[Bug middle-end/31309] 6 byte assignment at end of structure reads/writes past end of structure causing SEGV when that memory is not accessable.

["matz at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #18 from matz at gcc dot gnu dot org  2008-01-09 10:32 -------
It seems that this should work too: bitsize is 8 in this example IIRC, so
copy_mode would be byte_mode, which is what we need here.  Additionally using
word_mode when we can seems worthwhile too.

One thing would bother me, though: If TYPE_ALIGN(type) is very funny, then
bitsize is too, so that mode_for_size could return BLKmode, so word_mode would
be chosen to copy, which might then trip over the same problem.  But for that
TYPE_ALIGN must be so funny like 3 bits, i.e. something for which no INT mode
exists.  I believe it's impossible that TYPE_ALIGN is neither a multiple of
word_mode nor a number for which mode_for_size ever returns BLKmode (i.e.
either it returns BLKmode, but then only for very large aligns, which are
multiples of word_mode, or it will return something != BLKmode, which we then
can use).

IOW: it's impossible that TYPE_ALIGN ever is something like 3*BITS_PER_UNIT.

So, with that "argumentation" I think this would work, except for my probably
theoretical fear from comment #9, that the mode argument for extract_bit_field
is only a suggestion.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31309