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(gcc-295.info)Tagging Insns

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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 (Note: 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.