Do any C or C++ compilers optimize within define macros?

Question

Let's say I have the following in C or C++:

#include <math.h>
#define ROWS 15
#define COLS 16
#define COEFF 0.15
#define NODES (ROWS*COLS)
#define A_CONSTANT (COEFF*(sqrt(NODES)))

Then, I go and use NODES and A_CONSTANT somewhere deep within many nested loops (i.e. used many times). Clearly, both have numeric values that can be ascertained at compile-time, but do compilers actually do it? At run-time, will the CPU have to evaluate 15*16 every time it sees NODES, or will the compiler statically put 240 there? Similarly, will the CPU have to evaluate a square root every time it sees A_CONSTANT?

My guess is that the ROWS*COLS multiplication is optimized out but nothing else is. Integer multiplication is built into the language but sqrt is a library function. If this is indeed the case, is there any way to get a magic number equivalent to A_CONSTANT such that the square root is evaluated only once at run-time?

Answer 1

Macro definitions are expanded by simple textual substitution into the source code before it's handed to the compiler proper, which may do optimization. A compiler will generate exactly the same code for the expressions NODES, ROWS*COLS and 15*16 (and I can't think of a single one that will do the multiplication every time round the loop with optimization enabled).

As for A_CONSTANT, the fact that it is a macro again doesn't matter; what matters is whether the compiler is smart enough to figure out that sqrt of a constant is a constant (assuming that's sqrt from <math.h>). I know GCC is smart enough and I expect other production-quality compilers to be smart enough as well.

Answer 2

Anything in a #define is inserted into the source as a pre-compile step which means that once the code is compiled the macros have basically disappeared and the code is compiled as usual. Whether or not it is optimized depends on your code, compiler and complier settings.