1. Exception handling porting in
GCC
Shiva Chen
shiva0217@gmail.com
MAY 2013

2. Why need exception handling ?
 Exceptions provide a way to react to
exceptional circumstances (like runtime
errors)
 Transferring control to special functions called
handlers
 Make system more stable

3. Example
// exceptions
#include <iostream>
using namespace std;
int
main ()
{
try { throw 20; }
catch (int e)
{
cout << "An exception occurred. Exception Nr. " << e << endl;
}
return 0;
}

4. Unwinding
 After throwing a exception object, exception library will do
unwinding frame to find the handler which could handle the
exception object.
 There are two way to do unwinding
 Set jump/long jump (SJLJ) unwinding
 Don’t know about the frame info
 restore all registers saved in the jump-buffer by setjmp
 Smaller code size but slower (store all register when enter each frame)
 Configure gcc with --enable-sjlj-exceptions
 Dwarf2 unwinding
 Obtain eh_rame, gcc_except_table to record frame relative info
 Could know which register will really corrupt and restore the register which
really needed
 Code size bigger but faster

5. Dwarf2 unwinding
 Startup code to support dwarf2 unwinding
_init
call frame_dummy
call __register_frame_info_base
call __do_global_ctors
call atexit (__do_global_dtors)
__register_frame_info_base
Called from crtbegin.o to register the unwind info for an object
libgcc/config.host
extra_parts="crti.o crtn.o crtbegin.o crtend.o crtbeginS.o crtendS.o“
Which will use libgcc/crtstuff.c to build startup code

6. Dwarf2 unwinding relative porting
 There are three thing for Dwarf2 unwinding are
ABI specific which need porting code to handle
this part
 Pass exception info to exception handler
Prologue need preserve original register value
 Adjust stack pointer from unwind frame to handler
 Modify return value to exception handler
After unwinding, unwinding library will modify return value to
handler and jump to handler by function return
Return value could be in
 Return register
 Frame stack

7. Dwarf2 unwinding relative porting
 Pass exception info to exception handler
 Define EH_RETURN_DATA_REGNO
The register to pass exception info
Prologue need push original value to the stack before setting
exception info
 Adjust stack pointer from unwind frame to handler
 Define EH_RETURN_STACKADJ_RTX
The register to contain adjust offset
 Modify return value to exception handler
 Define EH_RETURN_HANDLER_RTX or eh_return
pattern
To modify return value to handler

8. eh_return pattern of arm
arm_set_return_address (rtx source, rtx scratch)
{
…
/* if return address in LR_REGNUM
emit a move to change return address to handler */
if ((saved_regs & (1 << LR_REGNUM)) == 0)
emit_move_insn (gen_rtx_REG (Pmode, LR_REGNUM), source);
/* return value in stack */
else
{
}
source contain return value

9. eh_return pattern of arm
arm_set_return_address (rtx source, rtx scratch)
{
…
/* return value in stack */
else
{
if (frame_pointer_needed)
addr = plus_constant(hard_frame_pointer_rtx, -4);
else
{ /* LR will be the first saved register. */
delta = offsets->outgoing_args - (offsets->frame + 4);
If (delta >= 4096)
{
emit_insn (gen_addsi3 (scratch, stack_pointer_rtx, GEN_INT (delta & ~4095)));
addr = scratch; delta &= 4095; }
else
addr = stack_pointer_rtx;
addr = plus_constant (addr, delta);
}
emit_move_insn (gen_frame_mem (Pmode, addr), source);
}
}
source contain return value
Use fp to find return address in stack
Emit memory move to modify return address in stack
Find the return address in stack

10. Sections for dwarf2 unwinding
 .eh_frame
 Contains the information necessary to pop back to the
state of the machine registers and the stack at any point
higher up the call stack
 .gcc_except_table
 Contains information about the exception handling
"langing pads" the locations of handlers.
 This is necessary so as to know when to stop
unwinding.
 http://gcc.gnu.org/ml/gcc-help/2010-09/msg00116.html

11. Reference
 C++ ABI for Itanium: Exception handling
 http://refspecs.linuxfoundation.org/abi-eh-
1.22.htm
 sjlj vs dw2 in GCC
 http://stackoverflow.com/questions/318383/exce
ption-handling-models-of-gcc