Loop fusion in llvm polly

[Raja Patra]

Hi all,

We are trying for loop fusion on nested loops. Firstly trying with -mllvm -polly-opt-fusion=max and simple -mllvm -polly flag, is this the way to do loop fusion in polly or some other flags or pragmas are available for loop transformation from polly side. Please guide.

Thanks,

Raja

[Michael Kruse]

The -polly-opt-fusion option maps to ISL's serialize_sccs option,
which only relates to fusion on the outermost level. Unfortunately
there is no single fuse option. Also checkout the
-polly-opt-maximize-bands, -polly-opt-outer-coincidence and ISL's
schedule_whole_component (-polly-isl-arg=--schedule-whole-component=0)
options.

Michael

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[Raja Patra]

Hi Michael,

Thanks for your reply. Does using clang pragmas for loop transformation a better option here, as the nested loops have vector intrinsics and non affine expressions ? Please guide me.

Thanks,

Siddharth

[Michael Kruse]

Depends. Vector intrinsics should not be a problem. Non-affine
expression can be enabled using -polly-allow-nonaffine. However, there
is no guarantee that ISL's rescheduling heuristic finds the
optimization you were looking for.
Using the Polly implementation of loop transformation pragmas, would
need to make Polly recognize the SCoP anyway, using mechanisms such as
-polly-allow-nonaffine, it only really replaces the rescheduling
heuristic.

Michael

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[Raja Patra]

Hi Michael,

Does using the clang pragmas discussed here - https://www.youtube.com/watch?v=RhwjZS9PSI8 would be of any use for loop transformations ?

Thanks

[Michael Kruse]

The talk refers to the implementation here:
https://reviews.llvm.org/D69088. Unfortunately there was no interest
by reviewers. Instead, I continued development of a fork here:
https://github.com/sollve/llvm-project/tree/pragma-clang-loop which
also includes the Polly changes and more loop transformations,
including loop fusion:
https://github.com/SOLLVE/llvm-project/blob/pragma-clang-loop/clang/test/TransformPragma/pragma-id-fuse.c

Michael

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