|GNU Compiler Collection (GCC) Internals: 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:
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
eq_attr (see below),
symbol_ref, simple arithmetic expressions, and
overrides on specific instructions (see Tagging Insns).
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
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
expression is that of the default expression.
test expressions can have one of the following forms:
This test is true if i is nonzero and false otherwise.
(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
The constraints operand is ignored and should be the null string.
The test is true if C expression c-expr is true. In non-constant attributes, c-expr has access to the following variables:
The rtl instruction under test.
define_insn alternative that insn matches.
See Output Statement.
An array of insn’s rtl operands.
c-expr behaves like the condition in a C
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
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
(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.
(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
(see Output Statement) and the values must be small integers. For
(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.
The value of an
attr_flag expression is true if the flag
specified by name is true for the
insn currently being
name is a string specifying one of a fixed set of flags to test.
Test the flags
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"))])
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.
The value of another attribute is returned. This is most useful
for numeric attributes, as
produce more efficient code for non-numeric attributes.