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


On Fri, Oct 10, 2014 at 10:38 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> 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?

Yes, that works for me.

Thanks,
Richard.

> --
> Eric Botcazou


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