GNU Compiler Collection (GCC) Internals: Expressions

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16.19.2 Attribute Expressions

RTL expressions used to define attributes use the codes described above plus a few specific to attribute definitions, to be discussed below. Attribute value expressions must have one of the following forms:

(const_int i)

The integer i specifies the value of a numeric attribute. i must be non-negative.

The value of a numeric attribute can be specified either with a const_int, or as an integer represented as a string in const_string, eq_attr (see below), attr, symbol_ref, simple arithmetic expressions, and set_attr overrides on specific instructions (see Tagging Insns).

(const_string value)

The string value specifies a constant attribute value. If value is specified as ‘"*"’, it means that the default value of the attribute is to be used for the insn containing this expression. ‘"*"’ obviously cannot be used in the default expression of a define_attr.

If the attribute whose value is being specified is numeric, value must be a string containing a non-negative integer (normally const_int would be used in this case). Otherwise, it must contain one of the valid values for the attribute.

(if_then_else test true-value false-value)

test specifies an attribute test, whose format is defined below. The value of this expression is true-value if test is true, otherwise it is false-value.

(cond [test1 value1 …] default)

The first operand of this expression is a vector containing an even number of expressions and consisting of pairs of test and value expressions. The value of the cond expression is that of the value corresponding to the first true test expression. If none of the test expressions are true, the value of the cond expression is that of the default expression.

test expressions can have one of the following forms:

(const_int i)

This test is true if i is nonzero and false otherwise.

(not test)

(ior test1 test2)

(and test1 test2)

These tests are true if the indicated logical function is true.

(match_operand:m n pred constraints)

This test is true if operand n of the insn whose attribute value is being determined has mode m (this part of the test is ignored if m is VOIDmode) and the function specified by the string pred returns a nonzero value when passed operand n and mode m (this part of the test is ignored if pred is the null string).

The constraints operand is ignored and should be the null string.

(match_test c-expr)

The test is true if C expression c-expr is true. In non-constant attributes, c-expr has access to the following variables:

insn

The rtl instruction under test.

which_alternative

The define_insn alternative that insn matches. See Output Statement.

operands

An array of insn’s rtl operands.

c-expr behaves like the condition in a C if statement, so there is no need to explicitly convert the expression into a boolean 0 or 1 value. For example, the following two tests are equivalent:

(match_test "x & 2")
(match_test "(x & 2) != 0")

(le arith1 arith2)

(leu arith1 arith2)

(lt arith1 arith2)

(ltu arith1 arith2)

(gt arith1 arith2)

(gtu arith1 arith2)

(ge arith1 arith2)

(geu arith1 arith2)

(ne arith1 arith2)

(eq arith1 arith2)

These tests are true if the indicated comparison of the two arithmetic expressions is true. Arithmetic expressions are formed with plus, minus, mult, div, mod, abs, neg, and, ior, xor, not, ashift, lshiftrt, and ashiftrt expressions.

const_int and symbol_ref are always valid terms (see Insn Lengths,for additional forms). symbol_ref is a string denoting a C expression that yields an int when evaluated by the ‘get_attr_…’ routine. It should normally be a global variable.

(eq_attr name value)

name is a string specifying the name of an attribute.

value is a string that is either a valid value for attribute name, a comma-separated list of values, or ‘!’ followed by a value or list. If value does not begin with a ‘!’, this test is true if the value of the name attribute of the current insn is in the list specified by value. If value begins with a ‘!’, this test is true if the attribute’s value is not in the specified list.

For example,

(eq_attr "type" "load,store")

is equivalent to

(ior (eq_attr "type" "load") (eq_attr "type" "store"))

If name specifies an attribute of ‘alternative’, it refers to the value of the compiler variable which_alternative (see Output Statement) and the values must be small integers. For example,

(eq_attr "alternative" "2,3")

is equivalent to

(ior (eq (symbol_ref "which_alternative") (const_int 2))
     (eq (symbol_ref "which_alternative") (const_int 3)))

Note that, for most attributes, an eq_attr test is simplified in cases where the value of the attribute being tested is known for all insns matching a particular pattern. This is by far the most common case.

(attr_flag name)

The value of an attr_flag expression is true if the flag specified by name is true for the insn currently being scheduled.

name is a string specifying one of a fixed set of flags to test. Test the flags forward and backward to determine the direction of a conditional branch.

This example describes a conditional branch delay slot which can be nullified for forward branches that are taken (annul-true) or for backward branches which are not taken (annul-false).

(define_delay (eq_attr "type" "cbranch")
  [(eq_attr "in_branch_delay" "true")
   (and (eq_attr "in_branch_delay" "true")
        (attr_flag "forward"))
   (and (eq_attr "in_branch_delay" "true")
        (attr_flag "backward"))])

The forward and backward flags are false if the current insn being scheduled is not a conditional branch.

attr_flag is only used during delay slot scheduling and has no meaning to other passes of the compiler.

(attr name)

The value of another attribute is returned. This is most useful for numeric attributes, as eq_attr and attr_flag produce more efficient code for non-numeric attributes.

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