Re: Ada subtypes and base types (was: Bootstrap failure on trunk: x86_64-linux-gnu)

[Jeffrey A Law <law at redhat dot com>]

On Mon, 2006-02-20 at 22:00 +0100, Richard Guenther wrote:
> On 2/20/06, Jeffrey A Law <law@redhat.com> wrote:
> > On Sun, 2006-02-19 at 20:43 +0100, Laurent GUERBY wrote:
> > > On Sun, 2006-02-19 at 14:23 -0500, Richard Kenner wrote:
> > > >     "Second, for a given integer type (such as
> > > >     natural___XDLU_0_2147483647), the type for the nodes in TYPE_MIN_VALUE
> > > >     and TYPE_MAX_VALUE really should be a natural___XDLU_0_2147483647.
> > > >     ie, the type of an integer constant should be the same as the type of
> > > >     its min/max values."
> > > >
> > > > No, the type of the bounds of a subtype should be the *base type*.  That's
> > > > how the tree has always looked, as far back as  I can remember.
> > >
> > > This is because intermediate computations can produce results
> > > outside the subtype range but within the base type range (RM 3.5(6)),
> > > right?
> > >
> > >  type T1 is range 0 .. 127;
> > >  -- Compiler will choose some type for T'Base, likely to be -128..127
> > >  -- but could be Integer (implementation dependant)
> > >  subtype T is T1 range 0 .. 100;
> > >  R : T := 100+X-X;
> > >  -- guaranteed work as long 100+X<=T'Base'Last and 100-X>=T'Base'First
> > Which leaves us with a very fundamental issue.  Namely that we can not
> > use TYPE_MIN_VALUE or TYPE_MAX_VALUE for ranges.  That's lame,
> > incredibly lame.  This nonsense really should be isolated within the
> > Ada front-end.
> 
> Indeed.  Ada should in this case generate
> 
>   R = (T)( (basetype)100 + (basetype)X - (basetype)X )
> 
> i.e. carry out all arithmetic explicitly in the basetype and only for stores
> and loads use the subtype.
I'd tend to agree, furthermore, if a pass starts wiping out those
type conversions, then we've got a bug.  I could believe that
such bugs exist as those conversions might be seen as useless
(particularly if the basetype and the real type differ only in
their TYPE_MIN_VALUE/TYPE_MAX_VALUE -- ie, they have the same
signedness and precision).  That case ought to be easy enough to
detect though.

Jeff