[PM/LoopUnswitch] Detect irreducible control flow within loops and skip unswitching non-trivial edges.

Summary:
This fixes the bug pointed out in review with non-trivial unswitching.

This also provides a basis that should make it pretty easy to finish
fleshing out a routine to scan an entire function body for irreducible
control flow, but this patch remains minimal for disabling loop
unswitch.

Reviewers: sanjoy, fedor.sergeev

Subscribers: mcrosier, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D45754

llvm-svn: 330357

1 files changed, 30 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll b/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll
index 51cdebe70e0..651ba19c427 100644
--- a/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch.ll
@@ -2350,3 +2350,33 @@ loop_exit:
 ; CHECK-NEXT:    %[[LCSSA:.*]] = phi i32 [ %[[V]], %loop_begin ]
 ; CHECK-NEXT:    ret i32 %[[LCSSA]]
 }
+
+; Negative test: we do not switch when the loop contains unstructured control
+; flows as it would significantly complicate the process as novel loops might
+; be formed, etc.
+define void @test_no_unswitch_unstructured_cfg(i1* %ptr, i1 %cond) {
+; CHECK-LABEL: @test_no_unswitch_unstructured_cfg(
+entry:
+  br label %loop_begin
+
+loop_begin:
+  br i1 %cond, label %loop_left, label %loop_right
+
+loop_left:
+  %v1 = load i1, i1* %ptr
+  br i1 %v1, label %loop_right, label %loop_merge
+
+loop_right:
+  %v2 = load i1, i1* %ptr
+  br i1 %v2, label %loop_left, label %loop_merge
+
+loop_merge:
+  %v3 = load i1, i1* %ptr
+  br i1 %v3, label %loop_latch, label %loop_exit
+
+loop_latch:
+  br label %loop_begin
+
+loop_exit:
+  ret void
+}