On VIEW_CONVERT_EXPRs and TBAA

[Richard Guenther <rguenther@suse.de>]

While looking at PR38747 again I was diving into the alias.c code
and tried to confirm that it does what I think it does for 
VIEW_CONVERT_EXPR.  And in fact it basically does.

In general there are odds between what get_alias_set does if you
pass it a reference tree compared to what it does if you pass it
a type.  In particular whether it returns the alias-set of the
innermost or the outermost aliased component is not really consistent.

But - onto that VIEW_CONVERT_EXPR.  VIEW_CONVERT_EXPR is basically
ignored / skipped, which means that if you have C++ code that does
*(type2 *)&X and translate it to VIEW_CONVERT_EXPR <type2> (X) then
you have generated wrong code (this is PR38747, forwprop does that).

With VIEW_CONVERT_EXPR you can also easily create the situation
where for a reference tree, let it be VIEW_CONVERT_EXPR <T2> (X.a).b
like commonly seen in Ada, the alias-set of the outermost component
is not a subset of that of the innermost one (the relationship that
is usually assured to be true by the record_component_aliases
machinery).  You are probably safe here if only your frontend generates
such conversions and it is very consistent on how it balances
V_C_Es with accesses through pointers (because in the above case
if .b is an addressable component an access via a pointer to type
of b wouldn't necessarily alias the cited reference tree).

The alias-oracle tries to use both the outermost and innermost
alias-sets for disambiguations, but relies on the outermost alias-set
being a subset of the innermost one - a relationship that V_C_E can
break.

So, what I'd like to know is in which circumstances the Ada
frontend uses VIEW_CONVERT_EXPRs and what counter-measures it
applies to ensure consistent TBAA.

Thanks,
Richard.