[cfe-dev] C++11: new builtin to allow constexpr to be applied to performance-critical functions

[Andy Gibbs]

On Saturday, October 20, 2012 7:50 AM, Chandler Carruth wrote:
> [...snip...] Let me hypothesize a different interface:
>
> This stays the same...
> constexpr int constexpr_strncmp(const char *p, const char *q, size_t n) {
>  return !n ? 0 : *p != *q ? *p - *q : !*p ? 0 : constexpr_strncmp(p+1, 
> q+1, n-1);
> }
>
>
> But here we do something different on the actual declaration:
> [[constexpr_alias(constexpr_strncmp)]]
> int strncmp(const char *p, const char *q, size_t n);
>
> When parsing the *declaration* of this function, we lookup the function
> name passed to constexpr_alias. We must find a constexpr function with an
> identical signature. Then, at function invocation substitution of strncmp,
> we instead substitute the body of constexpr_strncmp.
>
> This seems more direct (no redirection in the code), and it also provides
> a specific advantage of allowing this to be easily added to an existing
> declaration in a declaration-only header file without impacting or
> changing the name of the runtime executed body or definition.

I'd be very happy with this solution.  I come across precisely the problem
raised by Richard on a very regular basis and have different workarounds
for both gcc and clang.  I'd love to see something "standard" emerging!

For my side, I'd still like some way of declaring a function to be used
only in a constexpr environment, meaning that the compiler gives an error
up front when a function is then used in a non-constexpr environment.  The
above proposal will provide a link-time error if the non-constexpr function
is not defined, which is half-way there.  Perhaps using the "unavailable"
attribute in conjunction with "constexpr_alias" would be the compile-time
solution...

Cheers

Andy