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

[Drew Kersnar via llvm-commits]

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@@ -642,10 +702,54 @@ std::vector<Chain> Vectorizer::splitChainByContiguity(Chain &C) {
                       << *Prev.Inst << " (ends at offset " << PrevReadEnd
                       << ") -> " << *It->Inst << " (starts at offset "
                       << It->OffsetFromLeader << ")\n");
-    if (AreContiguous)
+
+    if (AreContiguous) {
       CurChain.push_back(*It);
-    else
-      Ret.push_back({*It});
+      continue;
+    }
+
+    // For now, we aren't filling gaps between load/stores of different sizes.
+    // Additionally, as a conservative heuristic, we only fill gaps of 1-2
+    // elements. Generating loads/stores with too many unused bytes has a side
+    // effect of increasing register pressure (on NVIDIA targets at least),
+    // which could cancel out the benefits of reducing number of load/stores.
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dakersnar wrote:

I don't currently envision supporting larger gaps, so I think the refactor to a loop should only happen if/when we change our minds on that.

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