Could type punning signed to unsigned integers make bounds checking faster by eliminating the need for >= comparison?

Question

Say I had a really performance-critical loop in my program where I need to check if a point was inside a rectangle, but I know at compile time that the lower bounds are always going to be 0, like the following: (x >= 0 && y >= 0 && x < width && y < height)

Could I eliminate the first two comparisons by type-punning x and y to unsigned integers (for instance with something like reinterpret_cast<>() or a union in C++), since the sign bit would guarantee any negative number would turn into an unsigned int large enough to fail the bounds check? If so, how would you go about implementing this in C++ or another language? Could you gain any performance improvement by doing this?

Answer 1

Yes, it's a perfectly valid optimization when you're testing a signed integer and the lower bound is zero. In fact it's such a common optimization that your compiler will almost certainly do it automatically; obfuscating your code by doing it yourself is very likely to be a pointless premature optimization.

I just tested this on GCC 4.9, and confirmed by inspecting the generated assembly code that it performs this optimization automatically at -O1 and above. I would expect all modern compilers to do the same.