PointerLikeTypeTraits: Standardize NumLowBitsAvailable on static constexpr rather than anonymous enum

This is (more?) usable by GDB pretty printers and seems nicer to write.

There's one tricky caveat that in C++14 (LLVM's codebase today) the
static constexpr member declaration is not a definition - so odr use of
this constant requires an out of line definition, which won't be
provided (that'd make all these trait classes more annoyidng/expensive
to maintain). But the use of this constant in the library implementation
is/should always be in a non-odr context - only two unit tests needed to
be touched to cope with this/avoid odr using these constants.

Based on/expanded from D72590 by Christian Sigg.

include/llvm/ADT/IntervalMap.h

@@ -491,7 +491,7 @@ class NodeRef {
   struct CacheAlignedPointerTraits {
     static inline void *getAsVoidPointer(void *P) { return P; }
     static inline void *getFromVoidPointer(void *P) { return P; }
-    enum { NumLowBitsAvailable = Log2CacheLine };
+    static constexpr int NumLowBitsAvailable = Log2CacheLine;
   };
   PointerIntPair<void*, Log2CacheLine, unsigned, CacheAlignedPointerTraits> pip;
 

include/llvm/ADT/PointerEmbeddedInt.h

@@ -94,7 +94,7 @@ struct PointerLikeTypeTraits<PointerEmbeddedInt<IntT, Bits>> {
     return T(reinterpret_cast<uintptr_t>(P), typename T::RawValueTag());
   }
 
-  enum { NumLowBitsAvailable = T::Shift };
+  static constexpr int NumLowBitsAvailable = T::Shift;
 };
 
 // Teach DenseMap how to use PointerEmbeddedInt objects as keys if the Int type

include/llvm/ADT/PointerIntPair.h

@@ -235,7 +235,8 @@ struct PointerLikeTypeTraits<
     return PointerIntPair<PointerTy, IntBits, IntType>::getFromOpaqueValue(P);
   }
 
-  enum { NumLowBitsAvailable = PtrTraits::NumLowBitsAvailable - IntBits };
+  static constexpr int NumLowBitsAvailable =
+      PtrTraits::NumLowBitsAvailable - IntBits;
 };
 
 } // end namespace llvm

include/llvm/IR/Use.h

@@ -69,7 +69,7 @@ public:
       return (User *)P;
     }
 
-    enum { NumLowBitsAvailable = 1 };
+    static constexpr int NumLowBitsAvailable = 1;
   };
 
   // A type for the word following an array of hung-off Uses in memory, which is
@@ -85,7 +85,7 @@ public:
       return (Use **)P;
     }
 
-    enum { NumLowBitsAvailable = 2 };
+    static constexpr int NumLowBitsAvailable = 2;
   };
 
 private:

include/llvm/Support/PointerLikeTypeTraits.h

@@ -56,7 +56,8 @@ template <typename T> struct PointerLikeTypeTraits<T *> {
   static inline void *getAsVoidPointer(T *P) { return P; }
   static inline T *getFromVoidPointer(void *P) { return static_cast<T *>(P); }
 
-  enum { NumLowBitsAvailable = detail::ConstantLog2<alignof(T)>::value };
+  static constexpr int NumLowBitsAvailable =
+      detail::ConstantLog2<alignof(T)>::value;
 };
 
 template <> struct PointerLikeTypeTraits<void *> {
@@ -70,7 +71,7 @@ template <> struct PointerLikeTypeTraits<void *> {
   ///
   /// All clients should use assertions to do a run-time check to ensure that
   /// this is actually true.
-  enum { NumLowBitsAvailable = 2 };
+  static constexpr int NumLowBitsAvailable = 2;
 };
 
 // Provide PointerLikeTypeTraits for const things.
@@ -83,7 +84,7 @@ template <typename T> struct PointerLikeTypeTraits<const T> {
   static inline const T getFromVoidPointer(const void *P) {
     return NonConst::getFromVoidPointer(const_cast<void *>(P));
   }
-  enum { NumLowBitsAvailable = NonConst::NumLowBitsAvailable };
+  static constexpr int NumLowBitsAvailable = NonConst::NumLowBitsAvailable;
 };
 
 // Provide PointerLikeTypeTraits for const pointers.
@@ -96,7 +97,7 @@ template <typename T> struct PointerLikeTypeTraits<const T *> {
   static inline const T *getFromVoidPointer(const void *P) {
     return NonConst::getFromVoidPointer(const_cast<void *>(P));
   }
-  enum { NumLowBitsAvailable = NonConst::NumLowBitsAvailable };
+  static constexpr int NumLowBitsAvailable = NonConst::NumLowBitsAvailable;
 };
 
