From 3e0a1f38828a309fda3e9b89bb2e9ffa5ba6387d Mon Sep 17 00:00:00 2001 From: Mike Frysinger 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. --- include/sys/cdefs.h | 8 ++++++++ libc/stdlib/malloc/heap.h | 2 +- libc/stdlib/malloc/malloc.h | 18 +++++++++++++----- 3 files changed, 22 insertions(+), 6 deletions(-) diff --git a/include/sys/cdefs.h b/include/sys/cdefs.h index fb53924e9..8daac15ef 100644 --- a/include/sys/cdefs.h +++ b/include/sys/cdefs.h @@ -189,6 +189,14 @@ # define __attribute__(xyz) /* Ignore */ #endif +/* We make this a no-op unless it can be used as both a variable and + a type attribute. gcc 2.8 is known to support both. */ +#if __GNUC_PREREQ (2,8) +# define __attribute_aligned__(size) __attribute__ ((__aligned__ (size))) +#else +# define __attribute_aligned__(size) /* Ignore */ +#endif + /* At some point during the gcc 2.96 development the `malloc' attribute for functions was introduced. We don't want to use it unconditionally (although this would be possible) since it generates warnings. */ 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 */ -/* 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) \ -- cgit v1.2.3