[PM/LoopUnswitch] When using the new SimpleLoopUnswitch pass, schedule

loop-cleanup passes at the beginning of the loop pass pipeline, and
re-enqueue loops after even trivial unswitching.

This will allow us to much more consistently avoid simplifying code
while doing trivial unswitching. I've also added a test case that
specifically shows effective iteration using this technique.

I've unconditionally updated the new PM as that is always using the
SimpleLoopUnswitch pass, and I've made the pipeline changes for the old
PM conditional on using this new unswitch pass. I added a bunch of
comments to the loop pass pipeline in the old PM to make it more clear
what is going on when reviewing.

Hopefully this will unblock doing *partial* unswitching instead of just
full unswitching.

Differential Revision: https://reviews.llvm.org/D47408

llvm-svn: 333493

1 files changed, 13 insertions, 5 deletions

diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index 8b9962af38c..9fdcbc5a234 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -390,13 +390,21 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   // Add the primary loop simplification pipeline.
   // FIXME: Currently this is split into two loop pass pipelines because we run
-  // some function passes in between them. These can and should be replaced by
-  // loop pass equivalenst but those aren't ready yet. Specifically,
-  // `SimplifyCFGPass` and `InstCombinePass` are used. We have
-  // `LoopSimplifyCFGPass` which isn't yet powerful enough, and the closest to
-  // the other we have is `LoopInstSimplify`.
+  // some function passes in between them. These can and should be removed
+  // and/or replaced by scheduling the loop pass equivalents in the correct
+  // positions. But those equivalent passes aren't powerful enough yet.
+  // Specifically, `SimplifyCFGPass` and `InstCombinePass` are currently still
+  // used. We have `LoopSimplifyCFGPass` which isn't yet powerful enough yet to
+  // fully replace `SimplifyCFGPass`, and the closest to the other we have is
+  // `LoopInstSimplify`.
   LoopPassManager LPM1(DebugLogging), LPM2(DebugLogging);
 
+  // Simplify the loop body. We do this initially to clean up after other loop
+  // passes run, either when iterating on a loop or on inner loops with
+  // implications on the outer loop.
+  LPM1.addPass(LoopInstSimplifyPass());
+  LPM1.addPass(LoopSimplifyCFGPass());
+
   // Rotate Loop - disable header duplication at -Oz
   LPM1.addPass(LoopRotatePass(Level != Oz));
   LPM1.addPass(LICMPass());