allocator: use the compiler to compute header size

Rather than trying to compute the size of the magazine header manually,
use the compiler's builtin `sizeof` operator.

Author: compnerd

src/allocator.c

@@ -603,8 +603,6 @@ _dispatch_alloc_init(void)
 
 	dispatch_assert(sizeof(bitmap_t) == BYTES_PER_BITMAP);
 	dispatch_assert(sizeof(bitmap_t) == BYTES_PER_SUPERMAP);
-	dispatch_assert(sizeof(struct dispatch_magazine_header_s) ==
-			SIZEOF_HEADER);
 
 	dispatch_assert(sizeof(struct dispatch_continuation_s) <=
 			DISPATCH_CONTINUATION_SIZE);
@@ -614,8 +612,6 @@ _dispatch_alloc_init(void)
 	dispatch_assert(sizeof(struct dispatch_magazine_s) == BYTES_PER_MAGAZINE);
 
 	// The header and maps sizes should match what we computed.
-	dispatch_assert(SIZEOF_HEADER ==
-			sizeof(((struct dispatch_magazine_s *)0x0)->header));
 	dispatch_assert(SIZEOF_MAPS ==
 			sizeof(((struct dispatch_magazine_s *)0x0)->maps));
 

src/allocator_internal.h

@@ -147,11 +147,7 @@ typedef unsigned long bitmap_t;
 
 #define PADDING_TO_CONTINUATION_SIZE(x) (ROUND_UP_TO_CONTINUATION_SIZE(x) - (x))
 
-#if defined(__LP64__)
-#define SIZEOF_HEADER 16
-#else
-#define SIZEOF_HEADER 8
-#endif
+#define SIZEOF_HEADER (sizeof(struct dispatch_magazine_header_s))
 
 #define SIZEOF_SUPERMAPS (BYTES_PER_SUPERMAP * SUPERMAPS_PER_MAGAZINE)
 #define SIZEOF_MAPS (BYTES_PER_BITMAP * BITMAPS_PER_SUPERMAP * \