[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, 41 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch-iteration.ll b/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch-iteration.ll
new file mode 100644
index 00000000000..7ccd89a3606
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/trivial-unswitch-iteration.ll
@@ -0,0 +1,41 @@
+; RUN: opt -passes='loop(loop-instsimplify,simplify-cfg,unswitch),verify<loops>' -S < %s | FileCheck %s
+
+declare void @some_func() noreturn
+
+define i32 @test1(i32* %var, i1 %cond1, i1 %cond2) {
+; CHECK-LABEL: @test1(
+entry:
+  br label %loop_begin
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 %{{.*}}, label %entry.split, label %loop_exit.split
+;
+; CHECK:       entry.split:
+; CHECK-NEXT:    br i1 %{{.*}}, label %entry.split.split, label %loop_exit
+;
+; CHECK:       entry.split.split:
+; CHECK-NEXT:    br label %do_something
+
+loop_begin:
+  br i1 %cond1, label %continue, label %loop_exit ; first trivial condition
+
+continue:
+  %var_val = load i32, i32* %var
+  %var_cond = trunc i32 %var_val to i1
+  %maybe_cond = select i1 %cond1, i1 %cond2, i1 %var_cond
+  br i1 %maybe_cond, label %do_something, label %loop_exit ; second trivial condition
+
+do_something:
+  call void @some_func() noreturn nounwind
+  br label %loop_begin
+; CHECK:       do_something:
+; CHECK-NEXT:    call
+; CHECK-NEXT:    br label %do_something
+
+loop_exit:
+  ret i32 0
+; CHECK:       loop_exit:
+; CHECK-NEXT:    br label %loop_exit.split
+;
+; CHECK:       loop_exit.split:
+; CHECK-NEXT:    ret
+}
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