Re: [patch] Fix miscompilation of gnat1 in LTO bootstrap

[Eric Botcazou <ebotcazou at adacore dot com>]

> I can't see how this can work with LTO.  We need a middle-end way
> to represent the alias relation of those types.  At least I can't see how
> your simple patch covers all cases here?

It covers what I think is the most prominent case (unconstrained array types), 
the other cases are far less annoying in practice.

> With LTO we preserve TYPE_ALIAS_SET == 0, so another way to
> fix this (and which I'd like more) is to do your patch in the Ada frontend,
> that is, use alias-set zero for all types you relate if flag_lto.

Really Big Hammer if you ask me.

> Another way is to make LTO canonical type merging handle the
> case of type_contains_placeholder_p "better", that is by treating
> two types with those equivalent more easily.  For arrays this simply
> means hashing and comparing non-constant TYPE_DOMAIN the
> same / as equal.  There is already some code handling PLACEHODER_EXPR
> special, but it doesn't seem to be enough (why in this case)?

Yes, gimple_canonical_types_compatible_p will return true for a couple of 
unconstrained (i.e. type_contains_placeholder_p) array types with the same 
component type.  The problem here is for an unconstrained array type and a 
constrained (regular) array type with the same component type.  Can I modify 
gimple_canonical_types_compatible_p to return true in that case as well?

-- 
Eric Botcazou