[cfe-dev] AST Representation of Conversions

[Douglas Gregor]

On Jul 28, 2009, at 7:16 PM, Eli Friedman wrote:

> On Tue, Jul 28, 2009 at 4:00 PM, Douglas Gregor<dgregor at apple.com>  
> wrote:
>> ImplicitConversionSequence is quite heavy, and I don't think that
>> clients need that much information. It seems to me that we could get
>> away with adding an enum (covering all the kinds of conversions you
>> mentioned above) and a declaration (that points to a constructor or
>> conversion function). The Expr class already has enough spare bits to
>> store the enum, so CastExpr (and its descendents) would only have to
>> grow by a single pointer. That, IMO, is an acceptable trade-off,  
>> since
>> we'll be making CodeGen easier for C and possibly for C++.
>
> I can't see how a pointer which would always be null in C code could
> possibly make CodeGen easier for C code...

The enum makes CodeGen easier for C code, since we don't need to  
figure out what kind of conversion to perform. The constructor/ 
conversion function pointer, of course, won't help C at all.

> Note that we can always recalculate the appropriate declaration; it
> might make sense to put the relevant code into the AST so it can be
> shared between Sema and CodeGen.


It's expensive to recalculate, because you'll have to perform overload  
resolution again. That's far too much code to move into AST.

	- Doug