### 17.2 Everything about Instruction Patterns

A `define_insn` expression is used to define instruction patterns to which insns may be matched. A `define_insn` expression contains an incomplete RTL expression, with pieces to be filled in later, operand constraints that restrict how the pieces can be filled in, and an output template or C code to generate the assembler output.

A `define_insn` is an RTL expression containing four or five operands:

1. An optional name n. When a name is present, the compiler automically generates a C++ function ‘`gen_n`’ that takes the operands of the instruction as arguments and returns the instruction’s rtx pattern. The compiler also assigns the instruction a unique code ‘`CODE_FOR_n`’, with all such codes belonging to an enum called `insn_code`.

   These names serve one of two purposes. The first is to indicate that the instruction performs a certain standard job for the RTL-generation pass of the compiler, such as a move, an addition, or a conditional jump. The second is to help the target generate certain target-specific operations, such as when implementing target-specific intrinsic functions.

   It is better to prefix target-specific names with the name of the target, to avoid any clash with current or future standard names.

   The absence of a name is indicated by writing an empty string where the name should go. Nameless instruction patterns are never used for generating RTL code, but they may permit several simpler insns to be combined later on.

   For the purpose of debugging the compiler, you may also specify a name beginning with the ‘`*`’ character. Such a name is used only for identifying the instruction in RTL dumps; it is equivalent to having a nameless pattern for all other purposes. Names beginning with the ‘`*`’ character are not required to be unique.

   The name may also have the form ‘`@n`’. This has the same effect as a name ‘`n`’, but in addition tells the compiler to generate further helper functions; see Parameterized Names for details.

2. The RTL template: This is a vector of incomplete RTL expressions which describe the semantics of the instruction (see RTL Template). It is incomplete because it may contain `match_operand`, `match_operator`, and `match_dup` expressions that stand for operands of the instruction.

   If the vector has multiple elements, the RTL template is treated as a `parallel` expression.

3. The condition: This is a string which contains a C expression. When the compiler attempts to match RTL against a pattern, the condition is evaluated. If the condition evaluates to `true`, the match is permitted. The condition may be an empty string, which is treated as always `true`.

   For a named pattern, the condition may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run.

   For nameless patterns, the condition is applied only when matching an individual insn, and only after the insn has matched the pattern’s recognition template. The insn’s operands may be found in the vector `operands`.

   An instruction condition cannot become more restrictive as compilation progresses. If the condition accepts a particular RTL instruction at one stage of compilation, it must continue to accept that instruction until the final pass. For example, ‘`!reload_completed`’ and ‘`can_create_pseudo_p ()`’ are both invalid instruction conditions, because they are true during the earlier RTL passes and false during the later ones. For the same reason, if a condition accepts an instruction before register allocation, it cannot later try to control register allocation by excluding certain register or value combinations.

   Although a condition cannot become more restrictive as compilation progresses, the condition for a nameless pattern can become more permissive. For example, a nameless instruction can require ‘`reload_completed`’ to be true, in which case it only matches after register allocation.

4. The output template or output statement: This is either a string, or a fragment of C code which returns a string.

   When simple substitution isn’t general enough, you can specify a piece of C code to compute the output. See Output Statement.

5. The insn attributes: This is an optional vector containing the values of attributes for insns matching this pattern (see Insn Attributes).