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| <a name="Conditional-Execution-1"></a> |
| <h3 class="section">16.20 Conditional Execution</h3> |
| <a name="index-conditional-execution"></a> |
| <a name="index-predication"></a> |
| |
| <p>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 |
| <samp>.md</samp> file. An alternative is the <code>define_cond_exec</code> template. |
| </p> |
| <a name="index-define_005fcond_005fexec"></a> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_cond_exec |
| [<var>predicate-pattern</var>] |
| "<var>condition</var>" |
| "<var>output-template</var>" |
| "<var>optional-insn-attribues</var>") |
| </pre></div> |
| |
| <p><var>predicate-pattern</var> is the condition that must be true for the |
| insn to be executed at runtime and should match a relational operator. |
| One can use <code>match_operator</code> to match several relational operators |
| at once. Any <code>match_operand</code> operands must have no more than one |
| alternative. |
| </p> |
| <p><var>condition</var> is a C expression that must be true for the generated |
| pattern to match. |
| </p> |
| <a name="index-current_005finsn_005fpredicate"></a> |
| <p><var>output-template</var> is a string similar to the <code>define_insn</code> |
| output template (see <a href="Output-Template.html#Output-Template">Output Template</a>), except that the ‘<samp>*</samp>’ |
| and ‘<samp>@</samp>’ 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 |
| <code>current_insn_predicate</code> that will contain the entire predicate |
| if the current insn is predicated, and will otherwise be <code>NULL</code>. |
| </p> |
| <p><var>optional-insn-attributes</var> 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. |
| </p> |
| <p>When <code>define_cond_exec</code> is used, an implicit reference to |
| the <code>predicable</code> instruction attribute is made. |
| See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>. This attribute must be a boolean (i.e. have |
| exactly two elements in its <var>list-of-values</var>), with the possible |
| values being <code>no</code> and <code>yes</code>. 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 |
| <code>enabled</code> attribute (see <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>), which |
| should also allow only <code>no</code> and <code>yes</code> as its values. |
| </p> |
| <p>For each <code>define_insn</code> for which the <code>predicable</code> |
| attribute is true, a new <code>define_insn</code> pattern will be |
| generated that matches a predicated version of the instruction. |
| For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(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")))] |
| "<var>test1</var>" |
| "add %2,%1,%0") |
| |
| (define_cond_exec |
| [(ne (match_operand:CC 0 "register_operand" "c") |
| (const_int 0))] |
| "<var>test2</var>" |
| "(%0)") |
| </pre></div> |
| |
| <p>generates a new pattern |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(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"))))] |
| "(<var>test2</var>) && (<var>test1</var>)" |
| "(%3) add %2,%1,%0") |
| </pre></div> |
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