llvm dev - Feb 2014 - [LLVMdev] [cfe-dev] Unwind behaviour in Clang/LLVM

On Thu, Feb 13, 2014 at 5:52 PM, Renato Golin <renato.golin at linaro.org>
wrote:
> On 13 February 2014 13:47, Evgeniy Stepanov <eugenis at google.com>
wrote:
>> Hm, I see that -funwind-tables on arm-linux-androideabi target
>> replaces this "cantunwind" with a proper unwind table.
>> Hence http://llvm-reviews.chandlerc.com/D2762.
>
> If Android is using EHABI (I think it is), the default now is to
> output full tables all the time, everywhere. This will change to be
> the same as x86 soon.
It does use EHABI, but at r201326:
$ cat 1.cc
int f() {}
$ ./bin/clang++ -target arm-linux-androideabi 1.cc -c
$ readelf -u 1.o
0x0 <_Z1fv>: 0x1 [cantunwind]
$ ./bin/clang++ -target arm-linux-androideabi 1.cc -c -funwind-tables
$ readelf -u 1.o
0x0 <_Z1fv>: 0x8000b0b0
  Compact model 0
  0x00      vsp = vsp + 4
  0xb0      finish
  0xb0      finish

Hi Evgeniy,

I have run in to the some unwind handling table issue recently.  After some
investigation, I believe that the correct behavior is to generate
[cantunwind] with and without the -funwind-tables.  Thus, I am afraid that
-funwind-tables might not be a good solution for your use case.  (i.e. your
program might fall in an infinite loop during stack unwinding.)

I will send the details for discussion ASAP.

Sincerely,
Logan


On Thu, Feb 13, 2014 at 10:03 PM, Evgeniy Stepanov <eugenis at
google.com>wrote:
> On Thu, Feb 13, 2014 at 5:52 PM, Renato Golin <renato.golin at
linaro.org>
> wrote:
> > On 13 February 2014 13:47, Evgeniy Stepanov <eugenis at
google.com> wrote:
> >> Hm, I see that -funwind-tables on arm-linux-androideabi target
> >> replaces this "cantunwind" with a proper unwind table.
> >> Hence http://llvm-reviews.chandlerc.com/D2762.
> >
> > If Android is using EHABI (I think it is), the default now is to
> > output full tables all the time, everywhere. This will change to be
> > the same as x86 soon.
>
> It does use EHABI, but at r201326:
> $ cat 1.cc
> int f() {}
> $ ./bin/clang++ -target arm-linux-androideabi 1.cc -c
> $ readelf -u 1.o
> 0x0 <_Z1fv>: 0x1 [cantunwind]
> $ ./bin/clang++ -target arm-linux-androideabi 1.cc -c -funwind-tables
> $ readelf -u 1.o
> 0x0 <_Z1fv>: 0x8000b0b0
>   Compact model 0
>   0x00      vsp = vsp + 4
>   0xb0      finish
>   0xb0      finish
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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I'd love to hear more details. Are you saying that this infinite loop
is a limitation of EHABI table format, and not something that can be
fixed in the compiler?
Meanwhile, please notice that gcc behavior matches current clang
behavior that I described above. We would not want to create an
incompatibility.


On Fri, Feb 14, 2014 at 8:42 PM, Logan Chien <tzuhsiang.chien at
gmail.com> wrote:
> Hi Evgeniy,
>
> I have run in to the some unwind handling table issue recently.  After some
> investigation, I believe that the correct behavior is to generate
> [cantunwind] with and without the -funwind-tables.  Thus, I am afraid that
> -funwind-tables might not be a good solution for your use case.  (i.e. your
> program might fall in an infinite loop during stack unwinding.)
>
> I will send the details for discussion ASAP.
>
> Sincerely,
> Logan
>
>
> On Thu, Feb 13, 2014 at 10:03 PM, Evgeniy Stepanov <eugenis at
google.com>
> wrote:
>>
>> On Thu, Feb 13, 2014 at 5:52 PM, Renato Golin <renato.golin at
linaro.org>
>> wrote:
>> > On 13 February 2014 13:47, Evgeniy Stepanov <eugenis at
google.com> wrote:
>> >> Hm, I see that -funwind-tables on arm-linux-androideabi target
>> >> replaces this "cantunwind" with a proper unwind
table.
>> >> Hence http://llvm-reviews.chandlerc.com/D2762.
>> >
>> > If Android is using EHABI (I think it is), the default now is to
>> > output full tables all the time, everywhere. This will change to
be
>> > the same as x86 soon.
>>
>> It does use EHABI, but at r201326:
>> $ cat 1.cc
>> int f() {}
>> $ ./bin/clang++ -target arm-linux-androideabi 1.cc -c
>> $ readelf -u 1.o
>> 0x0 <_Z1fv>: 0x1 [cantunwind]
>> $ ./bin/clang++ -target arm-linux-androideabi 1.cc -c -funwind-tables
>> $ readelf -u 1.o
>> 0x0 <_Z1fv>: 0x8000b0b0
>>   Compact model 0
>>   0x00      vsp = vsp + 4
>>   0xb0      finish
>>   0xb0      finish
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>