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

[Chandler Carruth]

On Fri, Oct 19, 2012 at 10:04 PM, Richard Smith <richard at metafoo.co.uk>wrote:

> [Crossposted to both GCC and Clang dev lists]
>
> Hi,
>
> One issue facing library authors wanting to use C++11's constexpr feature
> is that the same implementation must be provided for both the case of
> function invocation substitution and for execution at runtime. Due to the
> constraints on constexpr function definitions, this can force an
> implementation of a library function to be inefficient. To counteract this,
> I'd like to propose the addition of a builtin:
>
>   bool __builtin_constexpr_p()
>
> This builtin would only be supported within constexpr function
> definitions. If the containing function is undergoing function invocation
> substitution, it returns true. Otherwise, it returns false. Hence we can
> implement library functions with a pattern like:
>
>   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);
>   }
>   __attribute__((always_inline)) constexpr int my_strncmp(const char *p,
> const char *q, size_t n) {
>     return __builtin_constexpr_p() ? constexpr_strncmp(p, q, n) :
> strncmp(p, q, n);
>   }
>
> Does this seem reasonable?
>

Yes, especially the primary functionality. However, I have some concerns
about the interface. 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.
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