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<a name="Exception-Handling-Support"></a>
<h4 class="subsection">18.9.2 Exception Handling Support</h4>
<a name="index-exception-handling-1"></a>

<dl>
<dt><a name="index-EH_005fRETURN_005fDATA_005fREGNO"></a>Macro: <strong>EH_RETURN_DATA_REGNO</strong> <em>(<var>N</var>)</em></dt>
<dd><p>A C expression whose value is the <var>N</var>th register number used for
data by exception handlers, or <code>INVALID_REGNUM</code> if fewer than
<var>N</var> registers are usable.
</p>
<p>The exception handling library routines communicate with the exception
handlers via a set of agreed upon registers.  Ideally these registers
should be call-clobbered; it is possible to use call-saved registers,
but may negatively impact code size.  The target must support at least
2 data registers, but should define 4 if there are enough free registers.
</p>
<p>You must define this macro if you want to support call frame exception
handling like that provided by DWARF 2.
</p></dd></dl>

<dl>
<dt><a name="index-EH_005fRETURN_005fSTACKADJ_005fRTX"></a>Macro: <strong>EH_RETURN_STACKADJ_RTX</strong></dt>
<dd><p>A C expression whose value is RTL representing a location in which
to store a stack adjustment to be applied before function return.
This is used to unwind the stack to an exception handler&rsquo;s call frame.
It will be assigned zero on code paths that return normally.
</p>
<p>Typically this is a call-clobbered hard register that is otherwise
untouched by the epilogue, but could also be a stack slot.
</p>
<p>Do not define this macro if the stack pointer is saved and restored
by the regular prolog and epilog code in the call frame itself; in
this case, the exception handling library routines will update the
stack location to be restored in place.  Otherwise, you must define
this macro if you want to support call frame exception handling like
that provided by DWARF 2.
</p></dd></dl>

<dl>
<dt><a name="index-EH_005fRETURN_005fHANDLER_005fRTX"></a>Macro: <strong>EH_RETURN_HANDLER_RTX</strong></dt>
<dd><p>A C expression whose value is RTL representing a location in which
to store the address of an exception handler to which we should
return.  It will not be assigned on code paths that return normally.
</p>
<p>Typically this is the location in the call frame at which the normal
return address is stored.  For targets that return by popping an
address off the stack, this might be a memory address just below
the <em>target</em> call frame rather than inside the current call
frame.  If defined, <code>EH_RETURN_STACKADJ_RTX</code> will have already
been assigned, so it may be used to calculate the location of the
target call frame.
</p>
<p>Some targets have more complex requirements than storing to an
address calculable during initial code generation.  In that case
the <code>eh_return</code> instruction pattern should be used instead.
</p>
<p>If you want to support call frame exception handling, you must
define either this macro or the <code>eh_return</code> instruction pattern.
</p></dd></dl>

<dl>
<dt><a name="index-RETURN_005fADDR_005fOFFSET"></a>Macro: <strong>RETURN_ADDR_OFFSET</strong></dt>
<dd><p>If defined, an integer-valued C expression for which rtl will be generated
to add it to the exception handler address before it is searched in the
exception handling tables, and to subtract it again from the address before
using it to return to the exception handler.
</p></dd></dl>

<dl>
<dt><a name="index-ASM_005fPREFERRED_005fEH_005fDATA_005fFORMAT"></a>Macro: <strong>ASM_PREFERRED_EH_DATA_FORMAT</strong> <em>(<var>code</var>, <var>global</var>)</em></dt>
<dd><p>This macro chooses the encoding of pointers embedded in the exception
handling sections.  If at all possible, this should be defined such
that the exception handling section will not require dynamic relocations,
and so may be read-only.
</p>
<p><var>code</var> is 0 for data, 1 for code labels, 2 for function pointers.
<var>global</var> is true if the symbol may be affected by dynamic relocations.
The macro should return a combination of the <code>DW_EH_PE_*</code> defines
as found in <samp>dwarf2.h</samp>.
</p>
<p>If this macro is not defined, pointers will not be encoded but
represented directly.
</p></dd></dl>

