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Re: Canonical type nodes, or, comptypes considered harmful
On Tue, 7 Nov 2006, Doug Gregor wrote:
> On 11/7/06, Dale Johannesen <dalej@apple.com> wrote:
> >
> > On Nov 7, 2006, at 11:47 AM, Douglas Gregor wrote:
> >
> > > I just read Nathan's discussion [1] on changing GCC's type system
> > > to use canonical type nodes, where the comparison between two types
> > > requires only a pointer comparison. Right now, we use "comptypes",
> > > which typically needs to do deep structural checks to determine if
> > > two types are equivalent, because we often clone _TYPE nodes.
> >
> > One difficulty is that compatibility of types in C is not transitive,
> > especially when you're compiling more than one translation unit at a
> > time.
> > See the thread "IMA vs tree-ssa" in Feb-Mar 2004. Geoff Keating and
> > Joseph Myers give good examples.
>
> In that discussion, there was mention of submitting a defect report to
> the C commit about this intransitivity. Has this defect report been
> resolved?
DR#314 still shows as open.
In C, there are several other sources of intransitivity, such as
incomplete array types (int[10] and int[5] both compatible with int[] but
not with each other), unprototyped functions and enums (two enums
compatible with the same integer type are not compatible with each other).
--
Joseph S. Myers
joseph@codesourcery.com