[SimplifyCFG] Fix for PR34219: Preserve alignment after merging conditional stores.

Summary:
If SimplifyCFG pass is able to merge conditional stores into single one,
it loses the alignment. This may lead to incorrect codegen. Patch
sets the alignment of the new instruction if it is set in the original
one.

Reviewers: jmolloy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36841

llvm-svn: 312030

1 files changed, 30 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 1204a94a32a..1ee1e388def 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2860,7 +2860,8 @@ static Value *ensureValueAvailableInSuccessor(Value *V, BasicBlock *BB,
 static bool mergeConditionalStoreToAddress(BasicBlock *PTB, BasicBlock *PFB,
                                            BasicBlock *QTB, BasicBlock *QFB,
                                            BasicBlock *PostBB, Value *Address,
-                                           bool InvertPCond, bool InvertQCond) {
+                                           bool InvertPCond, bool InvertQCond,
+                                           const DataLayout &DL) {
   auto IsaBitcastOfPointerType = [](const Instruction &I) {
     return Operator::getOpcode(&I) == Instruction::BitCast &&
            I.getType()->isPointerTy();
@@ -2966,6 +2967,29 @@ static bool mergeConditionalStoreToAddress(BasicBlock *PTB, BasicBlock *PFB,
   PStore->getAAMetadata(AAMD, /*Merge=*/false);
   PStore->getAAMetadata(AAMD, /*Merge=*/true);
   SI->setAAMetadata(AAMD);
+  unsigned PAlignment = PStore->getAlignment();
+  unsigned QAlignment = QStore->getAlignment();
+  unsigned TypeAlignment =
+      DL.getABITypeAlignment(SI->getValueOperand()->getType());
+  unsigned MinAlignment;
+  unsigned MaxAlignment;
+  std::tie(MinAlignment, MaxAlignment) = std::minmax(PAlignment, QAlignment);
+  // Choose the minimum alignment. If we could prove both stores execute, we
+  // could use biggest one.  In this case, though, we only know that one of the
+  // stores executes.  And we don't know it's safe to take the alignment from a
+  // store that doesn't execute.
+  if (MinAlignment != 0) {
+    // Choose the minimum of all non-zero alignments.
+    SI->setAlignment(MinAlignment);
+  } else if (MaxAlignment != 0) {
+    // Choose the minimal alignment between the non-zero alignment and the ABI
+    // default alignment for the type of the stored value.
+    SI->setAlignment(std::min(MaxAlignment, TypeAlignment));
+  } else {
+    // If both alignments are zero, use ABI default alignment for the type of
+    // the stored value.
+    SI->setAlignment(TypeAlignment);
+  }
 
   QStore->eraseFromParent();
   PStore->eraseFromParent();
@@ -2973,7 +2997,8 @@ static bool mergeConditionalStoreToAddress(BasicBlock *PTB, BasicBlock *PFB,
   return true;
 }
 
-static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI) {
+static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI,
+                                   const DataLayout &DL) {
   // The intention here is to find diamonds or triangles (see below) where each
   // conditional block contains a store to the same address. Both of these
   // stores are conditional, so they can't be unconditionally sunk. But it may
@@ -3076,7 +3101,7 @@ static bool mergeConditionalStores(BranchInst *PBI, BranchInst *QBI) {
   bool Changed = false;
   for (auto *Address : CommonAddresses)
     Changed |= mergeConditionalStoreToAddress(
-        PTB, PFB, QTB, QFB, PostBB, Address, InvertPCond, InvertQCond);
+        PTB, PFB, QTB, QFB, PostBB, Address, InvertPCond, InvertQCond, DL);
   return Changed;
 }
 
@@ -3141,7 +3166,7 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
   // If both branches are conditional and both contain stores to the same
   // address, remove the stores from the conditionals and create a conditional
   // merged store at the end.
-  if (MergeCondStores && mergeConditionalStores(PBI, BI))
+  if (MergeCondStores && mergeConditionalStores(PBI, BI, DL))
     return true;
 
   // If this is a conditional branch in an empty block, and if any
@@ -5829,7 +5854,7 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
     if (BasicBlock *PrevBB = allPredecessorsComeFromSameSource(BB))
       if (BranchInst *PBI = dyn_cast<BranchInst>(PrevBB->getTerminator()))
         if (PBI != BI && PBI->isConditional())
-          if (mergeConditionalStores(PBI, BI))
+          if (mergeConditionalStores(PBI, BI, DL))
             return SimplifyCFG(BB, TTI, BonusInstThreshold, AC) | true;
 
   return false;