gcc complex constant folding

Question

It seems that gcc has some limitation on complex constant folding. Here is an example:

static inline unsigned int DJBHash(const char *str)
{
   int i;
   uns         


        

Answer 1

The OP is interested in constant-folding in C, but just for its C++ sibling: in C++14, you can simply put constexpr in front of both functions, and modify the loop to to compensate for strlen() not being constexpr

#include

static inline constexpr unsigned int DJBHash(const char *str)
{
   unsigned int hash = 5381;

   for(auto i = 0; i < 512; ++i) {
      if (*str == '\0') return hash;
      hash = ((hash << 5) + hash) + static_cast(*str);   
   }

   return hash;
}

constexpr unsigned int f(void)
{   
    return DJBHash("01234567890123456");
}

int main()
{
    constexpr auto h = f(); 
    std::cout << std::hex << h << "\n"; // 88a7b505
}

Live Example using Clang 3.4 SVN with -std=c++1y.

NOTE: the current Clang implementation does not properly run with a while(*str != '\0'). Instead, a finite loop of 512 with a return condition inside does the job.