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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 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)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)(cond [test1 value1 ...] default)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)(not test)(ior test1 test2)(and test1 test2)(match_operand:m n pred constraints)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.
(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)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)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)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. Test the flags very_likely,
likely, very_unlikely, and unlikely to determine
if a conditional branch is expected to be taken.
If the very_likely flag is true, then the likely flag is also
true. Likewise for the very_unlikely and unlikely flags.
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.
The very_likely and likely flags are true if the
insn being scheduled is not a conditional branch.
The very_unlikely and unlikely flags are false if the
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)eq_attr and attr_flag
produce more efficient code for non-numeric attributes.