[cfe-dev] Porting libcxxabi / Unwind to Windows / MingW 32 bit

[Yaron Keren]

I see. It is different than the gcc runtime libgcc.

libgcc always uses the set of FDE which was registered by __register_frame.
This data is private to every process since unwind runs in user space as
part of the process.

Here, the OS first locates which set of FDE is relevant.
Does this mean that in OS-X unwind runs in system space and holds FDE data
for all processes?

Yaron




2013/10/31 Nick Kledzik <kledzik at apple.com>

>
> On Oct 30, 2013, at 3:22 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>
> Hi,
>
> You are exactly correct, LocalAddressSpace::findUnwindSections does
> nothing #if !APPLE so the output value is random.
>
> LocalAddressSpace::findOtherFDE and LocalAddressSpace::findFunctionName
> also currently do nothing. Are these also required for regular exceptions?
>
> No.
>
> findOtherFDE() is only need if the OS has an alternate way to locate FDEs
> such as those for JITed code.
>
> findFunctionNam() is used in debug builds to print out the name of
> functions as the unwinding happens.
>
> -Nick
>
>
>
> 2013/10/31 Nick Kledzik <kledzik at apple.com>
>
>> Yaron,
>>
>> Each separately linked image (program, DLLs, etc) has its own set of FDEs
>> (in its .eh_frame) section.  The method
>> LocalAddressSpace::findUnwindSections() needs to be updated in an OS
>> specific way to: 1) find the image for a given pc value, and 2) find the
>> .eh_frame section bounds for that image.
>>
>> -Nick
>>
>>
>> On Oct 30, 2013, at 3:03 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>
>> Hi Nick,
>>
>> I looked at the disassembly, unw_getcontext() gets the correct IP into
>> the context and later setInfoBasedOnIPRegister gets it right into "pc".
>> This is OK.
>>
>> The trouble is, the whole address space is nowhere near the fde values.
>> For example,
>>
>> p =          0x66179070
>> <libunwind::DwarfFDECache<libunwind::LocalAddressSpace>::_initialBuffer>
>> pc =         0x6604e7d9
>> fde =         0x40801c0
>> p->ip_start = 0x4080010
>> p->ip_end =   0x408010c
>>
>> p is consistent with pc and local vars and addresses in unwind.dll
>> (0x66YYYYYY)
>> fde is consistent with the ip ranges and with the caller of unwind.dll
>> (the throwing function) address space (0x40800YY)
>>
>> they live in different memory ranges, the fde addresses are in caller
>> range whereas the unwind addresses are in the DLL range so when it tries to
>> find its IP in the fde tables it can't find.
>>
>> It's as if the DLL is supposed to load in other memory range, some linker
>> option required?
>>
>> Yaron
>>
>>
>>
>> 2013/10/29 Nick Kledzik <kledzik at apple.com>
>>
>>>
>>> On Oct 29, 2013, at 12:11 AM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>>
>>> > Hello,
>>> >
>>> > I'm trying to make libcxxabi / Unwind work on Windows / MingW 32 bit.
>>> Attached is a patch to make it compile without warnings and run without
>>> crashing.
>>> >
>>> > However the functionality isn't working yet: upon exception, the IP
>>> seems to be completely out of range the FDE tables. This may be related to
>>> the memory model / compilation options???
>>> Take a look at UnwindRegisterRestore.s.  When an exception is thrown,
>>> unw_getcontext() is called which saves all registers and the return address
>>> into a register struct.  Then the FDE info is used to step through each
>>> frame and modify the register set.  At which step is the IP register out of
>>> bounds?
>>>
>>> It might be that the C files need to be compiled with -fexceptions so
>>> that they have unwind tables.
>>>
>>> -Nick
>>>
>>>
>>>
>>
>>
>
>
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