Comment by eru
14 hours ago
> It is not on the list, but #define CMP(X, Y) (((X) > (Y)) - ((X) < (Y))) is an efficient way to do generic comparisons for things that want UNIX-style comparators. If you compare the output against 0 to check for some form of greater than, less than or equality, the compiler should automatically simplify it. For example, CMP(X, Y) > 0 is simplified to (X > Y) by a compiler.
I guess this only applies when the compiler knows what version of > you are using?
Eg it might not work in C++ when < and > are overloaded for eg strings?
My comment had been meant for C, but it should apply to C++ too even when operator overloading is used, provided the comparisons are simple and inlined. If you add overloads for the > and < operators in your string example to a place where they would inline, and the overload compares .length(), this should simplify. For example, godbolt shows that CMP(X, Y) == 0 is optimized to one mov instruction and one cmp instruction despite operator overloads when I implement your string example:
https://godbolt.org/z/nGbPhz86q
If you did not inline the operator overloads and had them in another compilation unit, do not expect this to simplify (unless you use LTO).
If you have compound comparators in the operator overloads (such that on equality in one field, it considers a second for a tie breaker), I would not expect it to simplify, although the compiler could surprise me.
The compiler would resolve that before the optimiser.