[llvm] [SandboxVec][BottomUpVec] Implement InstrMaps (PR #122848)

[via llvm-commits]

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@@ -148,6 +165,57 @@ class Pack final : public LegalityResultWithReason {
   }
 };
 
+/// Describes how to collect the values needed by each lane.
+class CollectDescr {
+public:
+  /// Describes how to get a value element. If the value is a vector then it
+  /// also provides the index to extract it from.
+  class ExtractElementDescr {
+    Value *V;
+    /// The index in `V` that the value can be extracted from.
+    /// This is nullopt if we need to use `V` as a whole.
+    std::optional<int> ExtractIdx;
+
+  public:
+    ExtractElementDescr(Value *V, int ExtractIdx)
+        : V(V), ExtractIdx(ExtractIdx) {}
+    ExtractElementDescr(Value *V) : V(V), ExtractIdx(std::nullopt) {}
+    Value *getValue() const { return V; }
+    bool needsExtract() const { return ExtractIdx.has_value(); }
+    int getExtractIdx() const { return *ExtractIdx; }
+  };
+
+  using DescrVecT = SmallVector<ExtractElementDescr, 4>;
+  DescrVecT Descrs;
+
+public:
+  CollectDescr(SmallVectorImpl<ExtractElementDescr> &&Descrs)
+      : Descrs(std::move(Descrs)) {}
+  std::optional<std::pair<Value *, bool>> getSingleInput() const {
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vporpo wrote:

This function tells us whether all the scalars come from a single vector and if we need a shuffle to get them in the right order. In a future patch this will also return the actual shuffle mask we need to get the elements in the right order.

I added a comment.

https://github.com/llvm/llvm-project/pull/122848