Add a simple type-safe bit-mangling pointer union class. This allows

you to do things like:

  ///    PointerUnion<int*, float*> P;
  ///    P = (int*)0;
  ///    printf("%d %d", P.is<int*>(), P.is<float*>()); // prints "1 0"
  ///    X = P.get<int*>();     // ok.
  ///    Y = P.get<float*>();   // runtime assertion failure.
  ///    Z = P.get<double*>();  // does not compile.
  ///    P = (float*)0;
  ///    Y = P.get<float*>();   // ok.
  ///    X = P.get<int*>();     // runtime assertion failure.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@67987 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/PointerUnion.h

@@ -0,0 +1,132 @@
+//===- llvm/ADT/PointerUnion.h - Discriminated Union of 2 Ptrs --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the PointerUnion class, which is a discriminated union of
+// pointer types.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_POINTERUNION_H
+#define LLVM_ADT_POINTERUNION_H
+
+#include "llvm/ADT/PointerIntPair.h"
+
+namespace llvm {
+
+  /// getPointerUnionTypeNum - If the argument has type PT1* or PT2* return
+  /// false or true respectively.
+  template <typename PT1, typename PT2>
+  static inline bool getPointerUnionTypeNum(PT1 *P) { return false; }
+  template <typename PT1, typename PT2>
+  static inline bool getPointerUnionTypeNum(PT2 *P) { return true; }
+  // Enable, if we could use static_assert.
+  //template <typename PT1, typename PT2>
+  //static inline bool getPointerUnionTypeNum(...) { abort() }
+  
+  
+  /// Provide PointerLikeTypeTraits for void* that is used by PointerUnion
+  /// for the two template arguments.
+  template <typename PT1, typename PT2>
+  class PointerUnionUIntTraits {
+  public:
+    static inline void *getAsVoidPointer(void *P) { return P; }
+    static inline void *getFromVoidPointer(void *P) { return P; }
+    enum {
+      PT1BitsAv = PointerLikeTypeTraits<PT1>::NumLowBitsAvailable,
+      PT2BitsAv = PointerLikeTypeTraits<PT2>::NumLowBitsAvailable,
+      NumLowBitsAvailable = PT1BitsAv < PT2BitsAv ? PT1BitsAv : PT2BitsAv
+    };
+  };
+  
+  /// PointerUnion - This implements a discriminated union of two pointer types,
+  /// and keeps the discriminator bit-mangled into the low bits of the pointer.
+  /// This allows the implementation to be extremely efficient in space, but
+  /// permits a very natural and type-safe API.
+  ///
+  /// Common use patterns would be something like this:
+  ///    PointerUnion<int*, float*> P;
+  ///    P = (int*)0;
+  ///    printf("%d %d", P.is<int*>(), P.is<float*>());  // prints "1 0"
+  ///    X = P.get<int*>();     // ok.
+  ///    Y = P.get<float*>();   // runtime assertion failure.
+  ///    Z = P.get<double*>();  // does not compile.
+  ///    P = (float*)0;
+  ///    Y = P.get<float*>();   // ok.
+  ///    X = P.get<int*>();     // runtime assertion failure.
+  template <typename PT1, typename PT2>
+  class PointerUnion {
+  public:
+    typedef PointerIntPair<void*, 1, bool, 
+                           PointerUnionUIntTraits<PT1,PT2> > ValTy;
+  private:
+    ValTy Val;
+  public:
+    PointerUnion() {}
+    
+    PointerUnion(PT1 V) {
+      Val.setPointer(V);
+      Val.setInt(0);
+    }
+    PointerUnion(PT2 V) {
+      Val.setPointer(V);
+      Val.setInt(1);
+    }
+    
+    template<typename T>
+    int is() const {
+      return Val.getInt() == ::llvm::getPointerUnionTypeNum<PT1, PT2>((T*)0);
+    }
+    template<typename T>
+    T get() const {
+      assert(is<T>() && "Invalid accessor called");
+      return static_cast<T>(Val.getPointer());
+    }
+    
+    const PointerUnion &operator=(const PT1 &RHS) {
+      Val.setPointer(RHS);
+      Val.setInt(0);
+      return *this;
+    }
+    const PointerUnion &operator=(const PT2 &RHS) {
+      Val.setPointer(RHS);
+      Val.setInt(1);
+      return *this;
+    }
+    
+    void *getOpaqueValue() const { return Val.getOpaqueValue(); }
+    static PointerUnion getFromOpaqueValue(void *VP) {
+      PointerUnion V;
+      V.Val = ValTy::getFromOpaqueValue(VP);
+      return V;
+    }
+  };
+  
+  // Teach SmallPtrSet that PointerIntPair is "basically a pointer", that has
+  // # low bits available = min(PT1bits,PT2bits)-1.
+  template<typename PT1, typename PT2>
+  class PointerLikeTypeTraits<PointerUnion<PT1, PT2> > {
+  public:
+    static inline void *
+    getAsVoidPointer(const PointerUnion<PT1, PT2> &P) {
+      return P.getOpaqueValue();
+    }
+    static inline PointerUnion<PT1, PT2>
+    getFromVoidPointer(void *P) {
+      return PointerUnion<PT1, PT2>::getFromOpaqueValue(P);
+    }
+    
+    // The number of bits available are the min of the two pointer types.
+    enum {
+      NumLowBitsAvailable = 
+        PointerUnion<PT1,PT2>::ValTy::NumLowBitsAvailable
+    };
+  };
+}
+
+#endif