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.
What do you do for member functions? Name the member with or without
qualification? Can the constexpr version be private?
Jeffrey
