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

@@ -147,12 +147,6 @@ 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_SUPERMAPS (BYTES_PER_SUPERMAP * SUPERMAPS_PER_MAGAZINE)
 #define SIZEOF_MAPS (BYTES_PER_BITMAP * BITMAPS_PER_SUPERMAP * \
 		SUPERMAPS_PER_MAGAZINE)
@@ -162,12 +156,12 @@ typedef unsigned long bitmap_t;
 // we want to align the maps to a continuation size, but we must also have proper padding
 // so that we can perform first_bitmap_in_same_page()
 #define SUPERMAPS_TO_MAPS_PADDING (PADDING_TO_BITMAP_ALIGNMENT_AND_CONTINUATION_SIZE( \
-		SIZEOF_SUPERMAPS + HEADER_TO_SUPERMAPS_PADDING + SIZEOF_HEADER))
+		SIZEOF_SUPERMAPS + HEADER_TO_SUPERMAPS_PADDING + sizeof(struct dispatch_magazine_header_s)))
 
 #define MAPS_TO_FPMAPS_PADDING (PADDING_TO_CONTINUATION_SIZE(SIZEOF_MAPS))
 
 #define BYTES_LEFT_IN_FIRST_PAGE (BYTES_PER_PAGE - \
-		(SIZEOF_HEADER + HEADER_TO_SUPERMAPS_PADDING + SIZEOF_SUPERMAPS + \
+		(sizeof(struct dispatch_magazine_header_s) + HEADER_TO_SUPERMAPS_PADDING + SIZEOF_SUPERMAPS + \
 		SUPERMAPS_TO_MAPS_PADDING + SIZEOF_MAPS + MAPS_TO_FPMAPS_PADDING))
 
 #if PACK_FIRST_PAGE_WITH_CONTINUATIONS