Merge pull request #104 from CTSRD-CHERI/for-upstream-cheri-prep

Further CHERI prep

src/mem/allocconfig.h

@@ -110,8 +110,9 @@ namespace snmalloc
 #endif
 
   // Minimum allocation size is space for two pointers.
-  static constexpr size_t MIN_ALLOC_BITS = bits::is64() ? 4 : 3;
-  static constexpr size_t MIN_ALLOC_SIZE = 1 << MIN_ALLOC_BITS;
+  static_assert(bits::next_pow2_const(sizeof(void*)) == sizeof(void*));
+  static constexpr size_t MIN_ALLOC_SIZE = 2 * sizeof(void*);
+  static constexpr size_t MIN_ALLOC_BITS = bits::ctz_const(MIN_ALLOC_SIZE);
 
   // Slabs are 64 KiB unless constrained to 16 KiB.
   static constexpr size_t SLAB_BITS = ADDRESS_SPACE_CONSTRAINED ? 14 : 16;

src/mem/metaslab.h

@@ -142,12 +142,12 @@ namespace snmalloc
 
     static Slab* get_slab(void* p)
     {
-      return pointer_cast<Slab>(address_cast(p) & SLAB_MASK);
+      return pointer_align_down<SLAB_SIZE, Slab>(p);
     }
 
     static bool is_short(Slab* p)
     {
-      return (address_cast(p) & SUPERSLAB_MASK) == address_cast(p);
+      return pointer_align_down<SUPERSLAB_SIZE>(p) == p;
     }
 
     /**

src/mem/pagemap.h

@@ -191,7 +191,7 @@ namespace snmalloc
           return std::pair(nullptr, 0);
 
         shift -= BITS_PER_INDEX_LEVEL;
-        ix = (addr >> shift) & ENTRIES_MASK;
+        ix = (static_cast<size_t>(addr) >> shift) & ENTRIES_MASK;
         e = &value->entries[ix];
 
         if constexpr (INDEX_LEVELS == 1)
@@ -211,7 +211,7 @@ namespace snmalloc
         return std::pair(nullptr, 0);
 
       shift -= BITS_FOR_LEAF;
-      ix = (addr >> shift) & LEAF_MASK;
+      ix = (static_cast<size_t>(addr) >> shift) & LEAF_MASK;
       return std::pair(leaf, ix);
     }
 
@@ -387,7 +387,7 @@ namespace snmalloc
      */
     size_t index_for_address(uintptr_t p)
     {
-      return reinterpret_cast<size_t>((p >> SHIFT) & (OS_PAGE_SIZE - 1));
+      return bits::align_down(static_cast<size_t>(p) >> SHIFT, OS_PAGE_SIZE);
     }
 
     /**