FoldBranchToCommonDest merges branches into a single branch with or/and of the condition. It has a heuristics for estimating when some of the dependencies are processed by out-of-order processors. This patch adds another rule to the heuristics that says that if the "BonusInstruction" that we speculatively execute is used by the condition of the second branch then it is okay to hoist it. This change exposes more opportunities for other passes to transform the code. It does not matter that much that we if-convert the code because the selectiondag builder splits or/and branches into multiple branches when profitable.

llvm-svn: 194524

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index d56bb329319..ff50b12cdb3 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2090,8 +2090,13 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
     // Ensure that any values used in the bonus instruction are also used
     // by the terminator of the predecessor.  This means that those values
     // must already have been resolved, so we won't be inhibiting the
-    // out-of-order core by speculating them earlier.
-    if (BonusInst) {
+    // out-of-order core by speculating them earlier. We also allow
+    // instructions that are used by the terminator's condition because it
+    // exposes more merging opportunities.
+    bool UsedByBranch = (BonusInst && BonusInst->hasOneUse() &&
+                         *BonusInst->use_begin() == Cond);
+
+    if (BonusInst && !UsedByBranch) {
       // Collect the values used by the bonus inst
       SmallPtrSet<Value*, 4> UsedValues;
       for (Instruction::op_iterator OI = BonusInst->op_begin(),