 // Provide PointerLikeTypeTraits for uintptr_t.
@@ -108,7 +109,7 @@ template <> struct PointerLikeTypeTraits<uintptr_t> {
     return reinterpret_cast<uintptr_t>(P);
   }
   // No bits are available!
-  enum { NumLowBitsAvailable = 0 };
+  static constexpr int NumLowBitsAvailable = 0;
 };
 
 /// Provide suitable custom traits struct for function pointers.
@@ -121,7 +122,8 @@ template <> struct PointerLikeTypeTraits<uintptr_t> {
 /// potentially use alignment attributes on functions to satisfy that.
 template <int Alignment, typename FunctionPointerT>
 struct FunctionPointerLikeTypeTraits {
-  enum { NumLowBitsAvailable = detail::ConstantLog2<Alignment>::value };
+  static constexpr int NumLowBitsAvailable =
+      detail::ConstantLog2<Alignment>::value;
   static inline void *getAsVoidPointer(FunctionPointerT P) {
     assert((reinterpret_cast<uintptr_t>(P) &
             ~((uintptr_t)-1 << NumLowBitsAvailable)) == 0 &&

lib/Analysis/GlobalsModRef.cpp

@@ -77,7 +77,7 @@ class GlobalsAAResult::FunctionInfo {
     static inline AlignedMap *getFromVoidPointer(void *P) {
       return (AlignedMap *)P;
     }
-    enum { NumLowBitsAvailable = 3 };
+    static constexpr int NumLowBitsAvailable = 3;
     static_assert(alignof(AlignedMap) >= (1 << NumLowBitsAvailable),
                   "AlignedMap insufficiently aligned to have enough low bits.");
   };

unittests/ADT/PointerEmbeddedIntTest.cpp

@@ -17,8 +17,8 @@ TEST(PointerEmbeddedIntTest, Basic) {
 
   EXPECT_EQ(42, I);
   EXPECT_EQ(43, I + 1);
-  EXPECT_EQ(sizeof(uintptr_t) * CHAR_BIT - CHAR_BIT,
-            PointerLikeTypeTraits<decltype(I)>::NumLowBitsAvailable);
+  EXPECT_EQ((int)sizeof(uintptr_t) * CHAR_BIT - CHAR_BIT,
+            (int)PointerLikeTypeTraits<decltype(I)>::NumLowBitsAvailable);
 
   EXPECT_FALSE(I == J);
   EXPECT_TRUE(I != J);

unittests/ADT/PointerIntPairTest.cpp

@@ -72,22 +72,22 @@ TEST(PointerIntPairTest, DefaultInitialize) {
   EXPECT_EQ(0U, Pair.getInt());
 }
 
+// In real code this would be a word-sized integer limited to 31 bits.
+struct Fixnum31 {
+  uintptr_t Value;
+};
+struct FixnumPointerTraits {
+  static inline void *getAsVoidPointer(Fixnum31 Num) {
+    return reinterpret_cast<void *>(Num.Value << NumLowBitsAvailable);
+  }
+  static inline Fixnum31 getFromVoidPointer(void *P) {
+    // In real code this would assert that the value is in range.
+    return {reinterpret_cast<uintptr_t>(P) >> NumLowBitsAvailable};
+  }
+  static constexpr int NumLowBitsAvailable =
+      std::numeric_limits<uintptr_t>::digits - 31;
+};
 TEST(PointerIntPairTest, ManyUnusedBits) {
-  // In real code this would be a word-sized integer limited to 31 bits.
-  struct Fixnum31 {
-    uintptr_t Value;
-  };
-  class FixnumPointerTraits {
-  public:
-    static inline void *getAsVoidPointer(Fixnum31 Num) {
-      return reinterpret_cast<void *>(Num.Value << NumLowBitsAvailable);
-    }
-    static inline Fixnum31 getFromVoidPointer(void *P) {
-      // In real code this would assert that the value is in range.
-      return { reinterpret_cast<uintptr_t>(P) >> NumLowBitsAvailable };
-    }
-    enum { NumLowBitsAvailable = std::numeric_limits<uintptr_t>::digits - 31 };
-  };
 
   PointerIntPair<Fixnum31, 1, bool, FixnumPointerTraits> pair;
   EXPECT_EQ((uintptr_t)0, pair.getPointer().Value);
@@ -98,7 +98,7 @@ TEST(PointerIntPairTest, ManyUnusedBits) {
   EXPECT_TRUE(pair.getInt());
 
   EXPECT_EQ(FixnumPointerTraits::NumLowBitsAvailable - 1,
-            PointerLikeTypeTraits<decltype(pair)>::NumLowBitsAvailable);
+            (int)PointerLikeTypeTraits<decltype(pair)>::NumLowBitsAvailable);
 
   static_assert(
       is_trivially_copyable<