Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2860,7 +2860,8 @@
 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 @@
   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 @@
   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 @@
   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 @@
   // 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 @@
     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;
Index: test/Transforms/SimplifyCFG/preserve-store-alignment.ll
===================================================================
--- test/Transforms/SimplifyCFG/preserve-store-alignment.ll
+++ test/Transforms/SimplifyCFG/preserve-store-alignment.ll
@@ -25,7 +25,7 @@
 ; CHECK-NEXT:    [[TMP7:%.*]] = xor i1 [[TOBOOL5]], true
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
 ; CHECK-NEXT:    br i1 [[TMP8]], label [[TMP9:%.*]], label [[TMP10:%.*]]
-; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*)
+; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*), align 8
 ; CHECK-NEXT:    br label [[TMP10]]
 ; CHECK:         ret i32 0
 ;
@@ -77,7 +77,7 @@
 ; CHECK-NEXT:    [[TMP7:%.*]] = xor i1 [[TOBOOL5]], true
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
 ; CHECK-NEXT:    br i1 [[TMP8]], label [[TMP9:%.*]], label [[TMP10:%.*]]
-; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*)
+; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*), align 8
 ; CHECK-NEXT:    br label [[TMP10]]
 ; CHECK:         ret i32 0
 ;
@@ -129,7 +129,7 @@
 ; CHECK-NEXT:    [[TMP7:%.*]] = xor i1 [[TOBOOL5]], true
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
 ; CHECK-NEXT:    br i1 [[TMP8]], label [[TMP9:%.*]], label [[TMP10:%.*]]
-; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*)
+; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*), align 8
 ; CHECK-NEXT:    br label [[TMP10]]
 ; CHECK:         ret i32 0
 ;
@@ -181,7 +181,7 @@
 ; CHECK-NEXT:    [[TMP7:%.*]] = xor i1 [[TOBOOL5]], true
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
 ; CHECK-NEXT:    br i1 [[TMP8]], label [[TMP9:%.*]], label [[TMP10:%.*]]
-; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*)
+; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*), align 16
 ; CHECK-NEXT:    br label [[TMP10]]
 ; CHECK:         ret i32 0
 ;
@@ -233,7 +233,7 @@
 ; CHECK-NEXT:    [[TMP7:%.*]] = xor i1 [[TOBOOL5]], true
 ; CHECK-NEXT:    [[TMP8:%.*]] = or i1 [[TMP6]], [[TMP7]]
 ; CHECK-NEXT:    br i1 [[TMP8]], label [[TMP9:%.*]], label [[TMP10:%.*]]
-; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*)
+; CHECK:         store <2 x i64> [[DOT]], <2 x i64>* bitcast (i64* getelementptr inbounds (%struct.Counters, %struct.Counters* @counters, i64 0, i32 1) to <2 x i64>*), align 16
 ; CHECK-NEXT:    br label [[TMP10]]
 ; CHECK:         ret i32 0
 ;