[llvm] [LoadStoreVectorizer] Fill gaps in load/store chains to enable vectorization (PR #159388)

[Drew Kersnar via llvm-commits]

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@@ -831,7 +905,59 @@ std::vector<Chain> Vectorizer::splitChainByAlignment(Chain &C) {
         }
       }
 
-      if (!IsAllowedAndFast(Alignment)) {
+      Chain ExtendingLoadsStores;
+      if (NumVecElems < TargetVF && NumVecElems % 2 != 0 && VecElemBits >= 8) {
+        // TargetVF may be a lot higher than NumVecElems,
+        // so only extend to the next power of 2.
+        assert(VecElemBits % 8 == 0);
+        unsigned VecElemBytes = VecElemBits / 8;
+        unsigned NewNumVecElems = PowerOf2Ceil(NumVecElems);
+        unsigned NewSizeBytes = VecElemBytes * NewNumVecElems;
+
+        assert(NewNumVecElems <= TargetVF);
+
+        LLVM_DEBUG(dbgs() << "LSV: attempting to extend chain of "
+                          << NumVecElems << " "
+                          << (IsLoadChain ? "loads" : "stores") << " to "
+                          << NewNumVecElems << " elements\n");
+        // Do not artificially increase the chain if it becomes misaligned,
+        // otherwise we may unnecessary split the chain when the target actually
+        // supports non-pow2 VF.
+        if (accessIsAllowedAndFast(NewSizeBytes, AS, Alignment, VecElemBits) &&
+            ((IsLoadChain &&
+              TTI.isLegalMaskedLoad(
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dakersnar wrote:

TODO: all these calls to isLegalMaskedLoad/Store need to be updated to pass in the new enum that represents a Constant mask that is being workshopped in the other PR.

https://github.com/llvm/llvm-project/pull/159388