On Mon, Sep 09, 2013 at 07:02:52PM +0100, Marc Glisse wrote:
> 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...
Yes, I had given an example below.
>
> > ...
> > 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?
At vec_init, I can recognize an idiom in order to generate vec_duplicate but
I can't really insist on the single lane load.. something like:
vc:V4SI[0] = c
vt:V4SI = vec_duplicate:V4SI (vec_select:SI vc:V4SI 0)
va:V4SI = vb:V4SI <op> vt:V4SI
Or is there any other way to do this?
Cheers
VP
>
> 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
>
