Re: On VIEW_CONVERT_EXPRs and TBAA

[Eric Botcazou <ebotcazou at adacore dot com>]

> 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