[llvm] [VectorCombine] Combine adjacent loads feeding as shuffle operands (PR #213007)

[Hari Limaye via llvm-commits]

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@@ -6374,6 +6378,210 @@ bool VectorCombine::shrinkLoadForShuffles(Instruction &I) {
   return false;
 }
 
+// Attempt to combine two adjacent fixed-length vector loads, that only feed
+// shufflevector instructions, into a single wider load, rewriting every such
+// shuffle so that operand 0 is the wide load and operand 1 is poison.
+// clang-format off
+// e.g.
+//   %loadA = load <16 x i8>, ptr %a
+//   %gep = getelementptr inbounds <16 x i8>, ptr %a, i64 1
+//   %loadB = load <16 x i8>, ptr %gep
+//   %shuffle0 = shufflevector <16 x i8> %loadA, <16 x i8> %loadB,
+//               <32 x i8> <...>
+//   %shuffle1 = shufflevector <16 x i8> %loadA, <16 x i8> %loadB,
+//               <32 x i8> <...>
+//
+// The fold would transform this to:
+//   %loadAB = load <32 x i8>, ptr %a
+//   %shuffle0 = shufflevector <32 x i8> %loadAB, <32 x i8> poison, 
----------------
hazzlim wrote:

Nit: trailing whitespace
```suggestion
//   %shuffle0 = shufflevector <32 x i8> %loadAB, <32 x i8> poison,
```

https://github.com/llvm/llvm-project/pull/213007