[llvm] [LoadStoreVectorizer] Propagate alignment through contiguous chain (PR #145733)

[Alex MacLean via llvm-commits]

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@@ -1634,3 +1645,27 @@ std::optional<APInt> Vectorizer::getConstantOffset(Value *PtrA, Value *PtrB,
         .sextOrTrunc(OrigBitWidth);
   return std::nullopt;
 }
+
+void Vectorizer::propagateBestAlignmentsInChain(ArrayRef<ChainElem> C) const {
+  ChainElem BestAlignedElem = C[0];
+  Align BestAlignSoFar = getLoadStoreAlignment(C[0].Inst);
+
+  for (const ChainElem &E : C) {
+    Align OrigAlign = getLoadStoreAlignment(E.Inst);
+    if (OrigAlign > BestAlignSoFar) {
+      BestAlignedElem = E;
+      BestAlignSoFar = OrigAlign;
+    }
+
+    APInt OffsetFromBestAlignedElem =
+        E.OffsetFromLeader - BestAlignedElem.OffsetFromLeader;
+    assert(OffsetFromBestAlignedElem.isNonNegative());
+    // commonAlignment is equivalent to a greatest common power-of-two divisor;
+    // it returns the largest power of 2 that divides both A and B.
+    Align NewAlign = commonAlignment(
+        BestAlignSoFar, OffsetFromBestAlignedElem.getLimitedValue());
+    if (NewAlign > OrigAlign)
+      setLoadStoreAlignment(E.Inst, NewAlign);
+  }
+  return;
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AlexMaclean wrote:

nit: can this return be removed?

https://github.com/llvm/llvm-project/pull/145733