On Sat, Oct 20, 2012 at 12:53 AM, 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. > > -Chandler > > PS: Sorry for the dup Clang folks, the GCC list doesn't like my mail client.
The way to solve this issue is: 1) to make most of the restrictions on constexpr functions evaporate. 2) resist the temptation of introducing a primitive to query the state of the optimizers. -- Gaby