Re: [PATCH v4 08/23] ewah: compressed bitmap implementation

[Jeff King <peff@peff.net>]

On Thu, Jan 23, 2014 at 11:52:06AM -0800, Jonathan Nieder wrote:

> > 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).
> 
> I had left out a cast to unsigned, producing an overflow.
> 
> My main worry about the patches is that they will probably run into
> an analagous problem to the one that v1.7.12-rc0~1^2~2 (block-sha1:
> avoid pointer conversion that violates alignment constraints,
> 2012-07-22) solved.  By casting the pointer to (uint32_t *) we are
> telling the compiler it is 32-bit aligned (C99 section 6.3.2.3).

Yeah, maybe. We go via memcpy, which takes a "void *", so that part is
good. However, the new code looks like:

  foo = align_ntohl(*(uint32_t *)ptr);

I think this probably works in practice because align_ntohl is inlined,
and any sane compiler will never actually load the variable. If we
change the signature of align_ntohl, we can do this:

  uint32_t align_ntohl(void *ptr)
  {
          uint32_t x;
          memcpy(x, ptr, sizeof(x));
          return ntohl(x);
  }

  ...

  foo = align_ntohl(ptr);

The memcpy solution is taken from read-cache.c, but as we noted, it
probably hasn't been used a lot. The blk_sha1 get_be may be faster, as
it converts as it reads. However, the bulk of the data is copied via
a single memcpy and then modified in place. I don't know if that would
be faster or not (for a big-endian system it probably is, since we can
omit the modification loop entirely).

-Peff