[SimplifyCFG] Teach merge conditional stores to handle cases where the PostBB has more than 2 predecessors by inserting a new block for the store.

Summary:
Currently merge conditional stores can't handle cases where PostBB (the block we need to move the store to) has more than 2 predecessors.

This patch removes that restriction by creating a new block with only the 2 predecessors we care about and an unconditional branch to the original block. This provides a place to put the store.

Reviewers: efriedma, jmolloy, ABataev

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39760

llvm-svn: 329142

1 files changed, 16 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 5dc9d2c3b71..389ec419b5c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2982,6 +2982,21 @@ static bool mergeConditionalStoreToAddress(BasicBlock *PTB, BasicBlock *PFB,
     if (&*I != PStore && I->mayReadOrWriteMemory())
       return false;
 
+  // If PostBB has more than two predecessors, we need to split it so we can
+  // sink the store.
+  if (std::next(pred_begin(PostBB), 2) != pred_end(PostBB)) {
+    // We know that QFB's only successor is PostBB. And QFB has a single
+    // predecessor. If QTB exists, then its only successor is also PostBB.
+    // If QTB does not exist, then QFB's only predecessor has a conditional
+    // branch to QFB and PostBB.
+    BasicBlock *TruePred = QTB ? QTB : QFB->getSinglePredecessor();
+    BasicBlock *NewBB = SplitBlockPredecessors(PostBB, { QFB, TruePred},
+                                               "condstore.split");
+    if (!NewBB)
+      return false;
+    PostBB = NewBB;
+  }
+
   // OK, we're going to sink the stores to PostBB. The store has to be
   // conditional though, so first create the predicate.
   Value *PCond = cast<BranchInst>(PFB->getSinglePredecessor()->getTerminator())
@@ -3117,7 +3132,7 @@ static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI,
   if ((PTB && !HasOnePredAndOneSucc(PTB, PBI->getParent(), QBI->getParent())) ||
       (QTB && !HasOnePredAndOneSucc(QTB, QBI->getParent(), PostBB)))
     return false;
-  if (!PostBB->hasNUses(2) || !QBI->getParent()->hasNUses(2))
+  if (!QBI->getParent()->hasNUses(2))
     return false;
 
   // OK, this is a sequence of two diamonds or triangles.