This doesn't have anything to do with Cocoa. You might have better luck taking
it up on the Xcode-users list.
(Sent from my iPad.)
--
Conrad Shultz
On Nov 9, 2011, at 9:39, Don Quixote de la Mancha <quix...@dulcineatech.com>
wrote:
> My iOS App is build with the Apple LLVM 3.0 compiler in Thumb mode.
> For armv7, I'm pretty sure that's actually Thumb-2.
>
> I'm reimplementing my two most time-consuming functions in ARM
> assembly code. The callers of these functions are Thumb, so I use
> Thumb to ARM interworking instructions to switch to ARM in the
> prologue of my functions so I have acesss to ARM's richer instruction
> set and greater number of registers. At function exit I use ARM to
> Thumb interworking to return to ARM mode.
>
> GDB's disassembly is correct for the Thumb code, but when I am in ARM
> mode, it disassembles the ARM instructions as if each one were a pair
> of completely nonsensical Thumb instructions. Is there some way I can
> tell GDB to switch to ARM disassembly, then upon returning to Thumb
> code, use the Thumb disassembler?
>
> Google is no help. There are apparently other forks of GDB that can
> do that, but I haven't figured out a way to do it with GDB.
>
> LLDB apparently supports ARM debugging, but it does not yet work on
> iOS devices in Xcode 4.2. When I choose the LLDB debugger in Product
> -> Edit Scheme, then set a breakpoint in my code, my App hangs before
> hitting the breakpoint.
>
> It's been a long time since I've done any assembly of any sort, so I
> am brushing up on the ARM calling conventions by implementing
> functions that take various parameters and return various types of
> results in both C and ARM assembly. The lower_case functions are C
> and the CamelCase functions are assembly. I call abiTest the very
> first thing from main(), and use assert() to ensure that it returns
> YES
>
> BOOL abiTest( void )
> {
> void_no_args();
> VoidNoArgs();
>
> if ( 42 != int_no_args() )
> return NO;
>
> if ( 42 != IntNoArgs() )
> return NO;
>
> return YES;
> }
>
> Here is the source for IntNoArgs. .thumb_func is a directive for the
> linker. My research seems to indicate that you want it even for ARM
> functions, if one mixes the two types of code
>
> .globl _IntNoArgs
> .align 2
> .code 16
> .thumb_func _IntNoArgs
>
> _IntNoArgs:
> @ int IntNoArgs( void );
> .loc 1 __LINE__ 0
>
> adr r0, Larm1 @ Larm1 is a PC-relative address. r0's low bit
> will be cleared
> bx r0 @ Switch to ARM mode then branch to Larm1.
> That's the next instruction
>
> .align 4
> .code 32
> Larm1:
> stmfd sp!, { r7, lr }
>
> mov r0, #42
>
> ldmfd sp!, { r7, lr }
> bx lr
>
> Here is how GDB disassembles the _IntNoArgs. The first two lines are
> correct, the remainder are completely wrong
>
> 0x000172c8 <+0000> add r0, pc, #0 (adr r0, 0x172cc <VoidNoArgs+4>)
> 0x000172ca <+0002> bx r0
> 0x000172cc <+0004> lsls r0, r0
> 0x000172ce <+0006> stmdb sp!, {r1, r3, r5}
> 0x000172d2 <+0010> b.n 0x17a16
> 0x000172d4 <+0012> lsls r0, r0
> 0x000172d6 <+0014> ldmia.w sp!, {r1, r2, r3, r4, r8, r9, r10, r11,
> r12, sp, lr, pc}
>
> The disassembly stops here because the ldmia.w instruction appears to
> be putting a new value into the program counter after taking it from
> the stack, thereby returning from the subroutine. After I step over
> this instruction with "si" the disassembly pane show:
>
> 0x000172d8 <+0016> vrhadd.u16 d14, d14, d31
>
> The si instruction always does the right thing, by advancing just one
> instruction whether we are in Thumb or ARM mode. So GDB _must_ know
> the current instruction set architecture, it's just that the
> disassembler is not getting that information.
>
> There is a bit in one of the ARM's register that indicates the current
> mode. Some forks of GDB have the ability to use the value of that bit
> when determining which ISA to disassemble as, but this is apparently
> not the case with Xcode 4.2's GDB.
>
> Xcode's GDB has a command "set arm disassembler" and its corresponding
> "show arm disassembler" that looks like it would help, but it doesn't.
> That apparently is meant to support other kinds of ARM variants than
> what the iOS devices use.
>
> "set fallback-mode" can be set to arm, thumb or auto in other forks of
> GDB, but not Xcodes. Ditto for "set disassembler-flavor".
>
> What I would REALLY REALLY REALLY like is a machine debugger that
> worked just like MacsBug did on the Classic Mac OS. While GDB is
> generally capable of doing assembler debugging, it totally sucks for
> that purpose. That's not anyone's fault, really, because it is
> designed for source debugging. A good assembly debugger is designed
> to do it that way from the ground up.
>
> Thanks for any help you can give me,
>
> Don Quixote
> --
> Don Quixote de la Mancha
> Dulcinea Technologies Corporation
> Software of Elegance and Beauty
> http://www.dulcineatech.com
> quix...@dulcineatech.com
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