Update to use a more efficient power of 2 check. (#274)

Author: mjp41

src/ds/address.h

@@ -49,7 +49,7 @@ namespace snmalloc
   template<size_t alignment>
   static inline bool is_aligned_block(void* p, size_t size)
   {
-    static_assert(bits::next_pow2_const(alignment) == alignment);
+    static_assert(bits::is_pow2(alignment));
 
     return ((address_cast(p) | size) & (alignment - 1)) == 0;
   }
@@ -62,7 +62,7 @@ namespace snmalloc
   SNMALLOC_FAST_PATH T* pointer_align_down(void* p)
   {
     static_assert(alignment > 0);
-    static_assert(bits::next_pow2_const(alignment) == alignment);
+    static_assert(bits::is_pow2(alignment));
     if constexpr (alignment == 1)
       return static_cast<T*>(p);
     else
@@ -84,7 +84,7 @@ namespace snmalloc
   inline T* pointer_align_up(void* p)
   {
     static_assert(alignment > 0);
-    static_assert(bits::next_pow2_const(alignment) == alignment);
+    static_assert(bits::is_pow2(alignment));
     if constexpr (alignment == 1)
       return static_cast<T*>(p);
     else
@@ -106,7 +106,7 @@ namespace snmalloc
   SNMALLOC_FAST_PATH T* pointer_align_down(void* p, size_t alignment)
   {
     SNMALLOC_ASSERT(alignment > 0);
-    SNMALLOC_ASSERT(bits::next_pow2(alignment) == alignment);
+    SNMALLOC_ASSERT(bits::is_pow2(alignment));
 #if __has_builtin(__builtin_align_down)
     return static_cast<T*>(__builtin_align_down(p, alignment));
 #else
@@ -123,7 +123,7 @@ namespace snmalloc
   inline T* pointer_align_up(void* p, size_t alignment)
   {
     SNMALLOC_ASSERT(alignment > 0);
-    SNMALLOC_ASSERT(bits::next_pow2(alignment) == alignment);
+    SNMALLOC_ASSERT(bits::is_pow2(alignment));
 #if __has_builtin(__builtin_align_up)
     return static_cast<T*>(__builtin_align_up(p, alignment));
 #else

src/ds/bits.h

@@ -211,6 +211,11 @@ namespace snmalloc
 #endif
     }
 
+    constexpr SNMALLOC_FAST_PATH bool is_pow2(size_t x)
+    {
+      return (x & (x - 1)) == 0;
+    }
+
     SNMALLOC_FAST_PATH size_t next_pow2(size_t x)
     {
       // Correct for numbers [0..MAX_SIZE >> 1).
@@ -244,7 +249,7 @@ namespace snmalloc
     constexpr SNMALLOC_FAST_PATH size_t
     align_down(size_t value, size_t alignment)
     {
-      SNMALLOC_ASSERT(next_pow2_const(alignment) == alignment);
+      SNMALLOC_ASSERT(is_pow2(alignment));
 
       size_t align_1 = alignment - 1;
       value &= ~align_1;
@@ -253,7 +258,7 @@ namespace snmalloc
 
     constexpr SNMALLOC_FAST_PATH size_t align_up(size_t value, size_t alignment)
     {
-      SNMALLOC_ASSERT(next_pow2_const(alignment) == alignment);
+      SNMALLOC_ASSERT(is_pow2(alignment));
 
       size_t align_1 = alignment - 1;
       value += align_1;

src/ds/helpers.h

@@ -52,8 +52,7 @@ namespace snmalloc
   template<size_t length, typename T>
   class Mod
   {
-    static_assert(
-      length == bits::next_pow2_const(length), "Must be a power of two.");
+    static_assert(bits::is_pow2(length), "Must be a power of two.");
 
   private:
     T value = 0;

src/mem/address_space.h

@@ -175,7 +175,7 @@ namespace snmalloc
     template<bool committed>
     void* reserve(size_t size)
     {
-      SNMALLOC_ASSERT(bits::next_pow2(size) == size);
+      SNMALLOC_ASSERT(bits::is_pow2(size));
       SNMALLOC_ASSERT(size >= sizeof(void*));
 
       if constexpr (pal_supports<AlignedAllocation, PAL>)

src/mem/sizeclass.h

@@ -183,7 +183,7 @@ namespace snmalloc
     // Client responsible for checking alignment is not zero
     SNMALLOC_ASSERT(alignment != 0);
     // Client responsible for checking alignment is a power of two
-    SNMALLOC_ASSERT(bits::next_pow2(alignment) == alignment);
+    SNMALLOC_ASSERT(bits::is_pow2(alignment));
 
     return ((alignment - 1) | (size - 1)) + 1;
   }

src/pal/pal.h

@@ -76,8 +76,7 @@ namespace snmalloc
   static constexpr size_t OS_PAGE_SIZE = Pal::page_size;
 
   static_assert(
-    bits::next_pow2_const(OS_PAGE_SIZE) == OS_PAGE_SIZE,
-    "OS_PAGE_SIZE must be a power of two");
+    bits::is_pow2(OS_PAGE_SIZE), "OS_PAGE_SIZE must be a power of two");
   static_assert(
     OS_PAGE_SIZE % Aal::smallest_page_size == 0,
     "The smallest architectural page size must divide OS_PAGE_SIZE");

src/pal/pal_bsd_aligned.h

@@ -32,7 +32,7 @@ namespace snmalloc
     static void* reserve_aligned(size_t size) noexcept
     {
       // Alignment must be a power of 2.
-      SNMALLOC_ASSERT(size == bits::next_pow2(size));
+      SNMALLOC_ASSERT(bits::is_pow2(size));
       SNMALLOC_ASSERT(size >= minimum_alloc_size);
 
       size_t log2align = bits::next_pow2_bits(size);

src/pal/pal_freebsd_kernel.h

@@ -62,7 +62,7 @@ namespace snmalloc
     template<bool committed>
     static void* reserve_aligned(size_t size) noexcept
     {
-      SNMALLOC_ASSERT(size == bits::next_pow2(size));
+      SNMALLOC_ASSERT(bits::is_pow2(size));
       SNMALLOC_ASSERT(size >= minimum_alloc_size);
       size_t align = size;
 

src/pal/pal_posix.h

@@ -212,7 +212,7 @@ namespace snmalloc
      */
     static std::pair<void*, size_t> reserve_at_least(size_t size) noexcept
     {
-      SNMALLOC_ASSERT(size == bits::next_pow2(size));
+      SNMALLOC_ASSERT(bits::is_pow2(size));
 
       // Magic number for over-allocating chosen by the Pal
       // These should be further refined based on experiments.

src/pal/pal_windows.h

@@ -185,7 +185,7 @@ namespace snmalloc
     template<bool committed>
     static void* reserve_aligned(size_t size) noexcept
     {
-      SNMALLOC_ASSERT(size == bits::next_pow2(size));
+      SNMALLOC_ASSERT(bits::is_pow2(size));
       SNMALLOC_ASSERT(size >= minimum_alloc_size);
 
       DWORD flags = MEM_RESERVE;
@@ -215,7 +215,7 @@ namespace snmalloc
 #  else
     static std::pair<void*, size_t> reserve_at_least(size_t size) noexcept
     {
-      SNMALLOC_ASSERT(size == bits::next_pow2(size));
+      SNMALLOC_ASSERT(bits::is_pow2(size));
 
       // Magic number for over-allocating chosen by the Pal
       // These should be further refined based on experiments.