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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 /docs
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.
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