Add VIEW_CONVERT_EXPR to operand_equal_p

[Eric Botcazou]

> Why is Ada fliddling with the modes? Is it only for packed structures?

Yes, in Ada packing or representation clauses are allowed to modify the type 
of components, so you can have e.g. a record type with size S1 and BLKmode and 
fields of this type with a packed version of this record type (with size S2<S1 
and integer mode) and assignments between the 2 views of the type done through 
VCEs.  Moreover, in Ada, packing or representation clauses are really part of 
the language and used quite a lot so you end up with extensive type fiddling.

> I was wondering how to produce VCE convesions of aggregates with C frontend
> at all (that is getting them synthetized by the middle-end) to get non-ada
> testcases.  Storing through union is never folded to one and I don't see
> any other obvious way of getting them.  Perhaps it may be possible to get
> them via inliner on incompatible parameter and LTO, but that seems to be
> the only case I can think of right now.

That makes sense, all the machinery implementing type fiddling for the Ada 
compiler is in gigi, not in stor-layout.c for example.

> I am testing the change to compare modes and revert the two expr.c changes.
> Lets see what is Richard's opinion. The whole concept of modes on aggregate
> types is bit funny post-tree-ssa days when we do SRA. I suppose they may be
> tied to calling conventions but should no longer be needed for code quality?

Ideally it should not be tied to calling conventions either, but it is known 
that some back-ends still use it for this purpose.

-- 
Eric Botcazou