Re: [PATCH 0/4] Add vector pair support to PowerPC attribute((vector_size(32)))

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Nov 20, 2023 at 9:56 AM Michael Meissner <meissner@linux.ibm.com> wrote:
>
> On Mon, Nov 20, 2023 at 08:24:35AM +0100, Richard Biener wrote:
> > I wouldn't expose the "fake" larger modes to the vectorizer but rather
> > adjust m_suggested_unroll_factor (which you already do to some extent).
>
> Thanks.  I figure I first need to fix the shuffle byes issue first and get a
> clean test run (with the flag enabled by default), before delving into the
> vectorization issues.
>
> But testing has shown that at least in the loop I was looking at, that using
> vector pair instructions (either through the built-ins I had previously posted
> or with these patches), that even if I turn off unrolling completely for the
> vector pair case, it still is faster than unrolling the loop 4 times for using
> vector types (or auto vectorization).  Note, of course the margin is much
> smaller in this case.

But unrolling 2 times or doubling the vector mode size results in exactly
the same - using a lager vectorization factor.

>
> vector double:           (a * b) + c, unroll 4         loop time: 0.55483
> vector double:           (a * b) + c, unroll default   loop time: 0.55638
> vector double:           (a * b) + c, unroll 0         loop time: 0.55686
> vector double:           (a * b) + c, unroll 2         loop time: 0.55772
>
> vector32, w/vector pair: (a * b) + c, unroll 4         loop time: 0.48257
> vector32, w/vector pair: (a * b) + c, unroll 2         loop time: 0.50782
> vector32, w/vector pair: (a * b) + c, unroll default   loop time: 0.50864
> vector32, w/vector pair: (a * b) + c, unroll 0         loop time: 0.52224
>
> Of course being micro-benchmarks, it doesn't mean that this translates to the
> behavior on actual code.

I guess the difference is from how RTL handles the larger modes vs.
more instructions with the smaller mode (if you don't immediately
expose the smaller modes during RTL expansion).

I'd compare assembly of vector double with unroll 2 and vector32 with unroll 0.

Richard.

>
>
> --
> Michael Meissner, IBM
> PO Box 98, Ayer, Massachusetts, USA, 01432
> email: meissner@linux.ibm.com