[CodeGen] Fix the computation of the alignment of split stores.

By Clement Courbet!

Backported from rG15488ff24b4a

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 003db39fe5f..7d77664fbf6 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -6857,12 +6857,20 @@ static bool splitMergedValStore(StoreInst &SI, const DataLayout &DL,
     Value *Addr = Builder.CreateBitCast(
         SI.getOperand(1),
         SplitStoreType->getPointerTo(SI.getPointerAddressSpace()));
-    if ((IsLE && Upper) || (!IsLE && !Upper))
+    const bool IsOffsetStore = (IsLE && Upper) || (!IsLE && !Upper);
+    if (IsOffsetStore)
       Addr = Builder.CreateGEP(
           SplitStoreType, Addr,
           ConstantInt::get(Type::getInt32Ty(SI.getContext()), 1));
+    MaybeAlign Alignment(SI.getAlignment());
+    if (IsOffsetStore && Alignment) {
+      // When splitting the store in half, naturally one half will retain the
+      // alignment of the original wider store, regardless of whether it was
+      // over-aligned or not, while the other will require adjustment.
+      Alignment = commonAlignment(Alignment, HalfValBitSize / 8);
+    }
     Builder.CreateAlignedStore(
-        V, Addr, Upper ? SI.getAlignment() / 2 : SI.getAlignment());
+        V, Addr, Alignment.hasValue() ? Alignment.getValue().value() : 0);
   };
 
   CreateSplitStore(LValue, false);