> 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.
> 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). 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.
--
Eric Botcazou
