On Wed, Jan 22, 2014 at 06:05:36PM -0800, Jonathan Nieder wrote:

> Jeff King wrote:
> > EWAH is a word-aligned compressed variant of a bitset (i.e. a data
> > structure that acts as a 0-indexed boolean array for many entries).
> I suspect that for some callers it's not word-aligned.

Yes, the mmap'd buffers aren't necessarily word-aligned. I don't think
we can fix that easily without changing the on-disk format (which comes
from JGit anyway). However, since we are memcpying the bulk of the data
into a newly allocated buffer (which must be aligned), we can do that
first, and then fix the endian-ness in place.

The only SPARC machine I have access to is running Solaris, but after
some slight wrestling with the BYTE_ORDER macros, I managed to get it to
compile and reproduced the bus error.

Here's a patch series (on top of jk/pack-bitmap, naturally) that lets
t5310 pass there. I assume the ARM problem is the same, though seeing
the failure in realloc() is unexpected. Can you try it on both your
platforms with these patches?

  [1/2]: compat: move unaligned helpers to bswap.h
  [2/2]: ewah: support platforms that require aligned reads

> Hopefully it's possible to get the alignment right in the caller
> and tweak the signature to require that instead of using unaligned
> reads like this.  There's still something wrong after this patch ---
> the new result is a NULL pointer dereference in t5310.7 "enumerate
> --objects (full bitmap)".

After my patches, t5310 runs fine for me. I didn't try your patch, but
mine are similar. Let me know if you still see the problem (there may
simply be a bug in yours, but I didn't see it).

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