The following patch adds support for bitfields to push_partial_def.
Previously pd.offset and pd.size were counted in bytes and maxsizei
in bits, now everything is counted in bits.

Not really sure how much of the further code can be outlined and moved, e.g.
the full def and partial def code doesn't have pretty much anything in
common (the partial defs case basically have some load bit range and a set
of store bit ranges that at least partially overlap and we need to handle
all the different cases, like negative pd.offset or non-negative, little vs.
bit endian, size so small that we need to preserve original bits on both
sides of the byte, size that fits or is too large.
Perhaps the storing of some value into a middle of existing buffer (i.e.
what push_partial_def now does in the loop) could, but the candidate for
sharing would be most likely store-merging rather than the other spots in
sccvn, and I think it is better not to touch store-merging at this stage.

Yes, I've thought about trying to do everything in place, but the code is
quite hard to understand and get right already now and if we tried to do the
optimize on the fly, it would need more special cases and would for gcov
coverage need more testcases to cover it.  Most of the time the sizes will
be small.  Furthermore, for bitfields native_encode_expr stores actually
number of bytes in the mode and not say actual bitsize rounded up to bytes,
so it wouldn't be just a matter of saving/restoring bytes at the start and
end, but we might need even 7 further bytes e.g. for __int128 bitfields.
Perhaps we could have just a fast path for the case where everything is byte
aligned and (for integral types the mode bitsize is equal to the size too)?

2020-02-24  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/93582
	* tree-ssa-sccvn.c (vn_walk_cb_data::push_partial_def): Consider
	pd.offset and pd.size to be counted in bits rather than bytes, add
	support for maxsizei that is not a multiple of BITS_PER_UNIT and
	handle bitfield stores and loads.
	(vn_reference_lookup_3): Don't call ranges_known_overlap_p with
	uncomparable quantities - bytes vs. bits.  Allow push_partial_def
	on offsets/sizes that aren't multiple of BITS_PER_UNIT and adjust
	pd.offset/pd.size to be counted in bits rather than bytes.
	Formatting fix.  Rename shadowed len variable to buflen.

	* gcc.dg/tree-ssa/pr93582-4.c: New test.
	* gcc.dg/tree-ssa/pr93582-5.c: New test.
	* gcc.dg/tree-ssa/pr93582-6.c: New test.
	* gcc.dg/tree-ssa/pr93582-7.c: New test.
	* gcc.dg/tree-ssa/pr93582-8.c: New test.