GNU Compiler Collection (GCC) Internals: Exception Handling

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#### 18.9.2 Exception Handling Support

Macro: EH_RETURN_DATA_REGNO (N)

  A C expression whose value is the Nth register number used for data by exception handlers, or `INVALID_REGNUM` if fewer than N registers are usable.

  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.

  You must define this macro if you want to support call frame exception handling like that provided by DWARF 2.

Macro: EH_RETURN_STACKADJ_RTX

  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’s call frame. It will be assigned zero on code paths that return normally.

  Typically this is a call-clobbered hard register that is otherwise untouched by the epilogue, but could also be a stack slot.

  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.

Macro: EH_RETURN_HANDLER_RTX

  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.

  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 target call frame rather than inside the current call frame. If defined, `EH_RETURN_STACKADJ_RTX` will have already been assigned, so it may be used to calculate the location of the target call frame.

  Some targets have more complex requirements than storing to an address calculable during initial code generation. In that case the `eh_return` instruction pattern should be used instead.

  If you want to support call frame exception handling, you must define either this macro or the `eh_return` instruction pattern.

Macro: RETURN_ADDR_OFFSET

  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.

Macro: ASM_PREFERRED_EH_DATA_FORMAT (code, global)

  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.

  code is 0 for data, 1 for code labels, 2 for function pointers. global is true if the symbol may be affected by dynamic relocations. The macro should return a combination of the `DW_EH_PE_*` defines as found in `dwarf2.h`.

  If this macro is not defined, pointers will not be encoded but represented directly.

Macro: ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX (file, encoding, size, addr, done)

  This macro allows the target to emit whatever special magic is required to represent the encoding chosen by `ASM_PREFERRED_EH_DATA_FORMAT`. Generic code takes care of pc-relative and indirect encodings; this must be defined if the target uses text-relative or data-relative encodings.

  This is a C statement that branches to done if the format was handled. encoding is the format chosen, size is the number of bytes that the format occupies, addr is the `SYMBOL_REF` to be emitted.

Macro: MD_FALLBACK_FRAME_STATE_FOR (context, fs)

  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.

  This macro is called from `uw_frame_state_for` in `unwind-dw2.c`, `unwind-dw2-xtensa.c` and `unwind-ia64.c`. context is an `_Unwind_Context`; fs is an `_Unwind_FrameState`. Examine `context->ra` for the address of the code being executed and `context->cfa` for the stack pointer value. If the frame can be decoded, the register save addresses should be updated in fs and the macro should evaluate to `_URC_NO_REASON`. If the frame cannot be decoded, the macro should evaluate to `_URC_END_OF_STACK`.

  For proper signal handling in Java this macro is accompanied by `MAKE_THROW_FRAME`, defined in `libjava/include/*-signal.h` headers.

Macro: MD_HANDLE_UNWABI (context, fs)

  This macro allows the target to add operating system specific code to the call-frame unwinder to handle the IA-64 `.unwabi` unwinding directive, usually used for signal or interrupt frames.

  This macro is called from `uw_update_context` in libgcc’s `unwind-ia64.c`. context is an `_Unwind_Context`; fs is an `_Unwind_FrameState`. Examine `fs->unwabi` for the abi and context in the `.unwabi` directive. If the `.unwabi` directive can be handled, the register save addresses should be updated in fs.

Macro: TARGET_USES_WEAK_UNWIND_INFO

  A C expression that evaluates to true if the target requires unwind info to be given comdat linkage. Define it to be `1` if comdat linkage is necessary. The default is `0`.

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