CODEWITHSUNDEEP
c
Danijel
Danijel

Reputation: 8616

Macro evaluation at compile time?

#define CONVERT(val, qin, qout) ((qin > qout) ? (val >> (qin-qout) ) : (val << (qout-qin)))

void f(void)
{
    ...
    y = CONVERT(x, 25, 31);
}

Is the above macro going to be evaluated by GCC at compile time? I mean the (qin > qout) part. I am trying to have only y = x << (31-25) after the compilation is done.

Upvotes: 0

Views: 336

Answers (1)

Lundin
Lundin

Reputation: 213711

The qin > qout part will end up as an integer constant expression in this case, which the compiler can optimize into 1 or 0 in theory.

In practice, it doesn't even do that. Looking at how real compilers actually treats your example, gcc and clang at -O0 (optimizations off) only generates the code for the left shift, omitting the conditional check and branch of ?:, as well as the second operand which is never true.

The machine code ends up boiling down to the equivalent of y = x << 6;, completely branch free.

Upvotes: 4

Related Questions