16.20 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")
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
define_cond_exec is used, an implicit reference to
predicable instruction attribute is made.
See Insn Attributes. This attribute must be boolean (i.e. have
exactly two elements in its list-of-values). Further, it must
not be used with complex expressions. That is, the default and all
uses in the insns must be a simple constant, not dependent on the
alternative or anything else.
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")