(gcc.info.gz) Machine Desc
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
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" ( 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
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. Output Template.
When simple substitution isn't general enough, you can specify a
piece of C code to compute the output. Output Statement.
5. Optionally, a vector containing the values of attributes for insns
matching this pattern. Insn Attributes.
(gcc.info.gz) Machine Desc
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