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<title>Exception Handling - GNU Compiler Collection (GCC) Internals</title>
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<a name="Exception-Handling"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Stack-Checking.html#Stack-Checking">Stack Checking</a>,
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Previous: <a rel="previous" accesskey="p" href="Frame-Layout.html#Frame-Layout">Frame Layout</a>,
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Up: <a rel="up" accesskey="u" href="Stack-and-Calling.html#Stack-and-Calling">Stack and Calling</a>
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<h4 class="subsection">17.9.2 Exception Handling Support</h4>
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<p><a name="index-exception-handling-4169"></a>
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<div class="defun">
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— Macro: <b>EH_RETURN_DATA_REGNO</b> (<var>N</var>)<var><a name="index-EH_005fRETURN_005fDATA_005fREGNO-4170"></a></var><br>
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<blockquote><p>A C expression whose value is the <var>N</var>th register number used for
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data by exception handlers, or <code>INVALID_REGNUM</code> if fewer than
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<var>N</var> registers are usable.
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<p>The exception handling library routines communicate with the exception
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handlers via a set of agreed upon registers. Ideally these registers
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should be call-clobbered; it is possible to use call-saved registers,
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but may negatively impact code size. The target must support at least
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2 data registers, but should define 4 if there are enough free registers.
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<p>You must define this macro if you want to support call frame exception
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handling like that provided by DWARF 2.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>EH_RETURN_STACKADJ_RTX</b><var><a name="index-EH_005fRETURN_005fSTACKADJ_005fRTX-4171"></a></var><br>
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<blockquote><p>A C expression whose value is RTL representing a location in which
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to store a stack adjustment to be applied before function return.
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This is used to unwind the stack to an exception handler's call frame.
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It will be assigned zero on code paths that return normally.
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<p>Typically this is a call-clobbered hard register that is otherwise
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untouched by the epilogue, but could also be a stack slot.
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<p>Do not define this macro if the stack pointer is saved and restored
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by the regular prolog and epilog code in the call frame itself; in
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this case, the exception handling library routines will update the
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stack location to be restored in place. Otherwise, you must define
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this macro if you want to support call frame exception handling like
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that provided by DWARF 2.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>EH_RETURN_HANDLER_RTX</b><var><a name="index-EH_005fRETURN_005fHANDLER_005fRTX-4172"></a></var><br>
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<blockquote><p>A C expression whose value is RTL representing a location in which
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to store the address of an exception handler to which we should
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return. It will not be assigned on code paths that return normally.
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<p>Typically this is the location in the call frame at which the normal
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return address is stored. For targets that return by popping an
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address off the stack, this might be a memory address just below
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the <em>target</em> call frame rather than inside the current call
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frame. If defined, <code>EH_RETURN_STACKADJ_RTX</code> will have already
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been assigned, so it may be used to calculate the location of the
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target call frame.
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<p>Some targets have more complex requirements than storing to an
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address calculable during initial code generation. In that case
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the <code>eh_return</code> instruction pattern should be used instead.
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<p>If you want to support call frame exception handling, you must
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define either this macro or the <code>eh_return</code> instruction pattern.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>RETURN_ADDR_OFFSET</b><var><a name="index-RETURN_005fADDR_005fOFFSET-4173"></a></var><br>
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<blockquote><p>If defined, an integer-valued C expression for which rtl will be generated
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to add it to the exception handler address before it is searched in the
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exception handling tables, and to subtract it again from the address before
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using it to return to the exception handler.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_PREFERRED_EH_DATA_FORMAT</b> (<var>code, global</var>)<var><a name="index-ASM_005fPREFERRED_005fEH_005fDATA_005fFORMAT-4174"></a></var><br>
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<blockquote><p>This macro chooses the encoding of pointers embedded in the exception
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handling sections. If at all possible, this should be defined such
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that the exception handling section will not require dynamic relocations,
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and so may be read-only.
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<p><var>code</var> is 0 for data, 1 for code labels, 2 for function pointers.
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<var>global</var> is true if the symbol may be affected by dynamic relocations.
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The macro should return a combination of the <code>DW_EH_PE_*</code> defines
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as found in <samp><span class="file">dwarf2.h</span></samp>.
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<p>If this macro is not defined, pointers will not be encoded but
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represented directly.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX</b> (<var>file, encoding, size, addr, done</var>)<var><a name="index-ASM_005fMAYBE_005fOUTPUT_005fENCODED_005fADDR_005fRTX-4175"></a></var><br>
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<blockquote><p>This macro allows the target to emit whatever special magic is required
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to represent the encoding chosen by <code>ASM_PREFERRED_EH_DATA_FORMAT</code>.
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Generic code takes care of pc-relative and indirect encodings; this must
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be defined if the target uses text-relative or data-relative encodings.
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<p>This is a C statement that branches to <var>done</var> if the format was
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handled. <var>encoding</var> is the format chosen, <var>size</var> is the number
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of bytes that the format occupies, <var>addr</var> is the <code>SYMBOL_REF</code>
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to be emitted.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>MD_FALLBACK_FRAME_STATE_FOR</b> (<var>context, fs</var>)<var><a name="index-MD_005fFALLBACK_005fFRAME_005fSTATE_005fFOR-4176"></a></var><br>
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<blockquote><p>This macro allows the target to add CPU and operating system specific
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code to the call-frame unwinder for use when there is no unwind data
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available. The most common reason to implement this macro is to unwind
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through signal frames.
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<p>This macro is called from <code>uw_frame_state_for</code> in
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<samp><span class="file">unwind-dw2.c</span></samp>, <samp><span class="file">unwind-dw2-xtensa.c</span></samp> and
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<samp><span class="file">unwind-ia64.c</span></samp>. <var>context</var> is an <code>_Unwind_Context</code>;
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<var>fs</var> is an <code>_Unwind_FrameState</code>. Examine <code>context->ra</code>
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for the address of the code being executed and <code>context->cfa</code> for
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the stack pointer value. If the frame can be decoded, the register
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save addresses should be updated in <var>fs</var> and the macro should
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evaluate to <code>_URC_NO_REASON</code>. If the frame cannot be decoded,
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the macro should evaluate to <code>_URC_END_OF_STACK</code>.
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<p>For proper signal handling in Java this macro is accompanied by
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<code>MAKE_THROW_FRAME</code>, defined in <samp><span class="file">libjava/include/*-signal.h</span></samp> headers.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>MD_HANDLE_UNWABI</b> (<var>context, fs</var>)<var><a name="index-MD_005fHANDLE_005fUNWABI-4177"></a></var><br>
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<blockquote><p>This macro allows the target to add operating system specific code to the
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call-frame unwinder to handle the IA-64 <code>.unwabi</code> unwinding directive,
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usually used for signal or interrupt frames.
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<p>This macro is called from <code>uw_update_context</code> in libgcc's
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<samp><span class="file">unwind-ia64.c</span></samp>. <var>context</var> is an <code>_Unwind_Context</code>;
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<var>fs</var> is an <code>_Unwind_FrameState</code>. Examine <code>fs->unwabi</code>
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for the abi and context in the <code>.unwabi</code> directive. If the
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<code>.unwabi</code> directive can be handled, the register save addresses should
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be updated in <var>fs</var>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>TARGET_USES_WEAK_UNWIND_INFO</b><var><a name="index-TARGET_005fUSES_005fWEAK_005fUNWIND_005fINFO-4178"></a></var><br>
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<blockquote><p>A C expression that evaluates to true if the target requires unwind
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info to be given comdat linkage. Define it to be <code>1</code> if comdat
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linkage is necessary. The default is <code>0</code>.
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</p></blockquote></div>
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