<dl>
<dt><a name="index-ASM_005fMAYBE_005fOUTPUT_005fENCODED_005fADDR_005fRTX"></a>Macro: <strong>ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX</strong> <em>(<var>file</var>, <var>encoding</var>, <var>size</var>, <var>addr</var>, <var>done</var>)</em></dt>
<dd><p>This macro allows the target to emit whatever special magic is required
to represent the encoding chosen by <code>ASM_PREFERRED_EH_DATA_FORMAT</code>.
Generic code takes care of pc-relative and indirect encodings; this must
be defined if the target uses text-relative or data-relative encodings.
</p>
<p>This is a C statement that branches to <var>done</var> if the format was
handled.  <var>encoding</var> is the format chosen, <var>size</var> is the number
of bytes that the format occupies, <var>addr</var> is the <code>SYMBOL_REF</code>
to be emitted.
</p></dd></dl>

<dl>
<dt><a name="index-MD_005fFALLBACK_005fFRAME_005fSTATE_005fFOR"></a>Macro: <strong>MD_FALLBACK_FRAME_STATE_FOR</strong> <em>(<var>context</var>, <var>fs</var>)</em></dt>
<dd><p>This macro allows the target to add CPU and operating system specific
code to the call-frame unwinder for use when there is no unwind data
available.  The most common reason to implement this macro is to unwind
through signal frames.
</p>
<p>This macro is called from <code>uw_frame_state_for</code> in
<samp>unwind-dw2.c</samp>, <samp>unwind-dw2-xtensa.c</samp> and
<samp>unwind-ia64.c</samp>.  <var>context</var> is an <code>_Unwind_Context</code>;
<var>fs</var> is an <code>_Unwind_FrameState</code>.  Examine <code>context-&gt;ra</code>
for the address of the code being executed and <code>context-&gt;cfa</code> for
the stack pointer value.  If the frame can be decoded, the register
save addresses should be updated in <var>fs</var> and the macro should
evaluate to <code>_URC_NO_REASON</code>.  If the frame cannot be decoded,
the macro should evaluate to <code>_URC_END_OF_STACK</code>.
</p>
<p>For proper signal handling in Java this macro is accompanied by
<code>MAKE_THROW_FRAME</code>, defined in <samp>libjava/include/*-signal.h</samp> headers.
</p></dd></dl>

<dl>
<dt><a name="index-MD_005fHANDLE_005fUNWABI"></a>Macro: <strong>MD_HANDLE_UNWABI</strong> <em>(<var>context</var>, <var>fs</var>)</em></dt>
<dd><p>This macro allows the target to add operating system specific code to the
call-frame unwinder to handle the IA-64 <code>.unwabi</code> unwinding directive,
usually used for signal or interrupt frames.
</p>
<p>This macro is called from <code>uw_update_context</code> in libgcc&rsquo;s
<samp>unwind-ia64.c</samp>.  <var>context</var> is an <code>_Unwind_Context</code>;
<var>fs</var> is an <code>_Unwind_FrameState</code>.  Examine <code>fs-&gt;unwabi</code>
for the abi and context in the <code>.unwabi</code> directive.  If the
<code>.unwabi</code> directive can be handled, the register save addresses should
be updated in <var>fs</var>.
</p></dd></dl>

<dl>
<dt><a name="index-TARGET_005fUSES_005fWEAK_005fUNWIND_005fINFO"></a>Macro: <strong>TARGET_USES_WEAK_UNWIND_INFO</strong></dt>
<dd><p>A C expression that evaluates to true if the target requires unwind
info to be given comdat linkage.  Define it to be <code>1</code> if comdat
linkage is necessary.  The default is <code>0</code>.
</p></dd></dl>

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