Conditional Execution - GNU Compiler Collection (GCC) Internals
Description: GNU Compiler Collection (GCC) Internals

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### 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` template.

         (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 alternative.

condition is a C expression that must be true for the generated pattern to match.

output-template is a string similar to the `define_insn` 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 `NULL`.

When `define_cond_exec` is used, an implicit reference to the `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.

For each `define_insn` for which the `predicable` attribute is true, a new `define_insn` pattern will be generated that matches a predicated version of the instruction. For example,

         (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")