### 16.2 Everything about Instruction Patterns

Each instruction pattern 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 pattern or C code to generate the assembler output, all wrapped up in a `define_insn` expression.

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

1. An optional name. The presence of a name indicate that this instruction pattern can perform a certain standard job for the RTL-generation pass of the compiler. This pass knows certain names and will use the instruction patterns with those names, if the names are defined in the machine description.

   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.

   Names that are not thus known and used in RTL-generation have no effect; they are equivalent to no name at all.

   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 entirely equivalent to having a nameless pattern for all other purposes.

2. The RTL template (see RTL Template) is a vector of incomplete RTL expressions which show what the instruction should look like. 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 only one element, that element is the template for the instruction pattern. If the vector has multiple elements, then the instruction pattern is a `parallel` expression containing the elements described.

3. A condition. This is a string which contains a C expression that is the final test to decide whether an insn body matches this pattern.

   For a named pattern, the condition (if present) 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`. For an insn where the condition has once matched, it can't be used to control register allocation, for example by excluding certain hard registers or hard register combinations.

4. The output template: a string that says how to output matching insns as assembler code. ‘`%`’ in this string specifies where to substitute the value of an operand. See Output Template.

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

5. Optionally, a vector containing the values of attributes for insns matching this pattern. See Insn Attributes.