[cfe-commits] r148374 - in /cfe/trunk: lib/AST/ExprConstant.cpp test/SemaCXX/constexpr-strlen.cpp

[Richard Smith]

On Thu, January 19, 2012 19:56, Howard Hinnant wrote:
> On Jan 19, 2012, at 2:30 PM, Matthieu Monrocq wrote:
>> Le 19 janvier 2012 17:52, Jonathan Sauer <jonathan.sauer at gmx.de> a écrit :
>>>> Do you know if the same apply (I would guess so) to std::strlen ?
>>>>
>>>>
>>>> It seems a pity that such a trivial function could not be constexpr as
>>>> making it constexpr is actually dead simple. I wonder if it would be
>>>> worth a Defect Report.
>>>>
>>> This would seem very reasonable to me...
>>>
>>
>> As well as a lot (all?) of the math functions in cmath. And clang's
>> built-ins (__builtin_sin etc), too.
>>
>>
>> What do you think?
>> Jonathan
>>
>>
>> I haven't followed the C11 proposal, is the "constexpr" idea addressed
>> there or would it be a specific C++ issue ?
>>
>> C++ imports a massive amount of C functions, and keeping their signature
>> as-is will be a hindrance, I don't fancy the idea of recoding them all in
>> pure C++ to allow such optimizations! It seems like at least the `std::`
>> versions could be made `constexpr` when possible.
>>
>> I am putting Howard in copy since this seems something libc++ would be
>> interested in, and perhaps his C++ committee experience could help us here.
>>
>>
>> -- Matthieu
>>
>
> I have not come up to speed on constexpr.  However, I do know that if it is
> the case that no conforming program could detect (or behave differently --
> besides performance) if the constexpr is added to a C function, then it
> should be ok.

It is not the case; whether an expression is a constant expression can be
detected in a SFINAE context:


template<typename T> T declval();

template<typename T, T V> struct Value {
  constexpr static T value = value;
};

template<typename F, F f,
         typename ...A, A ...a,
         int = (f(a...), 0)>
constexpr bool isConstexpr(Value<F, f> fn, Value<A, a> ...args) { return true; }

template<typename ...T>
constexpr bool isConstexpr(...) { return false; }

int f(int);
constexpr int g(int n) { return n; }

#define VALUE(x) Value<decltype(x), x>()
constexpr bool test1 = isConstexpr(VALUE(&f), VALUE(0));
constexpr bool test2 = isConstexpr(VALUE(&g), VALUE(0));
static_assert(!test1, "");
static_assert(test2, "");


- Richard