Improve code quality for alloc_size (#196)

* Improve code quality for alloc_size

* Made error noreturn.

* Update docs.

* Move annoation

Author: mjp41

README.md

@@ -160,7 +160,7 @@ your system.
 The PAL must implement the following methods:
 
 ```c++
-void error(const char* const str) noexcept;
+[[noreturn]] void error(const char* const str) noexcept;
 ```
 Report a fatal error and exit.
 

src/ds/defines.h

@@ -37,7 +37,7 @@
 namespace snmalloc
 {
   // Forwards reference so that the platform can define how to handle errors.
-  void error(const char* const str);
+  [[noreturn]] void error(const char* const str);
 } // namespace snmalloc
 
 #define TOSTRING(expr) TOSTRING2(expr)

src/mem/alloc.h

@@ -453,16 +453,19 @@ namespace snmalloc
 #endif
     }
 
-    static size_t alloc_size(const void* p)
+  private:
+    SNMALLOC_SLOW_PATH static size_t alloc_size_error()
+    {
+      error("Not allocated by this allocator");
+    }
+
+  public:
+    SNMALLOC_FAST_PATH static size_t alloc_size(const void* p)
     {
       // This must be called on an external pointer.
       size_t size = ChunkMap::get(address_cast(p));
 
-      if (size == 0)
-      {
-        error("Not allocated by this allocator");
-      }
-      else if (size == CMSuperslab)
+      if (likely(size == CMSuperslab))
       {
         Superslab* super = Superslab::get(p);
 
@@ -473,15 +476,21 @@ namespace snmalloc
 
         return sizeclass_to_size(meta.sizeclass);
       }
-      else if (size == CMMediumslab)
+
+      if (likely(size == CMMediumslab))
       {
         Mediumslab* slab = Mediumslab::get(p);
         // Reading a remote sizeclass won't fail, since the other allocator
         // can't reuse the slab, as we have no yet deallocated this pointer.
         return sizeclass_to_size(slab->get_sizeclass());
       }
 
-      return 1ULL << size;
+      if (likely(size != 0))
+      {
+        return 1ULL << size;
+      }
+
+      return alloc_size_error();
     }
 
     size_t get_id()

src/pal/pal.h

@@ -49,7 +49,7 @@ namespace snmalloc
     DefaultPal;
 #endif
 
-  SNMALLOC_SLOW_PATH inline void error(const char* const str)
+  [[noreturn]] SNMALLOC_SLOW_PATH inline void error(const char* const str)
   {
     Pal::error(str);
   }

src/pal/pal_freebsd_kernel.h

@@ -29,7 +29,7 @@ namespace snmalloc
      * PAL supports.
      */
     static constexpr uint64_t pal_features = AlignedAllocation;
-    void error(const char* const str)
+    [[noreturn]] void error(const char* const str)
     {
       panic("snmalloc error: %s", str);
     }

src/pal/pal_open_enclave.h

@@ -5,7 +5,7 @@
 extern "C" const void* __oe_get_heap_base();
 extern "C" const void* __oe_get_heap_end();
 extern "C" void* oe_memset_s(void* p, size_t p_size, int c, size_t size);
-extern "C" void oe_abort();
+extern "C" [[noreturn]] void oe_abort();
 
 namespace snmalloc
 {
@@ -19,7 +19,7 @@ namespace snmalloc
      * PAL supports.
      */
     static constexpr uint64_t pal_features = 0;
-    static void error(const char* const str)
+    [[noreturn]] static void error(const char* const str)
     {
       UNUSED(str);
       oe_abort();

src/pal/pal_posix.h

@@ -55,7 +55,7 @@ namespace snmalloc
     /**
      * Report a fatal error an exit.
      */
-    static void error(const char* const str) noexcept
+    [[noreturn]] static void error(const char* const str) noexcept
     {
       puts(str);
       print_stack_trace();

src/pal/pal_windows.h

@@ -105,7 +105,7 @@ namespace snmalloc
       low_memory_callbacks.register_notification(callback);
     }
 
-    static void error(const char* const str)
+    [[noreturn]] static void error(const char* const str)
     {
       puts(str);
       fflush(stdout);