I've seen this function
M2 = tmp ⊕ Pi. What does the circle with cross do?
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In logic and CompSci, $\oplus$ is used to denote the "exclusive or" or "XOR": $\quad x \lor y \land \lnot(x \land y)$.
In set theory, $\oplus$ denotes the disjoint union.
In linear algebra/vector analysis, it's used to denote the direct sum of two vector spaces.
It's also used to denote parity: see P Parity.
Clearly, the context in which it's used is crucial.
In $LaTeX$: use
$\oplus$ is often used for Directed Sum.
In Computer Science(especially cryptography), $\oplus$ usually means XOR, or bitwise addition.