GNU Compiler Collection (GCC) Internals: Define Subst Pattern Matching

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#### 16.21.2 Pattern Matching in `define_subst`

All expressions, allowed in `define_insn` or `define_expand`, are allowed in the input-template of `define_subst`, except `match_par_dup`, `match_scratch`, `match_parallel`. The meanings of expressions in the input-template were changed:

`match_operand` matches any expression (possibly, a subtree in RTL-template), if modes of the `match_operand` and this expression are the same, or mode of the `match_operand` is `VOIDmode`, or this expression is `match_dup`, `match_op_dup`. If the expression is `match_operand` too, and predicate of `match_operand` from the input pattern is not empty, then the predicates are compared. That can be used for more accurate filtering of accepted RTL-templates.

`match_operator` matches common operators (like `plus`, `minus`), `unspec`, `unspec_volatile` operators and `match_operator`s from the original pattern if the modes match and `match_operator` from the input pattern has the same number of operands as the operator from the original pattern.