[llvm-branch-commits] [PromoteMem2Reg] Optimize memory usage in PromoteMem2Reg (PR #142474)

[Florian Mayer via llvm-branch-commits]

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@@ -281,18 +281,44 @@ struct AllocaInfo {
   }
 };
 
+template <typename T> class VectorWithUndo {
+  SmallVector<T, 8> Vals;
+  SmallVector<std::pair<size_t, T>, 8> Undo;
+
+public:
+  void undo(size_t S) {
+    while (S < Undo.size()) {
+      Vals[Undo.back().first] = Undo.back().second;
+      Undo.pop_back();
+    }
+  }
+
+  void assign(size_t Sz, const T &Val) { Vals.assign(Sz, Val); }
+
+  size_t size() const { return Undo.size(); }
+
+  const T &operator[](size_t Idx) const { return Vals[Idx]; }
+
+  void set(size_t Idx, const T &Val) {
+    if (Vals[Idx] == Val)
+      return;
+    Undo.emplace_back(Idx, Vals[Idx]);
+    Vals[Idx] = Val;
+  }
+
+  void init(size_t Idx, const T &Val) { Vals[Idx] = Val; }
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fmayer wrote:

would it make sense to add asserts that this is not called after `set` or `undo`?

https://github.com/llvm/llvm-project/pull/142474