Re: Builtins in `std'

[Gabriel Dos Reis <gdr at codesourcery dot com>]

Mark Mitchell <mark@codesourcery.com> writes:

|   Those are driven by the necessacity of having ::fabs and std::fabs
|   referring to the same entity (ideally, the one provided by the C
|   library). 
| 
| I understand.  I expected that you would handle this in V3 by doing:
| 
|   namespace std {
|     extern "C" double fabs (double);
|   }
| 
| Then, since you probably want to take advantage of the builtin
| optimizations:
| 
|   namespace std {
|     extern "C" inline double fabs (double d) 
|       { return __builtin_fabs (d); }
|   }

OK.  But since we're violating the ODR here, is there a chance that
a user notices it? (I'm asking because I'm not sure)

| ... Note that there is no `#include <math>' here;

Ok, got it.

|   However, we need to ensure that `&::fabs == &std::fabs' evaluates to
|   true.  On the other hand we have to overload fabs() in std:: only.
| 
| Right.  Does my trick above work?

Yes, it is fine if can tell me that the definition provided by the C
library won't clash with the above.

|   | So, any objections to just not doing the insertion into `std'?
| 
|   Yes. #include <cmath> puts fabs in std:: only.
| 
| I don't think this is relevant -- but I could be wrong.  I *think*
| we're talking at cross-purposes.

Yes, I overlooked the <math.h> missing part.

| In particular, I'm trying to fix a regression on code like:
| 
|   extern "C" double fabs (double) {}
| 
|   namespace std { 
|     double fabs (double) {}
|   } 
| 
| (A user is allowed to do this; they might have a different library
| implementation or something.)

The above is not allowed.

| These two functions get the same mangled name.  The reason boils down
| to the fact that the compiler implicitly declared `std::fabs', rather
| than leaving that to the (most excellent) V3 headers to do.
| 
| To sum up, I think there's a different between C and C++.  In C, we
| want to make `fabs' a builtin, because uesrs are allowed to call it
| without including the relevant headers. 

100% agreed.

Furthermore, in C++ users are not allowed to add overloads in std::,
and since fabs() may have C-linkage, they are not allowed to do the
trick above.

| ... If we're going to optimize
| it, we need to make it a builtin.  However, in C++ you have to include
| the header first, or you're not writing legal C++.  And, that means
| that the compiler doesn't need to predeclare the function.
| 
| Does that make more sense now?

Yes, I now understand your point. But I don't think users are allowed
to do the example you're trying to fix.  
Or am I still talking about a different thing?

-- Gaby