[reprise] PIC and flat shared library support for m68k

Richard Sandiford richard at codesourcery.com
Fri Nov 17 07:37:12 UTC 2006


Mike Frysinger <vapier at gentoo.org> writes:
> this change was introduced as a fix on Blackfin ...

Right.  AIUI, the Blackfin issue was that heap structures are only
aligned in the same way that HEAP_GRANULARITY_TYPE is aligned, so the
alignment macros should reflect that.  Before the patch, the addresses
of heap objects were not always a multiple of either MALLOC_ALIGNMENT or
HEAP_GRANULARITY (the two macros must be the same), which triggers an
error in the debug checker.

I fully agree that __alignof__((double)) is the right value for most
targets, and my patch was supposed to keep that behaviour.  I.e. the
patch should be a no-op for targets like Blackfin, but fixes those
weird targets like m68k where __alignof__((double)) < sizeof (size_t).

Richard

> -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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