[llvm-dev] Enabling Loop Distribution Pass as default in the pipeline of new pass manager

[Michael Kruse via llvm-dev]

The LoopDistribute pass doesn't do anything unless it sees
llvm.loop.distribute.enable (`#pragma clang loop distribute(enable)`)
because it does not have a profitability heuristic. It cannot say
whether loop distribution is good for performance or not. What makes
it improve hmmer is that the distributed loops can be vectoried.
However, LoopDistribute is located before the vectorizer and cannot
say in advance whether a distributed loop will be vectorized or not.
If not, then it potentially only increased loop overhead.

To make -enable-loop-distribute on by default would mean that we could
consider loop distribution to be usually beneficial without causing
major regressions. We need a lot more data to support that conclusion.

Alternatively, we could consider loop-distribution a canonicalization.
A later LoopFuse would do the profitability heuristic to re-fuse loops
again if loop distribution did not gain anything.

Michael