From 3e0a1f38828a309fda3e9b89bb2e9ffa5ba6387d Mon Sep 17 00:00:00 2001
From: Mike Frysinger <vapier@gentoo.org>
Date: Mon, 29 Jan 2007 17:52:29 +0000
Subject: 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.
---
libc/stdlib/malloc/heap.h | 2 +-
libc/stdlib/malloc/malloc.h | 18 +++++++++++++-----
2 files changed, 14 insertions(+), 6 deletions(-)
(limited to 'libc/stdlib/malloc')
diff --git a/libc/stdlib/malloc/heap.h b/libc/stdlib/malloc/heap.h
index 23358d988..ecc432b8e 100644
--- a/libc/stdlib/malloc/heap.h
+++ b/libc/stdlib/malloc/heap.h
@@ -25,7 +25,7 @@
/* The heap allocates in multiples of, and aligned to, HEAP_GRANULARITY.
HEAP_GRANULARITY must be a power of 2. Malloc depends on this being the
same as MALLOC_ALIGNMENT. */
-#define HEAP_GRANULARITY_TYPE double
+#define HEAP_GRANULARITY_TYPE double __attribute_aligned__ (sizeof (size_t))
#define HEAP_GRANULARITY (__alignof__ (HEAP_GRANULARITY_TYPE))
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) \
--
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