编译器是否避免中间整体推广或转换？

Question

Consider a class like:

struct mystruct 
{
    constexpr operator char() {return x;}
    signed char x;
};

and an operation like:

mystruct m;
m.x = /* something at runtime */
int i = 3 * m + 45ULL * m;

Are compilers able to skip the temporary conversion to char and convert directly m to the required type in the 3 * m + 45ULL * m expression?

Answer 1

Seems like GCC version 5.3.0 is able to optimize the call to cast function, while Clang 3.7 is not as smart.

For this piece of code:

struct mystruct 
{
    constexpr operator char() const {return x;}
    signed char x;
} m;

void func(const int num) {
  m.x = num*2;
  int i = 3 * m + 45ULL * m;
}

you can check compiled assembly and compare them:

Clang with cast vs Clang with direct reference to field

Gcc with cast vs Gcc with direct reference to field

Though in a slightly different situation Clang does manage to optimize the call to cast function.