thus reducing the need for duplication and possibly improve
> reuse of previously loaded data.
>
> Appreciate your opinion on this.
>
> ---
>
> A phrase like this:
>
> for(i=0;i<4;i++)
> a[i] = b[i] <op> c[2];
>
> is usually vectorized as:
>
> va:V4SI = a[0:3]
> vb:V4SI = b[0:3]
> t = c[2]
> vc:V4SI = { t, t, t, t } // typically expanded as vec_duplicate at vec_init
> ...
> va:V4SI = vb:V4SI <op> vc:V4SI
>
> But this could be simplified further if a target has instructions that support
> indexed element as a parameter. For example an instruction like this:
>
> mul v0.4s, v1.4s, v2.4s[2]
>
> can perform multiplication of each element of v2.4s with the third element of
> v2.4s (specified as v2.4s[2]) and store the results in the corresponding
> elements of v0.4s.
>
> For this to happen, vectorizer needs to understand this idiom and treat the
> operand c[2] specially (and by taking in to consideration if the machine
> supports indexed element as an operand for <op> through a target hook or macro)
> and consider this as vectorizable statement without having to duplicate the
> elements explicitly.
>
> There are fews ways this could be represented at gimple:
>
> ...
> va:V4SI = vb:V4SI <op> VEC_DUPLICATE_EXPR (VEC_SELECT_EXPR (vc:V4SI 2))
> ...
>
> or by allowing a vectorizer treat an indexed element as a valid operand in a
> vectorizable statement:
Might as well allow any scalar then...
> ...
> va:V4SI = vb:V4SI <op> VEC_SELECT_EXPR (vc:V4SI 2)
> ...
>
> For the sake of explanation, the above two representations assumes that
> c[0:3] is loaded in vc for some other use and reused here. But when c[2] is the
> only use of 'c' then it may be safer to just load one element and use it like
> this:
>
> vc:V4SI[0] = c[2]
> va:V4SI = vb:V4SI <op> VEC_SELECT_EXPR (vc:V4SI 0)
>
> This could also mean that expressions involving scalar could be treated
> similarly. For example,
>
> for(i=0;i<4;i++)
> a[i] = b[i] <op> c
>
> could be vectorized as:
>
> vc:V4SI[0] = c
> va:V4SI = vb:V4SI <op> VEC_SELECT_EXPR (vc:V4SI 0)
>
> Such a change would also require new standard pattern names to be defined for
> each <op>.
>
> Alternatively, having something like this:
>
> ...
> vt:V4SI = VEC_DUPLICATE_EXPR (VEC_SELECT_EXPR (vc:V4SI 2))
> va:V4SI = vb:V4SI <op> vt:V4SI
> ...
>
> would remove the need to introduce several new standard pattern names but have
> just one to represent vec_duplicate(vec_select()) but ofcourse this will expect
> the target to have combiner patterns.
The cost estimation wouldn't be very good, but aren't combine patterns
enough for the whole thing? Don't you model your mul instruction as:
(mult:V4SI
(match_operand:V4SI)
(vec_duplicate:V4SI (vec_select:SI (match_operand:V4SI))))
anyway? Seems that combine should be able to handle it. What currently
happens that we fail to generate the right instruction?
In gimple, we already have BIT_FIELD_REF for vec_select and CONSTRUCTOR
for vec_duplicate, adding new nodes is always painful.
> This enhancement could possibly help further optimizing larger scenarios such
> as linear systems.
>
> Regards
> VP
--
Marc Glisse
next prev parent reply other threads:[~2013-09-09 18:02 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-09 17:25 Vidya Praveen
2013-09-09 18:02 ` Marc Glisse [this message]
2013-09-10 8:25 ` Richard Biener
2013-09-24 15:03 ` Vidya Praveen
2013-09-25 9:22 ` Richard Biener
2013-09-30 13:01 ` Vidya Praveen
2013-09-24 15:04 ` Vidya Praveen
2013-09-25 9:25 ` Richard Biener
2013-09-27 14:50 ` Vidya Praveen
2013-09-27 15:19 ` Vidya Praveen
2013-09-30 12:55 ` Vidya Praveen
2013-09-30 13:19 ` Richard Biener
2013-09-30 14:00 ` Vidya Praveen
2013-10-01 8:26 ` Richard Biener
2013-10-11 14:54 ` Vidya Praveen
2013-10-11 15:05 ` Jakub Jelinek
2013-12-04 17:07 ` Vidya Praveen
2013-10-14 8:05 ` Richard Biener
2013-12-04 16:10 ` Vidya Praveen
2013-12-06 11:48 ` Richard Biener
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