[reprise] PIC and flat shared library support for m68k

Mike Frysinger vapier at gentoo.org
Fri Nov 17 05:56:40 UTC 2006


this change was introduced as a fix on Blackfin ... Bernd would be able to 
explain the issue ...
-mike

On Friday 27 October 2006 10:15, Richard Sandiford wrote:
> 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.
>
> Tested on m68k.  Please install if OK.
>
> Richard
>
>
> Index: include/sys/cdefs.h
> ===================================================================
> --- include/sys/cdefs.h	(revision 16433)
> +++ include/sys/cdefs.h	(working copy)
> @@ -189,6 +189,14 @@ #define __warndecl(name, msg) extern voi
>  # define __attribute__(xyz)	/* Ignore */
>  #endif
>
> +/* __attribute_aligned__ is 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.  */
> Index: libc/stdlib/malloc/heap.h
> ===================================================================
> --- libc/stdlib/malloc/heap.h	(revision 16433)
> +++ libc/stdlib/malloc/heap.h	(working copy)
> @@ -24,7 +24,7 @@
>  /* 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))
>
>
> Index: libc/stdlib/malloc/malloc.h
> ===================================================================
> --- libc/stdlib/malloc/malloc.h	(revision 16433)
> +++ libc/stdlib/malloc/malloc.h	(working copy)
> @@ -11,8 +11,13 @@
>   * Written by Miles Bader <miles at 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 __putc(int c, FILE *stream) a
>
>
>  /* 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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