[llvm-dev] Exceptions not getting caught on bare-metal target

[Dominik Montada via llvm-dev]

Hi Anton,

thanks for the reply. I left some comments and questions below.

Am 03.07.20 um 17:48 schrieb Anton Korobeynikov:
> Dominik,
>
> Multiple things are involved:
>
> 1. You need to make sure libunwind knows where to look for EH tables.
> Usually some communication from dynamic linker or some other way of
> obtaining the address of EH table section is necessary
Since we are on a bare-metal target we only do static linking. We used 
the following snippet found in AddressSpace.hpp in our linker script:

.eh_frame :
{
     __eh_frame_start = .;
     KEEP(*(.eh_frame))
     __eh_frame_end = .;
}

.eh_frame_hdr :
{
     KEEP(*(.eh_frame_hdr))
}

__eh_frame_hdr_start = SIZEOF(.eh_frame_hdr) > 0 ? ADDR(.eh_frame_hdr) : 0;
__eh_frame_hdr_end = SIZEOF(.eh_frame_hdr) > 0 ? . : 0;

And unwind does find the EH tables. It just cannot find an entry which 
covers the given PC. But I don't know whether the tables or the PC (or 
both?) are incorrect.
> 2. You need to describe properly the location of all registers that
> need to be restored. If there is something special on your platform,
> then you need to implement this functionality in libunwind as well.
Thanks! I assume the correct place to implement this in our case would 
be stepWithDwarf? I saw some other backends doing some target-specific 
handling there as well.
>
> You may want to enable debug printing in libunwind – usually it helps a lot.

I already have all traces enabled and also uncommented some fprintf 
calls, but they just tell me that no entry covers the PC (which was 
already helpful in that I at least know what is going wrong, just not why).

Cheers,

Dominik

>
> On Fri, Jul 3, 2020 at 1:22 PM Dominik Montada via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi,
>>
>> We're working on adding exception handling support for a downstream
>> bare-metal target. I read through the LLVM exception handling docs [1]
>> and went through some patches from other backends to understand what
>> parts we need to implement.
>>
>> We're now at a point were it feels like it should work, but
>> unfortunately exceptions are still not getting caught. Our target uses
>> DWARF unwinding.
>>
>> The problem seems to be that findFDE cannot find a CIE which covers the
>> given PC. We're not sure why this is though. From [2] and other backend
>> implementations we gathered that unw_getcontext saves the PC of the
>> call-site, i.e. the return address of unw_getcontext and that is that we
>> are doing as well.
>>
>> Is there some other target hook in LLVM or libunwind that we need to
>> implement in order to get this to work? Or are we not emitting
>> enough/correct CFI instructions (currently we only emit .cfi_def_cfa in
>> the prolog, we don't have any callee-saved registers).
>>
>> Speaking of calling conventions: our target is special in that registers
>> are automatically pushed to a special save area upon executing a call,
>> and automatically restored when executing a return instruction. I assume
>> we need to implement some target specific code in stepWithDwarf to
>> handle this?
>>
>> Any help in this regard would be highly appreciated!
>>
>> Cheers,
>>
>> Dominik
>>
>> [1] https://llvm.org/docs/ExceptionHandling.html
>> [2] https://www.nongnu.org/libunwind/man/unw_getcontext(3).html
>>
>> --
>> ----------------------------------------------------------------------
>> Dominik Montada                   Email: dominik.montada at hightec-rt.com
>> HighTec EDV-Systeme GmbH          Phone: +49 681 92613 19
>> Europaallee 19                    Fax:   +49-681-92613-26
>> D-66113 Saarbrücken               WWW: http://www.hightec-rt.com
>>
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>>
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>
-- 
----------------------------------------------------------------------
Dominik Montada                   Email: dominik.montada at hightec-rt.com
HighTec EDV-Systeme GmbH          Phone: +49 681 92613 19
Europaallee 19                    Fax:   +49-681-92613-26
D-66113 Saarbrücken               WWW: http://www.hightec-rt.com

Managing Director: Vera Strothmann
Register Court: Saarbrücken, HRB 10445, VAT ID: DE 138344222

This e-mail may contain confidential and/or privileged information. If
you are not the intended recipient please notify the sender immediately
and destroy this e-mail. Any unauthorised copying, disclosure or
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