On Fri, Oct 19, 2012 at 10:04 PM, Richard Smith <rich...@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.
-Chandler
PS: Sorry for the dup Clang folks, the GCC list doesn't like my mail client.
