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   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. _exception-region-output:
   7
   8 Assembler Commands for Exception Regions
   9 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  10
  11 .. prevent bad page break with this line
  12
  13 This describes commands marking the start and the end of an exception
  14 region.
  15
  16 .. c:macro:: EH_FRAME_SECTION_NAME
  17
  18   If defined, a C string constant for the name of the section containing
  19   exception handling frame unwind information.  If not defined, GCC will
  20   provide a default definition if the target supports named sections.
  21   :samp:`crtstuff.c` uses this macro to switch to the appropriate section.
  22
  23   You should define this symbol if your target supports DWARF 2 frame
  24   unwind information and the default definition does not work.
  25
  26 .. c:macro:: EH_FRAME_THROUGH_COLLECT2
  27
  28   If defined, DWARF 2 frame unwind information will identified by
  29   specially named labels.  The collect2 process will locate these
  30   labels and generate code to register the frames.
  31
  32   This might be necessary, for instance, if the system linker will not
  33   place the eh_frames in-between the sentinals from :samp:`crtstuff.c`,
  34   or if the system linker does garbage collection and sections cannot
  35   be marked as not to be collected.
  36
  37 .. c:macro:: EH_TABLES_CAN_BE_READ_ONLY
  38
  39   Define this macro to 1 if your target is such that no frame unwind
  40   information encoding used with non-PIC code will ever require a
  41   runtime relocation, but the linker may not support merging read-only
  42   and read-write sections into a single read-write section.
  43
  44 .. c:macro:: MASK_RETURN_ADDR
  45
  46   An rtx used to mask the return address found via ``RETURN_ADDR_RTX``, so
  47   that it does not contain any extraneous set bits in it.
  48
  49 .. c:macro:: DWARF2_UNWIND_INFO
  50
  51   Define this macro to 0 if your target supports DWARF 2 frame unwind
  52   information, but it does not yet work with exception handling.
  53   Otherwise, if your target supports this information (if it defines
  54   ``INCOMING_RETURN_ADDR_RTX`` and ``OBJECT_FORMAT_ELF``),
  55   GCC will provide a default definition of 1.
  56
  57 .. function:: enum unwind_info_type TARGET_EXCEPT_UNWIND_INFO (struct gcc_options *opts)
  58
  59   This hook defines the mechanism that will be used for exception handling
  60   by the target.  If the target has ABI specified unwind tables, the hook
  61   should return ``UI_TARGET``.  If the target is to use the
  62   ``setjmp`` / ``longjmp`` -based exception handling scheme, the hook
  63   should return ``UI_SJLJ``.  If the target supports DWARF 2 frame unwind
  64   information, the hook should return ``UI_DWARF2``.
  65
  66   A target may, if exceptions are disabled, choose to return ``UI_NONE``.
  67   This may end up simplifying other parts of target-specific code.  The
  68   default implementation of this hook never returns ``UI_NONE``.
  69
  70   Note that the value returned by this hook should be constant.  It should
  71   not depend on anything except the command-line switches described by
  72   :samp:`{opts}`.  In particular, the
  73   setting ``UI_SJLJ`` must be fixed at compiler start-up as C pre-processor
  74   macros and builtin functions related to exception handling are set up
  75   depending on this setting.
  76
  77   The default implementation of the hook first honors the
  78   :option:`--enable-sjlj-exceptions` configure option, then
  79   ``DWARF2_UNWIND_INFO``, and finally defaults to ``UI_SJLJ``.  If
  80   ``DWARF2_UNWIND_INFO`` depends on command-line options, the target
  81   must define this hook so that :samp:`{opts}` is used correctly.
  82
  83 .. c:var:: bool TARGET_UNWIND_TABLES_DEFAULT
  84
  85   This variable should be set to ``true`` if the target ABI requires unwinding
  86   tables even when exceptions are not used.  It must not be modified by
  87   command-line option processing.
  88
  89 .. c:macro:: DONT_USE_BUILTIN_SETJMP
  90
  91   Define this macro to 1 if the ``setjmp`` / ``longjmp`` -based scheme
  92   should use the ``setjmp`` / ``longjmp`` functions from the C library
  93   instead of the ``__builtin_setjmp`` / ``__builtin_longjmp`` machinery.
  94
  95 .. c:macro:: JMP_BUF_SIZE
  96
  97   This macro has no effect unless ``DONT_USE_BUILTIN_SETJMP`` is also
  98   defined.  Define this macro if the default size of ``jmp_buf`` buffer
  99   for the ``setjmp`` / ``longjmp`` -based exception handling mechanism
 100   is not large enough, or if it is much too large.
 101   The default size is ``FIRST_PSEUDO_REGISTER * sizeof(void *)``.
 102
 103 .. c:macro:: DWARF_CIE_DATA_ALIGNMENT
 104
 105   This macro need only be defined if the target might save registers in the
 106   function prologue at an offset to the stack pointer that is not aligned to
 107   ``UNITS_PER_WORD``.  The definition should be the negative minimum
 108   alignment if ``STACK_GROWS_DOWNWARD`` is true, and the positive
 109   minimum alignment otherwise.  See :ref:`dwarf`.  Only applicable if
 110   the target supports DWARF 2 frame unwind information.
 111
 112 .. include:: ../tm.rst.in
 113   :start-after: [TARGET_TERMINATE_DW2_EH_FRAME_INFO]
 114   :end-before: [TARGET_TERMINATE_DW2_EH_FRAME_INFO]
 115
 116
 117 .. include:: ../tm.rst.in
 118   :start-after: [TARGET_DWARF_REGISTER_SPAN]
 119   :end-before: [TARGET_DWARF_REGISTER_SPAN]
 120
 121
 122 .. include:: ../tm.rst.in
 123   :start-after: [TARGET_DWARF_FRAME_REG_MODE]
 124   :end-before: [TARGET_DWARF_FRAME_REG_MODE]
 125
 126
 127 .. include:: ../tm.rst.in
 128   :start-after: [TARGET_INIT_DWARF_REG_SIZES_EXTRA]
 129   :end-before: [TARGET_INIT_DWARF_REG_SIZES_EXTRA]
 130
 131
 132 .. include:: ../tm.rst.in
 133   :start-after: [TARGET_ASM_TTYPE]
 134   :end-before: [TARGET_ASM_TTYPE]
 135
 136
 137 .. include:: ../tm.rst.in
 138   :start-after: [TARGET_ARM_EABI_UNWINDER]
 139   :end-before: [TARGET_ARM_EABI_UNWINDER]