[Support] Use CTLog2 from PointerLikeTypeTraits.h (NFC) (llvm#157790)

This patch replaces ConstantLog2 with CTLog2.  ConstantLog2 only
operates on alignment values, making the two interchangeable in this
context.  CTLog2 also has the benefit of a static_assert that ensures
its parameter is a power of two.

Author: kazutakahirata

clang/include/clang/AST/Expr.h

@@ -1038,7 +1038,7 @@ class Expr : public ValueStmt {
 // PointerLikeTypeTraits is specialized so it can be used with a forward-decl of
 // Expr. Verify that we got it right.
 static_assert(llvm::PointerLikeTypeTraits<Expr *>::NumLowBitsAvailable <=
-                  llvm::detail::ConstantLog2<alignof(Expr)>::value,
+                  llvm::CTLog2<alignof(Expr)>(),
               "PointerLikeTypeTraits<Expr*> assumes too much alignment.");
 
 using ConstantExprKind = Expr::ConstantExprKind;

llvm/include/llvm/Support/PointerLikeTypeTraits.h

@@ -15,8 +15,8 @@
 #define LLVM_SUPPORT_POINTERLIKETYPETRAITS_H
 
 #include "llvm/Support/DataTypes.h"
+#include "llvm/Support/MathExtras.h"
 #include <cassert>
-#include <type_traits>
 
 namespace llvm {
 
@@ -25,12 +25,6 @@ namespace llvm {
 template <typename T> struct PointerLikeTypeTraits;
 
 namespace detail {
-/// A tiny meta function to compute the log2 of a compile time constant.
-template <size_t N>
-struct ConstantLog2
-    : std::integral_constant<size_t, ConstantLog2<N / 2>::value + 1> {};
-template <> struct ConstantLog2<1> : std::integral_constant<size_t, 0> {};
-
 // Provide a trait to check if T is pointer-like.
 template <typename T, typename U = void> struct HasPointerLikeTypeTraits {
   static const bool value = false;
@@ -57,8 +51,7 @@ 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); }
 
-  static constexpr int NumLowBitsAvailable =
-      detail::ConstantLog2<alignof(T)>::value;
+  static constexpr int NumLowBitsAvailable = CTLog2<alignof(T)>();
 };
 
 template <> struct PointerLikeTypeTraits<void *> {
@@ -123,8 +116,7 @@ template <> struct PointerLikeTypeTraits<uintptr_t> {
 /// potentially use alignment attributes on functions to satisfy that.
 template <int Alignment, typename FunctionPointerT>
 struct FunctionPointerLikeTypeTraits {
-  static constexpr int NumLowBitsAvailable =
-      detail::ConstantLog2<Alignment>::value;
+  static constexpr int NumLowBitsAvailable = CTLog2<Alignment>();
   static inline void *getAsVoidPointer(FunctionPointerT P) {
     assert((reinterpret_cast<uintptr_t>(P) &
             ~((uintptr_t)-1 << NumLowBitsAvailable)) == 0 &&