On Oct 19, 2012, at 10:51 PM, Richard Smith <rich...@metafoo.co.uk> wrote:
>
> On Fri, Oct 19, 2012 at 10:50 PM, Chandler Carruth <chandl...@google.com>
> wrote:
> 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.
>
> I really like this approach.
I'd love to see this is in the pre-Bristol mailing with implementation
experience to back it up. :-) Nice job Richard and Chandler!
Howard
