[MBP] Don't move bottom block before header if it can't reduce taken branches

If bottom of block BB has only one successor OldTop, in most cases it is profitable to move it before OldTop, except the following case:

-->OldTop<-
|    .    |
|    .    |
|    .    |
---Pred   |
     |    |
    BB-----

Move BB before OldTop can't reduce the number of taken branches, this patch detects this case and prevent the moving.

Differential Revision: https://reviews.llvm.org/D57067

llvm-svn: 352236

1 files changed, 36 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/X86/code_placement_no_header_change.ll b/llvm/test/CodeGen/X86/code_placement_no_header_change.ll
new file mode 100644
index 00000000000..0275606568c
--- /dev/null
+++ b/llvm/test/CodeGen/X86/code_placement_no_header_change.ll
@@ -0,0 +1,36 @@
+; RUN: llc -mtriple=i686-linux < %s | FileCheck %s
+
+
+define i32 @bar(i32 %count) {
+; Test checks that basic block backedge2 is not moved before header,
+; because it can't reduce taken branches.
+; Later backedge1 and backedge2 is rotated before loop header.
+; CHECK-LABEL: bar
+; CHECK: %.entry
+; CHECK: %.backedge1
+; CHECK: %.backedge2
+; CHECK: %.header
+; CHECK: %.exit
+.entry:
+  %c = shl nsw i32 %count, 2
+  br label %.header
+
+.header:
+  %val1 = call i32 @foo()
+  %cond1 = icmp sgt i32 %val1, 1
+  br i1 %cond1, label %.exit, label %.backedge1
+
+.backedge1:
+  %val2 = call i32 @foo()
+  %cond2 = icmp sgt i32 %val2, 1
+  br i1 %cond2, label %.header, label %.backedge2
+
+.backedge2:
+  %val3 = call i32 @foo()
+  br label %.header
+
+.exit:
+  ret i32 %c
+}
+
+declare i32 @foo()