[SimplifyCFG] Fix a cost modeling oversight in branch commoning

The cost modeling was not accounting for the fact we were duplicating the instruction once per predecessor.  With a default threshold of 1, this meant we were actually creating #pred copies.

Adding to the fun, there is *absolutely no* test coverage for this.  Simply bailing for more than one predecessor passes all checked in tests.

llvm-svn: 341001

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 355410ad045..f0ed075cce6 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2539,6 +2539,8 @@ static bool extractPredSuccWeights(BranchInst *PBI, BranchInst *BI,
 bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
   BasicBlock *BB = BI->getParent();
 
+  const unsigned PredCount = pred_size(BB);
+
   Instruction *Cond = nullptr;
   if (BI->isConditional())
     Cond = dyn_cast<Instruction>(BI->getCondition());
@@ -2588,7 +2590,8 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
   // too many instructions and these involved instructions can be executed
   // unconditionally. We denote all involved instructions except the condition
   // as "bonus instructions", and only allow this transformation when the
-  // number of the bonus instructions does not exceed a certain threshold.
+  // number of the bonus instructions we'll need to create when cloning into
+  // each predecessor does not exceed a certain threshold. 
   unsigned NumBonusInsts = 0;
   for (auto I = BB->begin(); Cond != &*I; ++I) {
     // Ignore dbg intrinsics.
@@ -2603,7 +2606,10 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
     // I is used in the same BB. Since BI uses Cond and doesn't have more slots
     // to use any other instruction, User must be an instruction between next(I)
     // and Cond.
-    ++NumBonusInsts;
+
+    // Account for the cost of duplicating this instruction into each
+    // predecessor. 
+    NumBonusInsts += PredCount;
     // Early exits once we reach the limit.
     if (NumBonusInsts > BonusInstThreshold)
       return false;