[PATCH] Silence misguided GCC warning about alignment issues

[Johannes Schindelin]

Hi Denys,

On Mon, 7 Aug 2017, Denys Vlasenko wrote:

> On Mon, Aug 7, 2017 at 12:09 PM, Johannes Schindelin
> <johannes.schindelin at gmx.de> wrote:
> > When compiling xz_dec_stream.c with GCC 7.1.0, it complains thusly:
> >
> >         In function 'dec_stream_footer':
> >         error: dereferencing type-punned pointer will break strict-aliasing
> >               rules [-Werror=strict-aliasing]
> >            if (xz_crc32(s->temp.buf + 4, 6, 0) != get_le32(s->temp.buf))
> >                ^~
> >
> > The thing is, the `buf` field was put just after two fields of type
> > size_t for the express purpose of avoiding alignment issues, as per the
> > comment above the `temp` struct.
> >
> > Meaning: GCC gets this all wrong and should not complain. So let's force
> > it to be quiet for a couple of lines.
> 
> xz_crc32 is:
> 
> xz_crc32(const uint8_t *buf, size_t size, uint32_t crc)
> 
> I think gcc is not complaining about xz_crc32,
> it complains about get_le32!
> 
> Can you test this theory by splitting that statement in two?

I, too, thought at first that it is clearly about get_le32(), but GCC was
really adamant that it was the xz_crc32() call.

And a little further investigation suggests that GCC really meant
xz_crc32(), which is defined as a static function in
archival/libarchive/decompress_unxz.c that gets inlined by GCC because it
simply hands off to crc32_block_endian0() which in turn takes an
`uint32_t *` as first parameter. And that's where the type-punning is
broken.

Ciao,
Johannes