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<title>GNU Compiler Collection (GCC) Internals: GIMPLE Exception Handling</title>

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<a name="Exception-Handling-1"></a>
<h3 class="section">11.4 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 &lsquo;<samp>goto</samp>&rsquo;s, magic
labels, and EH regions.  Continuing to unwind at the end of a
cleanup is represented with a <code>GIMPLE_RESX</code>.
</p>

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