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Re: Ada subtypes and base types
- From: Laurent GUERBY <laurent at guerby dot net>
- To: Waldek Hebisch <hebisch at math dot uni dot wroc dot pl>
- Cc: Jeffrey A Law <law at redhat dot com>, gcc at gcc dot gnu dot org
- Date: Tue, 28 Feb 2006 21:28:37 +0100
- Subject: Re: Ada subtypes and base types
- References: <E1FDnj3email@example.com>
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.
> 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
> 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