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Re: On VIEW_CONVERT_EXPRs and TBAA
On Sat, 26 Sep 2009, Eric Botcazou wrote:
> > 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).
>
> I presume you're talking exclusively about the addressable case here, because
> in Ada the alias set of (the type of) the outermost component is not a subset
> of (that of) the innermost one by default (at least for scalar types, it is
> for aggregate types because of implementation limitations), there being a VCE
> or not; the field must explicitly be declared addressable.
Yes, addressable in the way that get_alias_set would not skip the
component as component_uses_parent_alias_set.
> > 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.
>
> The Ada compiler generates a lot of VIEW_CONVERT_EXPRs for conversions between
> aggregate (sub)types because it generates a lot of aggregate subtypes from a
> given aggregate type. These conversions are purely formal and their purpose
> is to ensure type consistency. The Ada compiler also uses VIEW_CONVERT_EXPR
> to implement up-casting
>
> type Base is tagged record
> I : Integer;
> end record;
>
> type Derived is new Base with record
> J : Integer;
> end record;
>
> D : Derived;
> Base (D).I
>
> is translated into VIEW_CONVERT_EXPR<base>(d).i (a questionable use of VCE, no
> doubt about that).
C++ for this case has a FIELD_DECL in Derived of type Base, so the
middle-end sees d.<anonymous>.i here (and thus also automatically
gets the TBAA hierarchy correct by means of recording component aliases).
The Ada way doesn't look too obscure to me.
> There are a few other, more obscur uses, but we get rid
> of the most controversial one (optimization barrier on scalars for VRP).
>
> The main countermeasure to ensure consistent TBAA is decl.c:relate_alias_sets
> which is aimed at doing the right thing when types are VCEd to each other.
Ok, thanks.
Richard.