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

On Thu, Feb 13, 2014 at 5:39 PM, Renato Golin <renato.golin at linaro.org>
wrote:
> On 13 February 2014 11:35, Evgeniy Stepanov <eugenis at google.com>
wrote:
>> Unwind table index '.ARM.exidx' at offset 0x818 contains 4
entries:
>> 0x5d4 <main>: 0x1 [cantunwind]
>
> This is exactly what I meant.
>
>
>> because the latter prevent any unwinding, breaking
>> debuggers/profilers/sanitizers.
>
> In that case, my argument stands again.
>
>
>> As I understand, the right way to enable basic unwind through any
>> function (without emitting landing pands etc, unless they are needed
>> for something else) is -funwind-tables. I'm going to add that in
the
>> driver for all sanitizers then.
>
> -funwind-tables are not yet understood by ARM, but it is for Dwarf EH.
> I plan on moving ARM to the same style as Dwarf, so you can assume it
> is correct.
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.

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.

--renato

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