Re: [objc-improvements] Just remembered something about multiple method signatures...

[Ziemowit Laski]

On Saturday, Sep 6, 2003, at 00:54 US/Pacific, David Ayers wrote:

> Ziemowit Laski wrote:
>
> > I guess my point is that arbitrarily picking a signature out of a pool
> > of contenders is fraught with problems, and I still believe that
> > the IMP fallback is a cleaner solution in this case.  I don't think
> > doing this breaks (codegen-wise) the counterexample you guys keep
> > discussing (i.e., where all the candidate methods return 
> > NSComparisonResult),
> > does it?
>
> It doesn't on my architecture and probably not on the G4, but it would 
> on 64 bit archs where small enums (i.e. NSComparisionResult) are not 
> the same size as id (I'm not sure how the G5 handles small enums).  In 
> fact it would probably turn into an outright compile time error.  I 
> just liked that example because it wasn't constructed.  (i.e. NSDate, 
> NSString and NSNumber have different prototypes for '-compare:'.)
>
> But maybe I can offer a more constructed example based on the 
> promotions caused by IMP that shows the issue on all architecures and 
> that given the intensive use of float with NSView subclasses in 
> OPENSTEP derivatives, make it a valid one.  I've attached an excerpt 
> of a test we're preparing for contribution.
>
> Given
> Class1: -(id)test1Float:(float)f int:(int)i;
> Class2: -(Object *)test1Float:(float)f int:(int)i;
>
> In the old case, you could get a warning about the compiler picking 
> the wrong prototype but the results were correct.

In this particular case, the compiler picked a method signature out of 
a hat that happened to work in that the intended codegen occurred.  But 
just because the "results were correct" in this case does not make the 
compiler correct. :-(  In fact, all you need to do here is add

  Class3: -(void)test1Float:(float)f int:(int)i;

to see just how "incorrect" things can get. :-)

>  But the new IMP promotion for id typed receivers will promote them to 
> doubles and clobber not only of the float but all following arguments 
> as well

I absolutely agree with you in that using the IMP prototype can also 
result in "bad" (i.e., unintended) codegen.  If you're given a set of 
competing method signatures, each of them with conflicting codegen 
requirements, there is absolutely no way you can win here. :-(  In the 
end, the user will simply have to type the receiver more strongly 
(possibly by casting it at the call site) to pick the intended 
signature, and I really don't see any other way out of this predicament.

So why did I bother with this IMP thing here?  There are two reasons:
  (1) I wanted a consistent failure mode.  In your test1Float:int: 
example above, you see how the compiler may wind up picking a different 
signature as more and more test1Float:int: methods are added to the 
mix.  The signature that gets picked may happen to "work" in some cases 
and not work in others (due to the float<->int promotions that both you 
and I discussed).  Of course, the IMP signature may also either work or 
not work :-); but at least now, it will do so consistently as new 
incarnations of test1Float:int: are pulled in from who knows where.  So 
choosing IMP preserves consistency of the outcome regardless of the 
number/kind of conflicting signatures chosen.  In addition to being 
"internally" consistent, choosing IMP here is consistent with what the 
compiler already does when there are _no_ method signatures available, 
which leads to the following elegant end-user pronouncement:

   Whenever the compiler cannot statically determine the argument types 
or return type of a method being called, it shall use the
   generic messenger signature (IMP) provided by the runtime system.

Obviously, we could wordsmith the foregoing some more, but as an 
end-user of the language, I'd love to have a fallback mechanism that is 
as simple, elegant and consistent as this.

  (2) After making its method signature selection, the compiler should 
be able to proceed as gracefully as possible.  These are, after all, 
warnings rather than errors.  However, if a signature with return type 
'void' is picked, the compiler will subsequently issue a hard error if 
the program attempts to use the return value of the message.  A milder 
version of this occurs when the compiler produces a string of warnings 
about incompatible pointer types of arguments; these warnings are 
purely a result of the fact that there was a conflict to begin with, 
and tend to confuse users (as evidenced by bug reports filed against 
Apple's gcc).  So again, the main theme here is consistency of 
experience: the compiler encountered a situation not to its liking, but 
should at least resolve it in the same fashion each time.

> I think that your suggestion about itteratively comparing the return 
> value and then the arguments before falling back to varadic processing 
> might be a good solution, as long as 'compatible' return 
> values/arguments are "merged" to a general type (such as id/class) 
> before falling back to varadic processing.  (The warnings should 
> remain, and if you like, with a flag to turn them off :-) )

Doing this will increase the probability (vs. both the current and the 
IMP approaches) that the compiler will happen to do "the right thing" 
(codegen-wise) when encountering conflicts, although of course it won't 
make it a certainty -- no scheme can do that.  The down side is that 
this scheme lacks the simple elegance (and hence end-user 
understandability) of the IMP approach. :-(

--Zem
--------------------------------------------------------------
Ziemowit Laski                 1 Infinite Loop, MS 301-2K
Mac OS X Compiler Group        Cupertino, CA USA  95014-2083
Apple Computer, Inc.           +1.408.974.6229  Fax .5477