Dale Johannesen 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.
For example:
http://gcc.gnu.org/ml/gcc/2004-02/msg01462.html
However, I still doubt that this is what the C committee actually
intended.
Transitivity of type equivalence is fundamental in every type system
(real and theoretical) with which I'm familiar. In C++, these examples
are not valid because the ODR, and, IIRC, in C you cannot produce them
in a single translation unit -- which is the case that most C
programmers think about. So, I'm of the opinion that we should discount
this issue.
I do think that canonical types (with equivalence classes, as Doug
suggests) would be a big win, for all of the reasons he suggests. We
have known for a long time that comptypes is a bottleneck in the C++
front end, and while some of that could be solved in other ways, making
it a near-free operation would be a huge benefit.
--
Mark Mitchell
CodeSourcery
[EMAIL PROTECTED]
(650) 331-3385 x713
