Re: [PATCH 6/6] Extend SLP permutation optimisations

[Richard Sandiford <richard.sandiford@arm.com>]

Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On 8/25/2022 7:07 AM, Richard Sandiford via Gcc-patches wrote:
>> Currently SLP tries to force permute operations "down" the graph
>> from loads in the hope of reducing the total number of permutations
>> needed or (in the best case) removing the need for the permutations
>> entirely.  This patch tries to extend it as follows:
>>
>> - Allow loads to take a different permutation from the one they
>>    started with, rather than choosing between "original permutation"
>>    and "no permutation".
>>
>> - Allow changes in both directions, if the target supports the
>>    reverse permutation.
>>
>> - Treat the placement of permutations as a two-way dataflow problem:
>>    after propagating information from leaves to roots (as now), propagate
>>    information back up the graph.
>>
>> - Take execution frequency into account when optimising for speed,
>>    so that (for example) permutations inside loops have a higher
>>    cost than permutations outside loops.
>>
>> - Try to reduce the total number of permutations when optimising for
>>    size, even if that increases the number of permutations on a given
>>    execution path.
>>
>> See the big block comment above vect_optimize_slp_pass for
>> a detailed description.
>>
>> The original motivation for doing this was to add a framework that would
>> allow other layout differences in future.  The two main ones are:
>>
>> - Make it easier to represent predicated operations, including
>>    predicated operations with gaps.  E.g.:
>>
>>       a[0] += 1;
>>       a[1] += 1;
>>       a[3] += 1;
>>
>>    could be a single load/add/store for SVE.  We could handle this
>>    by representing a layout such as { 0, 1, _, 2 } or { 0, 1, _, 3 }
>>    (depending on what's being counted).  We might need to move
>>    elements between lanes at various points, like with permutes.
>>
>>    (This would first mean adding support for stores with gaps.)
>>
>> - Make it easier to switch between an even/odd and unpermuted layout
>>    when switching between wide and narrow elements.  E.g. if a widening
>>    operation produces an even vector and an odd vector, we should try
>>    to keep operations on the wide elements in that order rather than
>>    force them to be permuted back "in order".
> Very cool.  Richi and I discussed this a bit a year or so ago -- 
> basically noting that bi-directional movement is really the way to go 
> and that the worst thing to do is push a permute down into the *middle* 
> of a computation chain since that will tend to break FMA generation.  
> Moving to the loads or stores or to another permute point ought to be 
> the goal.

Hmm, I hadn't thought specifically about the case of permutes
ending up in the middle of a fusable operation.  The series doesn't
address that directly.  If we have:

  permute(a) * permute(b) + c

then the tendency will still be to convert that into:

  permute(a * b) + c

Damn.  Another case to think about ;-)

I've pushed the series for now though (thanks to Richi for the reviews).

Richard