[cmake-developers] [PATCH] Macro generation for relaxed constexpr

[Stephen Kelly]

Jean-Michaël Celerier wrote:

>> I think there should be a test for the different allowed contexts of the
> ${prefix_arg}_RELAXED_CONSTEXPR and ${prefix_arg}_CONSTEXPR macros. Could
> you extend Tests/Module/WriteCompilerDetectionHeader with a test for that?
> 
> For sure, I'll do this asap.
> 
>> constexpr foo = ...;
> 
> If I read http://en.cppreference.com/w/cpp/language/constexpr
> correctly, there should be no differences in variable handling between
> C++11 and 14 for constexpr, the changes are only for function bodies.

Right.

Qt 5 provides a macro for this context which expands to either 'const' or 
'constexpr' depending on whether cxx_constexpr is available, and another 
macro which expands to either 'const' or 'constexpr' depending on whether 
cxx_relaxed_constexpr is available. 

Compare the assembly of the following when compiled with -std=c++98/11/14:

  #if __has_feature(cxx_constexpr)
  #define DECL_CONSTEXPR constexpr
  #else
  #define DECL_CONSTEXPR
  #endif

  #if __has_feature(cxx_relaxed_constexpr)
  #define DECL_RELAXED_CONSTEXPR constexpr
  #else
  #define DECL_RELAXED_CONSTEXPR
  #endif

  DECL_CONSTEXPR int getNumRegular()
  {
    return 42;
  }

  DECL_RELAXED_CONSTEXPR int getNumRelaxed()
  {
    int result = 0;
    for (int i = 0; i < 4; ++i)
      result += 10;
    return result + 2;
  }

  DECL_CONSTEXPR int betterIfConstexprParameter(int param)
  {
    return param / 2;
  }

  #if __has_feature(cxx_constexpr)
  #define CXX11_CONSTEXPR_VARIABLE constexpr
  #else
  #define CXX11_CONSTEXPR_VARIABLE const
  #endif

  #if __has_feature(cxx_relaxed_constexpr)
  #define CXX14_CONSTEXPR_VARIABLE constexpr
  #else
  #define CXX14_CONSTEXPR_VARIABLE const
  #endif

  int main()
  {
    CXX11_CONSTEXPR_VARIABLE int num1 = getNumRegular();
    CXX11_CONSTEXPR_VARIABLE int result1 = betterIfConstexprParameter(num1);
    CXX14_CONSTEXPR_VARIABLE int num2 = getNumRelaxed();
    CXX14_CONSTEXPR_VARIABLE int result2 = betterIfConstexprParameter(num2);
    return result1 - result2;
  }


$ wc -l 98.s 11.s 14.s 
 126 98.s
  90 11.s
  31 14.s


Of course, such macros don't belong in the same patch as the one you 
submitted and could be in a follow-up.

Thanks,

Steve.