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

[Richard Smith]

On Thu, January 19, 2012 16:52, Jonathan Sauer wrote:
>>> 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.

We can support __builtin_* in constant expressions as a conforming extension;
this is already how we behave for all the __builtin_*s providing C library
functions which we can constant-fold (namely, __builtin_strlen,
__builtin_huge_val*, __builtin_inf*, __builtin_nan*, __builtin_fabs* and
__builtin_copysign*).

If you want to add more such builtins to this list, the first step is to
implement the corresponding operation for llvm::APFloat. Support in clang
should then be trivial to add -- it'd also be great to add constant folding to
llvm for such cases.

Constant-folding (non-__builtin_)strlen can theoretically break conforming
code and is likely to lead to people accidentally writing non-portable code in
practice, and I've not found a clause in C++11 allowing the implementation to
mark more functions in the library as constexpr, so I'm reluctant to support
it. If you want to be non-portable, use __builtin_strlen.

- Richard