Swap 2 Variables

This problem can be seen everywhere, and almost every people knows the solution. In this post, I'm going to summarize 3 functions for swap.

Function 1: naiveSwap

Function 2: xorSwap

Function 3: addSwap

Summary:

	naiveSwap	xorSwap	addSwap
extra variable	need	not need	not need
check \(x \ne y\) (address)	not need	need	need
readability	good	not good	not good
overflow	no problem	no problem	potential problem
aliasing	fine	complicated	complicated
execution model	instruction pipeline	strictly sequential	strictly sequential

Situations for XOR use in practice:

• On a processor where the instruction set encoding permits the XOR swap to be encoded in a smaller number of bytes;
• In a region with high register pressure, it may allow the register allocator to avoid spilling a register;
• In Microcontrollers where available RAM is very limited.

Because these situations are rare, most optimizing compilers don't generate XOR swap code.

Because of the problem of overflow, the add-swap approach is used less often.


References:

Wikipedia item: XOR swap algorithm