18.20.9 Assembler Commands for Exception Regions

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


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.


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.


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.


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.


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.


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.


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.


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 *).


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 Macros for DWARF Output. Only applicable if the target supports DWARF 2 frame unwind information.


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.


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.