|GNU Compiler Collection (GCC) Internals: Conditional Execution|
A number of architectures provide for some form of conditional
execution, or predication. The hallmark of this feature is the
ability to nullify most of the instructions in the instruction set.
When the instruction set is large and not entirely symmetric, it
can be quite tedious to describe these forms directly in the
.md file. An alternative is the
(define_cond_exec [predicate-pattern] "condition" "output-template" "optional-insn-attribues")
predicate-pattern is the condition that must be true for the
insn to be executed at runtime and should match a relational operator.
One can use
match_operator to match several relational operators
at once. Any
match_operand operands must have no more than one
condition is a C expression that must be true for the generated pattern to match.
output-template is a string similar to the
output template (see Output Template), except that the ‘*’
and ‘@’ special cases do not apply. This is only useful if the
assembly text for the predicate is a simple prefix to the main insn.
In order to handle the general case, there is a global variable
current_insn_predicate that will contain the entire predicate
if the current insn is predicated, and will otherwise be
optional-insn-attributes is an optional vector of attributes that gets appended to the insn attributes of the produced cond_exec rtx. It can be used to add some distinguishing attribute to cond_exec rtxs produced that way. An example usage would be to use this attribute in conjunction with attributes on the main pattern to disable particular alternatives under certain conditions.
define_cond_exec is used, an implicit reference to
predicable instruction attribute is made.
See Insn Attributes. This attribute must be a boolean (i.e. have
exactly two elements in its list-of-values), with the possible
yes. The default and all uses in
the insns must be a simple constant, not a complex expressions. It
may, however, depend on the alternative, by using a comma-separated
list of values. If that is the case, the port should also define an
enabled attribute (see Disable Insn Alternatives), which
should also allow only
yes as its values.
define_insn for which the
attribute is true, a new
define_insn pattern will be
generated that matches a predicated version of the instruction.
(define_insn "addsi" [(set (match_operand:SI 0 "register_operand" "r") (plus:SI (match_operand:SI 1 "register_operand" "r") (match_operand:SI 2 "register_operand" "r")))] "test1" "add %2,%1,%0") (define_cond_exec [(ne (match_operand:CC 0 "register_operand" "c") (const_int 0))] "test2" "(%0)")
generates a new pattern
(define_insn "" [(cond_exec (ne (match_operand:CC 3 "register_operand" "c") (const_int 0)) (set (match_operand:SI 0 "register_operand" "r") (plus:SI (match_operand:SI 1 "register_operand" "r") (match_operand:SI 2 "register_operand" "r"))))] "(test2) && (test1)" "(%3) add %2,%1,%0")