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

On Thu, Jan 23, 2014 at 03:03:11PM -0500, Jeff King wrote:

quoted

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:

Actually, it is a little trickier than that. We actually take the
address in the macro. So even without inlining, we end up casting to
void. I still think this:

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

is a little more obvious, though. It does mean that everybody has to
pass a pointer, though, and on platforms where non-aligned reads are OK,
we do the cast ourselves. That means that:

  foo = align_ntohl(&bar);

will not be able to do any type-checking for "bar" (say, when we are
pulling "bar" straight out of a packed struct). I don't know how much
we care.

-Peff