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authorMike Frysinger <vapier@gentoo.org>2007-01-29 17:52:29 +0000
committerMike Frysinger <vapier@gentoo.org>2007-01-29 17:52:29 +0000
commit3e0a1f38828a309fda3e9b89bb2e9ffa5ba6387d (patch)
tree0a5b1459b5c9a5d738434662a36992407b8470c6 /libc/stdlib/malloc/malloc.h
parentc5db1d4612f80820cdf2123674c6771892ef5ea9 (diff)
Richard Sandiford writes:
However, retesting on m68k showed up a problem that had appeared in uClibc since the last time I tried. Specifically, revision 15785 did: -#define HEAP_GRANULARITY (sizeof (HEAP_GRANULARITY_TYPE)) +#define HEAP_GRANULARITY (__alignof__ (HEAP_GRANULARITY_TYPE)) -#define MALLOC_ALIGNMENT (sizeof (double)) +#define MALLOC_ALIGNMENT (__alignof__ (double)) The problem is that (a) MALLOC_HEADER_SIZE == MALLOC_ALIGNMENT (b) the header contains a size value of type size_t (c) sizeof (size_t) is 4 on m68k, but... (d) __alignof__ (double) is only 2 (the largest alignment used on m68k) So we only allocate 2 bytes for the 4-byte header, and the least significant 2 bytes of the size are in the user's area rather than the header. The patch below fixes that problem by redefining MALLOC_HEADER_SIZE to: MAX (MALLOC_ALIGNMENT, sizeof (size_t)) (but without the help of the MAX macro ;)). However, we really would like to have word alignment on Coldfire. It makes a big performance difference, and because we have to allocate a 4-byte header anyway, what wastage there is will be confined to the end of the allocated block. Any wastage will also be limited to 2 bytes per allocation compared to the current alignment. I've therefore used the __aligned__ type attribute to create a double type that has at least sizeof (size_t) bytes of alignment. I've introduced a new __attribute_aligned__ macro for this. It might seem silly protecting against old or non-GNU compilers here, but the extra alignment is only an optimisation, and having the macro is more in the spirit of the other attribute code.
Diffstat (limited to 'libc/stdlib/malloc/malloc.h')
-rw-r--r--libc/stdlib/malloc/malloc.h18
1 files changed, 13 insertions, 5 deletions
diff --git a/libc/stdlib/malloc/malloc.h b/libc/stdlib/malloc/malloc.h
index 747e8bf9e..78232d62d 100644
--- a/libc/stdlib/malloc/malloc.h
+++ b/libc/stdlib/malloc/malloc.h
@@ -11,8 +11,13 @@
* Written by Miles Bader <miles@gnu.org>
*/
-/* The alignment we guarantee for malloc return values. */
-#define MALLOC_ALIGNMENT (__alignof__ (double))
+/* The alignment we guarantee for malloc return values. We prefer this
+ to be at least sizeof (size_t) bytes because (a) we have to allocate
+ that many bytes for the header anyway and (b) guaranteeing word
+ alignment can be a significant win on targets like m68k and Coldfire,
+ where __alignof__(double) == 2. */
+#define MALLOC_ALIGNMENT \
+ __alignof__ (double __attribute_aligned__ (sizeof (size_t)))
/* The system pagesize... */
extern size_t __pagesize;
@@ -98,17 +103,20 @@ extern int __malloc_mmb_debug;
/* The size of a malloc allocation is stored in a size_t word
- MALLOC_ALIGNMENT bytes prior to the start address of the allocation:
+ MALLOC_HEADER_SIZE bytes prior to the start address of the allocation:
+--------+---------+-------------------+
| SIZE |(unused) | allocation ... |
+--------+---------+-------------------+
^ BASE ^ ADDR
- ^ ADDR - MALLOC_ALIGN
+ ^ ADDR - MALLOC_HEADER_SIZE
*/
/* The amount of extra space used by the malloc header. */
-#define MALLOC_HEADER_SIZE MALLOC_ALIGNMENT
+#define MALLOC_HEADER_SIZE \
+ (MALLOC_ALIGNMENT < sizeof (size_t) \
+ ? sizeof (size_t) \
+ : MALLOC_ALIGNMENT)
/* Set up the malloc header, and return the user address of a malloc block. */
#define MALLOC_SETUP(base, size) \