Switch lowering: Take branch weight into account when ordering for fall-through

Previously, the code would try to put a fall-through case last,
even if that meant moving a case with much higher branch weight
further down the chain.

Ordering by branch weight is most important, putting a fall-through
block last is secondary.

llvm-svn: 235942

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index d7edd2c1966..985a4dfd49d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -7678,11 +7678,12 @@ void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond,
       return a.Weight > b.Weight;
     });
 
-    // Rearrange the case blocks so that the last one falls through if possible.
-    // Start at the bottom as that's the case with the lowest weight.
-    // FIXME: Take branch probability into account.
+    // Rearrange the case blocks so that the last one falls through if possible
+    // without without changing the order of weights.
     for (CaseClusterIt I = W.LastCluster; I > W.FirstCluster; ) {
       --I;
+      if (I->Weight > W.LastCluster->Weight)
+        break;
       if (I->Kind == CC_Range && I->MBB == NextMBB) {
         std::swap(*I, *W.LastCluster);
         break;