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#### 16.19.3 Assigning Attribute Values to Insns

The value assigned to an attribute of an insn is primarily determined by which pattern is matched by that insn (or which `define_peephole` generated it). Every `define_insn` and `define_peephole` can have an optional last argument to specify the values of attributes for matching insns. The value of any attribute not specified in a particular insn is set to the default value for that attribute, as specified in its `define_attr`. Extensive use of default values for attributes permits the specification of the values for only one or two attributes in the definition of most insn patterns, as seen in the example in the next section.

The optional last argument of `define_insn` and `define_peephole` is a vector of expressions, each of which defines the value for a single attribute. The most general way of assigning an attribute's value is to use a `set` expression whose first operand is an `attr` expression giving the name of the attribute being set. The second operand of the `set` is an attribute expression (see Expressions) giving the value of the attribute.

When the attribute value depends on the ‘`alternative`’ attribute (i.e., which is the applicable alternative in the constraint of the insn), the `set_attr_alternative` expression can be used. It allows the specification of a vector of attribute expressions, one for each alternative.

When the generality of arbitrary attribute expressions is not required, the simpler `set_attr` expression can be used, which allows specifying a string giving either a single attribute value or a list of attribute values, one for each alternative.

The form of each of the above specifications is shown below. In each case, name is a string specifying the attribute to be set.

`(set_attr` name value-string`)`
  value-string is either a string giving the desired attribute value, or a string containing a comma-separated list giving the values for succeeding alternatives. The number of elements must match the number of alternatives in the constraint of the insn pattern.

  Note that it may be useful to specify ‘`*`’ for some alternative, in which case the attribute will assume its default value for insns matching that alternative.

`(set_attr_alternative` name`[`value1 value2`...])`
  Depending on the alternative of the insn, the value will be one of the specified values. This is a shorthand for using a `cond` with tests on the ‘`alternative`’ attribute.

`(set (attr` name`)` value`)`
  The first operand of this `set` must be the special RTL expression `attr`, whose sole operand is a string giving the name of the attribute being set. value is the value of the attribute.

The following shows three different ways of representing the same attribute value specification:

         (set_attr "type" "load,store,arith")

         (set_attr_alternative "type"
                               [(const_string "load") (const_string "store")
                                (const_string "arith")])

         (set (attr "type")
              (cond [(eq_attr "alternative" "1") (const_string "load")
                     (eq_attr "alternative" "2") (const_string "store")]
                    (const_string "arith")))

The `define_asm_attributes` expression provides a mechanism to specify the attributes assigned to insns produced from an `asm` statement. It has the form:

         (define_asm_attributes [attr-sets])

where attr-sets is specified the same as for both the `define_insn` and the `define_peephole` expressions.

These values will typically be the “worst case” attribute values. For example, they might indicate that the condition code will be clobbered.

A specification for a `length` attribute is handled specially. The way to compute the length of an `asm` insn is to multiply the length specified in the expression `define_asm_attributes` by the number of machine instructions specified in the `asm` statement, determined by counting the number of semicolons and newlines in the string. Therefore, the value of the `length` attribute specified in a `define_asm_attributes` should be the maximum possible length of a single machine instruction.