Switch lowering: use profile info to build weight-balanced binary search trees

This will cause hot nodes to appear closer to the root.

The literature says building the tree like this makes it a near-optimal (in
terms of search time given key frequencies) binary search tree. In LLVM's case,
we can do up to 3 comparisons in each leaf node, so it might be better to opt
for lower tree height in some cases; that's something to look into in the
future.

Differential Revision: http://reviews.llvm.org/D9318

llvm-svn: 236192

1 files changed, 39 insertions, 1 deletions

diff --git a/llvm/test/CodeGen/Generic/MachineBranchProb.ll b/llvm/test/CodeGen/Generic/MachineBranchProb.ll
index 83277c98989..f030775d535 100644
--- a/llvm/test/CodeGen/Generic/MachineBranchProb.ll
+++ b/llvm/test/CodeGen/Generic/MachineBranchProb.ll
@@ -5,7 +5,7 @@
 
 ; Make sure we have the correct weight attached to each successor.
 define i32 @test2(i32 %x) nounwind uwtable readnone ssp {
-; CHECK: Machine code for function test2:
+; CHECK-LABEL: Machine code for function test2:
 entry:
   %conv = sext i32 %x to i64
   switch i64 %conv, label %return [
@@ -33,3 +33,41 @@ return:
 }
 
 !0 = !{!"branch_weights", i32 7, i32 6, i32 4, i32 4, i32 64}
+
+
+declare void @g(i32)
+define void @left_leaning_weight_balanced_tree(i32 %x) {
+entry:
+  switch i32 %x, label %return [
+    i32 0,  label %bb0
+    i32 10, label %bb1
+    i32 20, label %bb2
+    i32 30, label %bb3
+    i32 40, label %bb4
+    i32 50, label %bb5
+  ], !prof !1
+bb0: tail call void @g(i32 0) br label %return
+bb1: tail call void @g(i32 1) br label %return
+bb2: tail call void @g(i32 2) br label %return
+bb3: tail call void @g(i32 3) br label %return
+bb4: tail call void @g(i32 4) br label %return
+bb5: tail call void @g(i32 5) br label %return
+return: ret void
+
+; Check that we set branch weights on the pivot cmp instruction correctly.
+; Cases {0,10,20,30} go on the left with weight 13; cases {40,50} go on the
+; right with weight 20.
+;
+; CHECK-LABEL: Machine code for function left_leaning_weight_balanced_tree:
+; CHECK: BB#0: derived from LLVM BB %entry
+; CHECK-NOT: Successors
+; CHECK: Successors according to CFG: BB#8(13) BB#9(20)
+}
+
+!1 = !{!"branch_weights",
+  ; Default:
+  i32 1,
+  ; Case 0, 10, 20:
+  i32 10, i32 1, i32 1,
+  ; Case 30, 40, 50:
+  i32 1, i32 10, i32 10}