iphone + LDC, a new ARM adventure

[Dan Olson]

Jacob Carlborg <doob at me.com> writes:

> On 2014-01-05 08:34, Dan Olson wrote:
>
>> Ah, I didn't connect SLJL with setjmp/longjmp. Good clue. Yes, it
>> appears arm-apple-darwin exceptions are setjmp based (confirmed by
>> compiling a c++ function with clang). Ok, I will dig in some more, see
>> where it leads me.
>
> Hmm, I would have thought iOS used DWARF-based exception handling.

Hi Jacob.

Yeah, I probably have called it wrong as I am just learning about
exception handling implementations.  clangc c++ exception handling for
C++ armv7-apple-darwin is emitting __gxx_personality_sj0 and plugs in
_Unwind_SjLj_Register/_Unwind_SjLj_Unregister.  Perhaps those are just
for catching C code longjmps?

For example, this C++ code:

    try {
        death();
    }
    catch (...) {
        puts("got it");
    }

produced (edited):

_main:
	.cfi_startproc
	.cfi_personality 155, L___gxx_personality_sj0$non_lazy_ptr
Leh_func_begin0:
	.cfi_lsda 16, Lexception0
@ BB#0:                                 @ %entry
	push	{r4, r5, r6, r7, lr}

--- snip ---

LPC0_1:
	add	r1, pc
	str.w	sp, [sp, #52]
	str	r1, [sp, #48]
	movs	r1, #1
	str	r1, [sp, #16]
	blx	__Unwind_SjLj_Register
Ltmp0:
	blx	__Z5deathv
Ltmp1:
LBB0_1:                                 @ %try.cont
	add	r0, sp, #12
	blx	__Unwind_SjLj_Unregister
	add	r4, sp, #64
	movs	r0, #0

--- snip function prologue ---

LBB0_5:                                 @ %invoke.cont2
Ltmp2:
	ldr	r0, [sp, #20]
	ldr	r1, [sp, #24]
	blx	___cxa_begin_catch
	movw	r0, :lower16:(L_.str-(LPC0_3+4))
	movt	r0, :upper16:(L_.str-(LPC0_3+4))
LPC0_3:
	add	r0, pc
	blx	_puts
	mov.w	r0, #-1
	str	r0, [sp, #16]
	blx	___cxa_end_catch
	b	LBB0_1

And I do see an exception table.

	.section	__TEXT,__gcc_except_tab
	.align	2
GCC_except_table0:
Lexception0:
L_LSDA_0:
	.byte	255                     @ @LPStart Encoding = omit
	.byte	155                     @ @TType Encoding = indirect pcrel sdata4
	.asciz	 "\212\200\200"         @ @TType base offset
	.byte	3                       @ Call site Encoding = udata4
	.byte	2                       @ Call site table length
	.byte	0                       @ >> Call Site 0 <<
                                        @   On exception at call site 0
	.byte	1                       @   Action: 1
	.byte	1                       @ >> Action Record 1 <<
                                        @   Catch TypeInfo 1
	.byte	0                       @   No further actions
                                        @ >> Catch TypeInfos <<
	.long	0                       @ TypeInfo 1
	.align	2


I read llvm source code for a while yesterday and really haven't wrapped
by brain around it yet.

Dan