On Sat, Oct 20, 2012 at 12:53 AM, Chandler Carruth <[email protected]> wrote:
> On Fri, Oct 19, 2012 at 10:04 PM, Richard Smith <[email protected]> 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
