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
