Could someone help explain what this C one liner does?

Question

I can usually figure out most C code but this one is over my head.

#define kroundup32(x) (--(x), (x)|=(x)>>1, (x)|=(x)>>2, (x)|=(x)>>4, (x)|=(x)>>8, (x)|=(x)>>16, ++(x))

an example usage would be something like:

int x = 57;
kroundup32(x);
//x is now 64

A few other examples are:

1 to 1
2 to 2
7 to 8
31 to 32
60 to 64
3000 to 4096

I know it's rounding an integer to it's nearest power of 2, but that's about as far as my knowledge goes.

Any explanations would be greatly appreciated.

Thanks

Answer 1

(--(x), (x)|=(x)>>1, (x)|=(x)>>2, (x)|=(x)>>4, (x)|=(x)>>8, (x)|=(x)>>16, ++(x))

1. Decrease x by 1
2. OR x with (x / 2).
3. OR x with (x / 4).
4. OR x with (x / 16).
5. OR x with (x / 256).
6. OR x with (x / 65536).
7. Increase x by 1.

For a 32-bit unsigned integer, this should move a value up to the closest power of 2 that is equal or greater. The OR sections set all the lower bits below the highest bit, so it ends up as a power of 2 minus one, then you add one back to it. It looks like it's somewhat optimized and therefore not very readable; doing it by bitwise operations and bit shifting alone, and as a macro (so no function call overhead).