GNU Compiler Collection (GCC) Internals: Exception Region Output

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#### 18.20.9 Assembler Commands for Exception Regions

This describes commands marking the start and the end of an exception region.

Macro: EH_FRAME_SECTION_NAME

  If defined, a C string constant for the name of the section containing exception handling frame unwind information. If not defined, GCC will provide a default definition if the target supports named sections. `crtstuff.c` uses this macro to switch to the appropriate section.

  You should define this symbol if your target supports DWARF 2 frame unwind information and the default definition does not work.

Macro: EH_FRAME_THROUGH_COLLECT2

  If defined, DWARF 2 frame unwind information will identified by specially named labels. The collect2 process will locate these labels and generate code to register the frames.

  This might be necessary, for instance, if the system linker will not place the eh_frames in-between the sentinals from `crtstuff.c`, or if the system linker does garbage collection and sections cannot be marked as not to be collected.

Macro: EH_TABLES_CAN_BE_READ_ONLY

  Define this macro to 1 if your target is such that no frame unwind information encoding used with non-PIC code will ever require a runtime relocation, but the linker may not support merging read-only and read-write sections into a single read-write section.

Macro: MASK_RETURN_ADDR

  An rtx used to mask the return address found via `RETURN_ADDR_RTX`, so that it does not contain any extraneous set bits in it.

Macro: DWARF2_UNWIND_INFO

  Define this macro to 0 if your target supports DWARF 2 frame unwind information, but it does not yet work with exception handling. Otherwise, if your target supports this information (if it defines `INCOMING_RETURN_ADDR_RTX` and `OBJECT_FORMAT_ELF`), GCC will provide a default definition of 1.

Common Target Hook: enum unwind_info_type TARGET_EXCEPT_UNWIND_INFO (struct gcc_options *opts)

  This hook defines the mechanism that will be used for exception handling by the target. If the target has ABI specified unwind tables, the hook should return `UI_TARGET`. If the target is to use the `setjmp`/`longjmp`-based exception handling scheme, the hook should return `UI_SJLJ`. If the target supports DWARF 2 frame unwind information, the hook should return `UI_DWARF2`.

  A target may, if exceptions are disabled, choose to return `UI_NONE`. This may end up simplifying other parts of target-specific code. The default implementation of this hook never returns `UI_NONE`.

  Note that the value returned by this hook should be constant. It should not depend on anything except the command-line switches described by opts. In particular, the setting `UI_SJLJ` must be fixed at compiler start-up as C pre-processor macros and builtin functions related to exception handling are set up depending on this setting.

  The default implementation of the hook first honors the `--enable-sjlj-exceptions` configure option, then `DWARF2_UNWIND_INFO`, and finally defaults to `UI_SJLJ`. If `DWARF2_UNWIND_INFO` depends on command-line options, the target must define this hook so that opts is used correctly.

Common Target Hook: bool TARGET_UNWIND_TABLES_DEFAULT

  This variable should be set to `true` if the target ABI requires unwinding tables even when exceptions are not used. It must not be modified by command-line option processing.

Macro: DONT_USE_BUILTIN_SETJMP

  Define this macro to 1 if the `setjmp`/`longjmp`-based scheme should use the `setjmp`/`longjmp` functions from the C library instead of the `__builtin_setjmp`/`__builtin_longjmp` machinery.

Macro: JMP_BUF_SIZE

  This macro has no effect unless `DONT_USE_BUILTIN_SETJMP` is also defined. Define this macro if the default size of `jmp_buf` buffer for the `setjmp`/`longjmp`-based exception handling mechanism is not large enough, or if it is much too large. The default size is `FIRST_PSEUDO_REGISTER * sizeof(void *)`.

Macro: DWARF_CIE_DATA_ALIGNMENT

  This macro need only be defined if the target might save registers in the function prologue at an offset to the stack pointer that is not aligned to `UNITS_PER_WORD`. The definition should be the negative minimum alignment if `STACK_GROWS_DOWNWARD` is true, and the positive minimum alignment otherwise. See DWARF. Only applicable if the target supports DWARF 2 frame unwind information.

Target Hook: bool TARGET_TERMINATE_DW2_EH_FRAME_INFO

  Contains the value true if the target should add a zero word onto the end of a Dwarf-2 frame info section when used for exception handling. Default value is false if `EH_FRAME_SECTION_NAME` is defined, and true otherwise.

Target Hook: rtx TARGET_DWARF_REGISTER_SPAN (rtx reg)

  Given a register, this hook should return a parallel of registers to represent where to find the register pieces. Define this hook if the register and its mode are represented in Dwarf in non-contiguous locations, or if the register should be represented in more than one register in Dwarf. Otherwise, this hook should return `NULL_RTX`. If not defined, the default is to return `NULL_RTX`.

Target Hook: machine_mode TARGET_DWARF_FRAME_REG_MODE (int regno)

  Given a register, this hook should return the mode which the corresponding Dwarf frame register should have. This is normally used to return a smaller mode than the raw mode to prevent call clobbered parts of a register altering the frame register size

Target Hook: void TARGET_INIT_DWARF_REG_SIZES_EXTRA (tree address)

  If some registers are represented in Dwarf-2 unwind information in multiple pieces, define this hook to fill in information about the sizes of those pieces in the table used by the unwinder at runtime. It will be called by `expand_builtin_init_dwarf_reg_sizes` after filling in a single size corresponding to each hard register; address is the address of the table.

Target Hook: bool TARGET_ASM_TTYPE (rtx sym)

  This hook is used to output a reference from a frame unwinding table to the type_info object identified by sym. It should return `true` if the reference was output. Returning `false` will cause the reference to be output using the normal Dwarf2 routines.

Target Hook: bool TARGET_ARM_EABI_UNWINDER

  This flag should be set to `true` on targets that use an ARM EABI based unwinding library, and `false` on other targets. This effects the format of unwinding tables, and how the unwinder in entered after running a cleanup. The default is `false`.

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