Re: Ada subtypes and base types

[Laurent GUERBY <laurent at guerby dot net>]

On Mon, 2006-02-27 at 20:08 +0100, Waldek Hebisch wrote:
> Jeffrey A Law wrote:
> > My suspicions appear to be correct.  This never triggers except for
> > Ada code and it's relatively common in Ada code.  No surprise since
> > I don't think any other front-end abuses TYPE_MAX_VALUE in the way
> > the Ada front-end does.  This wouldn't be the first time we've had
> > to hack up something in the generic optimizers to deal with the
> > broken TYPE_MAX_VALUE.
> 
> What do you mean by "abuse"?  

I'd like to know too :). Both Pascal and Ada came up with the same
representation of ranged types to the back-end, if anything else
should be used, by all mean let us know the proposal.

Laurent


> TYPE_MAX_VALUE means maximal value
> allowed by given type.  For range types it is clearily the upper
> bound of the range.  Of course, upper bound should be representable,
> so TYPE_MAX_VALUE <= (2**TYPE_PRECISION - 1) for unsigned types
> and TYPE_MAX_VALUE <= (2**(TYPE_PRECISION - 1) - 1) for signed types.
> However, if the language has non-trivial range types you can expect
> strict inequality.  Note, that if you do not allow strict inequality
> above, TYPE_MAX_VALUE would be redundant.
> 
> FYI GNU Pascal is using such representation for range types, so for
> example:
> 
> type t = 0..5;
> 
> will give you TYPE_PRECISION equal to 32 (this is an old decision
> which tries to increase speed at the cost of space, otherwise 8
> would be enough) and TYPE_MAX_VALUE equal to 5.
> 
> GNU Pascal promotes arguments of operators, so that arithmetic take
> place in "standard" types -- I belive Ada is doing the same.
> 
> BTW, setting TYPE_PRECISION to 3 for the type above used to cause
> wrong code, so the way above was forced by the backend.
> 
> If you think that such behaviour is "abuse" then why to have sparate
> TYPE_MAX_VALUE. How to represent range types so that optimizers
> will know about allowed ranges (and use them!)? And how about debug
> info?
>