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


On Tue, Oct 7, 2014 at 10:04 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> Testcase?  I think it would be better to handle this in the canonical type
>> merging code in lto.c - or how does it end up working without LTO?  That is,
>> what does the Ada frontend do to make sure get_alias_set handles this
>> correctly?
>
> It manages the alias sets, see gcc-interface/utils.c:relate_alias_sets.

Ugh :/

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?

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.

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)?

Thanks,
Richard.

> --
> Eric Botcazou


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