[clang] nolock/noalloc attributes (PR #84983)

[Shafik Yaghmour via cfe-commits]

================
@@ -4181,6 +4185,127 @@ class FunctionNoProtoType : public FunctionType, public llvm::FoldingSetNode {
   }
 };
 
+class FunctionEffect;
+class FunctionEffectSet;
+
+// It is the user's responsibility to keep this in set form: elements are
+// ordered and unique.
+// We could hide the mutating methods which are capable of breaking the
+// invariant, but they're needed and safe when used with STL set algorithms.
+class MutableFunctionEffectSet : public SmallVector<const FunctionEffect *, 4> {
+public:
+  using SmallVector::insert;
+  using SmallVector::SmallVector;
+
+  /// Maintains order/uniquenesss.
+  void insert(const FunctionEffect *effect);
+
+  MutableFunctionEffectSet &operator|=(FunctionEffectSet rhs);
+};
+
+class FunctionEffectSet {
+public:
+  // These sets will tend to be very small (1 element), so represent them as
+  // sorted vectors, which are compatible with the STL set algorithms. Using an
+  // array or vector also means the elements are contiguous, keeping iterators
+  // simple.
+
+private:
+  // 'Uniqued' refers to the set itself being uniqued. Storage is allocated
+  // separately. Use ArrayRef for its iterators. Subclass so as to be able to
+  // compare (it seems ArrayRef would silently convert itself to a vector for
+  // comparison?!).
+  class UniquedAndSortedFX : public llvm::ArrayRef<const FunctionEffect *> {
+  public:
+    using Base = llvm::ArrayRef<const FunctionEffect *>;
+
+    UniquedAndSortedFX(Base Array) : Base(Array) {}
+    UniquedAndSortedFX(const FunctionEffect **Ptr, size_t Len)
+        : Base(Ptr, Len) {}
+
+    bool operator<(const UniquedAndSortedFX &rhs) const;
+  };
+
+  // Could have used a TinyPtrVector if it were unique-able.
+  // Empty set has a null Impl.
+  llvm::PointerUnion<const FunctionEffect *, const UniquedAndSortedFX *> Impl;
+
+  explicit FunctionEffectSet(const FunctionEffect *Single) : Impl(Single) {}
+  explicit FunctionEffectSet(const UniquedAndSortedFX *Multi) : Impl(Multi) {}
+
+public:
+  using Differences =
+      SmallVector<std::pair<const FunctionEffect *, bool /*added*/>>;
----------------
shafik wrote:

```suggestion
      SmallVector<std::pair<const FunctionEffect *, /*added=*/bool>>;
```

https://github.com/llvm/llvm-project/pull/84983