V_MUST_DEF vs V_MAY_DEF ?

[QiangHuang]

    "V_MUST_DEF" and "V_MAY_DEF" , The two tags appear in the IL of
SSA in GCC.I know they have something to do with the virtual
operand,however,I wonder that what the exact distinction between them
is.

    Corresponding to the 2 tags, is there  "V_MUST_USE" or "V_MAY_USE"?

    Thanks a lot~^_^