[cxx-abi-dev] Mangling of function-to-pointer conversion

[Jason Merrill]

On 03/04/2011 12:17 PM, Jason Merrill wrote:
> A related case:
>
> template<typename T> int cmp1(T a, T b);
> template<typename T> int cmp2(T a, T b);
> template<typename T, int (*cmp)(T, T)> struct A { };
> template <typename T> void f (A<T,cmp1> &);
> void g()
> {
>   A<char,cmp1> a;
>   f(a);
> }
>
> How do we mangle 'cmp1' in the signature of f? The ABI doesn't say
> anything specific about how to mangle unresolved overloading.
>
> G++ currently crashes on this testcase, but some obvious fixes lead it
> to generate "4cmp1", i.e. <unresolved-name>, which seems consistent with
> the mangling in a dependent call.

> EDG 4.2 mangles it as adL_Z4cmp1, which doesn't seem to match anything
> in the spec; it seems to be treating the cmp1 template as an extern "C"
> function.

Clang also produces 4cmp1 for this testcase.  Curiously, if I adjust the 
testcase to

template<typename T> int cmp1(T a, T b);
int cmp2(char a, char b);
template<typename T, int (*cmp)(T, T)> struct A { };
template <typename T> void f (A<T,cmp1> &);
template <typename T> void f (A<T,cmp2> &);
void g()
{
   A<char,cmp1> a;
   f(a);
   A<char,cmp2> a2;
   f(a2);
}

For the second F I get three different results from the three compilers:

EDG gives _Z1fIcEvR1AIT_XadL_Z4cmp2ccEEE, which is consistent with its 
treatment of the cmp1 case (i.e. adding the unwritten address operation).

G++ with my patch gives _Z1fIcEvR1AIT_L_Z4cmp2ccEE, treating cmp2 as a 
simple literal.

Clang gives _Z1fIcEvR1AIT_XL_Z4cmp2ccEEE, wrapping the literal in the 
X/E expression delimiters.  This seems like a bug.

> I tried to create an analogous dependent call to see what it
> would do in that case:
>
> template <class T, int I> struct B { };
> template <class T> void h(B<T,sizeof(cmp1(T(),T()))>);
>
> void i()
> {
>   B<int,sizeof(int)> b;
>   h(b);
> }
>
> but EDG 4.2 rejects this testcase.

EDG 4.3 produces _Z1hIiEv1BIT_Xszcl4cmp1cvS1__EcvS1__EEEE for this 
testcase, as do GCC and Clang.

Jason