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| <a name="Exception-Handling-1"></a> |
| <h3 class="section">11.3 Exception Handling</h3> |
| <a name="index-GIMPLE-Exception-Handling"></a> |
| |
| <p>Other exception handling constructs are represented using |
| <code>GIMPLE_TRY_CATCH</code>. <code>GIMPLE_TRY_CATCH</code> has two operands. The |
| first operand is a sequence of statements to execute. If executing |
| these statements does not throw an exception, then the second operand |
| is ignored. Otherwise, if an exception is thrown, then the second |
| operand of the <code>GIMPLE_TRY_CATCH</code> is checked. The second |
| operand may have the following forms: |
| </p> |
| <ol> |
| <li> A sequence of statements to execute. When an exception occurs, |
| these statements are executed, and then the exception is rethrown. |
| |
| </li><li> A sequence of <code>GIMPLE_CATCH</code> statements. Each |
| <code>GIMPLE_CATCH</code> has a list of applicable exception types and |
| handler code. If the thrown exception matches one of the caught |
| types, the associated handler code is executed. If the handler |
| code falls off the bottom, execution continues after the original |
| <code>GIMPLE_TRY_CATCH</code>. |
| |
| </li><li> A <code>GIMPLE_EH_FILTER</code> statement. This has a list of |
| permitted exception types, and code to handle a match failure. If the |
| thrown exception does not match one of the allowed types, the |
| associated match failure code is executed. If the thrown exception |
| does match, it continues unwinding the stack looking for the next |
| handler. |
| |
| </li></ol> |
| |
| <p>Currently throwing an exception is not directly represented in |
| GIMPLE, since it is implemented by calling a function. At some |
| point in the future we will want to add some way to express that |
| the call will throw an exception of a known type. |
| </p> |
| <p>Just before running the optimizers, the compiler lowers the |
| high-level EH constructs above into a set of ‘<samp>goto</samp>’s, magic |
| labels, and EH regions. Continuing to unwind at the end of a |
| cleanup is represented with a <code>GIMPLE_RESX</code>. |
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