Tweak the loop rotation logic to check whether the loop is naturally

laid out in a form with a fallthrough into the header and a fallthrough
out of the bottom. In that case, leave the loop alone because any
rotation will introduce unnecessary branches. If either side looks like
it will require an explicit branch, then the rotation won't add any, do
it to ensure the branch occurs outside of the loop (if possible) and
maximize the benefit of the fallthrough in the bottom.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154806 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 32 insertions, 2 deletions

diff --git a/test/CodeGen/X86/block-placement.ll b/test/CodeGen/X86/block-placement.ll
index f3c9727605..9359fb8c12 100644
--- a/test/CodeGen/X86/block-placement.ll
+++ b/test/CodeGen/X86/block-placement.ll
@@ -122,14 +122,14 @@ define i32 @test_loop_early_exits(i32 %i, i32* %a) {
 ; Check that we sink early exit blocks out of loop bodies.
 ; CHECK: test_loop_early_exits:
 ; CHECK: %entry
+; CHECK: %body1
 ; CHECK: %body2
 ; CHECK: %body3
 ; CHECK: %body4
-; CHECK: %body1
+; CHECK: %exit
 ; CHECK: %bail1
 ; CHECK: %bail2
 ; CHECK: %bail3
-; CHECK: %exit
 
 entry:
   br label %body1
@@ -199,6 +199,36 @@ exit:
   ret i32 %base
 }
 
+define i32 @test_no_loop_rotate(i32 %i, i32* %a) {
+; Check that we don't try to rotate a loop which is already laid out with
+; fallthrough opportunities into the top and out of the bottom.
+; CHECK: test_no_loop_rotate:
+; CHECK: %entry
+; CHECK: %body0
+; CHECK: %body1
+; CHECK: %exit
+
+entry:
+  br label %body0
+
+body0:
+  %iv = phi i32 [ 0, %entry ], [ %next, %body1 ]
+  %base = phi i32 [ 0, %entry ], [ %sum, %body1 ]
+  %arrayidx = getelementptr inbounds i32* %a, i32 %iv
+  %0 = load i32* %arrayidx
+  %sum = add nsw i32 %0, %base
+  %bailcond1 = icmp eq i32 %sum, 42
+  br i1 %bailcond1, label %exit, label %body1
+
+body1:
+  %next = add i32 %iv, 1
+  %exitcond = icmp eq i32 %next, %i
+  br i1 %exitcond, label %exit, label %body0
+
+exit:
+  ret i32 %base
+}
+
 define void @test_loop_rotate_reversed_blocks() {
 ; This test case (greatly reduced from an Olden bencmark) ensures that the loop
 ; rotate implementation doesn't assume that loops are laid out in a particular