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(gcc-295.info)Patterns

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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.

  2. The "RTL template" (Note: 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'.

  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.  Note: Output Template.

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

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