[ADT] Make sure the PageSize is a power of two

Author: nikic

llvm/include/llvm/ADT/PagedVector.h

@@ -39,7 +39,8 @@ namespace llvm {
 /// while iterating, therefore materialising them and losing the gains in terms
 /// of memory usage this container provides. If you have such a use case, you
 /// probably want to use a normal std::vector or a llvm::SmallVector.
-template <typename T, size_t PageSize = 1024 / sizeof(T)> class PagedVector {
+template <typename T, size_t PageSize = PowerOf2Ceil(1024 / sizeof(T))>
+class PagedVector {
   static_assert(PageSize > 1, "PageSize must be greater than 0. Most likely "
                               "you want it to be greater than 16.");
   /// The actual number of elements in the vector which can be accessed.

llvm/include/llvm/Support/MathExtras.h

@@ -358,7 +358,7 @@ constexpr inline uint64_t NextPowerOf2(uint64_t A) {
 
 /// Returns the power of two which is greater than or equal to the given value.
 /// Essentially, it is a ceil operation across the domain of powers of two.
-inline uint64_t PowerOf2Ceil(uint64_t A) {
+constexpr inline uint64_t PowerOf2Ceil(uint64_t A) {
   if (!A)
     return 0;
   return NextPowerOf2(A - 1);