- /// IsUndef - True if this is a register def / use of "undef", i.e. register
- /// defined by an IMPLICIT_DEF. This is only valid on registers.
+ /// IsUndef - True if this register operand reads an "undef" value, i.e. the
+ /// read value doesn't matter. This flag can be set on both use and def
+ /// operands. On a sub-register def operand, it refers to the part of the
+ /// register that isn't written. On a full-register def operand, it is a
+ /// noop. See readsReg().
+ ///
+ /// This is only valid on registers.
+ ///
+ /// Note that an instruction may have multiple <undef> operands referring to
+ /// the same register. In that case, the instruction may depend on those
+ /// operands reading the same dont-care value. For example:
+ ///
+ /// %vreg1<def> = XOR %vreg2<undef>, %vreg2<undef>
+ ///
+ /// Any register can be used for %vreg2, and its value doesn't matter, but
+ /// the two operands must be the same register.
+ ///