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

[Drew Kersnar via llvm-commits]

================
@@ -1634,3 +1638,32 @@ std::optional<APInt> Vectorizer::getConstantOffset(Value *PtrA, Value *PtrB,
         .sextOrTrunc(OrigBitWidth);
   return std::nullopt;
 }
+
+void Vectorizer::propagateBestAlignmentsInChain(ArrayRef<ChainElem> C) const {
+  auto PropagateAlignments = [](auto ChainIt) {
+    ChainElem BestAlignedElem = *ChainIt.begin();
+    Align BestAlignSoFar = getLoadStoreAlignment(BestAlignedElem.Inst);
+
+    for (const ChainElem &E : ChainIt) {
+      Align OrigAlign = getLoadStoreAlignment(E.Inst);
+      if (OrigAlign > BestAlignSoFar) {
+        BestAlignedElem = E;
+        BestAlignSoFar = OrigAlign;
+        continue;
+      }
+
+      APInt DeltaFromBestAlignedElem =
+          APIntOps::abdu(E.OffsetFromLeader, BestAlignedElem.OffsetFromLeader);
+      // 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, DeltaFromBestAlignedElem.getLimitedValue());
+      if (NewAlign > OrigAlign)
+        setLoadStoreAlignment(E.Inst, NewAlign);
+    }
+  };
+
+  // Propagate forwards and backwards.
+  PropagateAlignments(C);
+  PropagateAlignments(reverse(C));
----------------
dakersnar wrote:

`test_forward_and_reverse` in the test file demonstrates why going backwards could _theoretically_ be useful. If you have a well-aligned element later in the chain, propagating the alignment backwards could improve earlier elements in the chain.

I haven't thought of a specific end-to-end test that would trigger this, but I think it can't hurt to include.

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