New Loop Distribution pass


New Loop Distribution pass

This implements the initial version as was proposed earlier this year
Since then Loop Access Analysis was split out from the Loop Vectorizer
and was made into a separate analysis pass. Loop Distribution becomes
the second user of this analysis.

The pass is off by default and can be enabled
with -enable-loop-distribution. There is currently no notion of
profitability; if there is a loop with dependence cycles, the pass will
try to split them off from other memory operations into a separate loop.

I decided to remove the control-dependence calculation from this first
version. This and the issues with the PDT are actively discussed so it
probably makes sense to treat it separately. Right now I just mark all
terminator instruction required which keeps identical CFGs for each
distributed loop. This seems to be working pretty well for 456.hmmer
where even though there is an empty if-then block in the distributed
loop initially, it gets completely removed.

The pass keeps DominatorTree and LoopInfo updated. I've tested this
with -loop-distribute-verify with the testsuite where we distribute ~90
loops. SimplifyLoop is violated in some cases and I have a FIXME
covering this.

Reviewers: hfinkel, nadav, aschwaighofer

Reviewed By: aschwaighofer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8831


anemetMay 14 2015, 5:05 AM
Differential Revision
D8831: New Loop Distribution pass
rL237357: Fixed some typos and broken links in source level debugging docs.