Re: [PATCH] powerpc: Remove backtrace implementation

[Tulio Magno Quites Machado Filho <tuliom@ascii.art.br>]

Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:

> On 12/02/2021 14:21, Florian Weimer wrote:
>> * Adhemerval Zanella:
>> 
>> Does this really work well (enough) on systems which do not build
>> everything with asynchronous unwind tables?  If I recall correctly, GCC
>> enabled them by default only quite recently.
>
> This should work as well any other architecture since it uses the libgcc_s
> routines.  Afaik the powerpc implementation was done as optimization, not
> for correctness.

I think Florian is correct.

GCC enabled -fasynchronous-unwind-tables by default on ppc* ELF targets on
version 8 [1].
So if you take a Debian 10 ppc64le, build the backtrace example from
linux-manpages [2] with gcc-7 or with a newer GCC using
'gcc-9 -g0 -fno-asynchronous-unwind-tables', this executable won't be able to
print the correct backtrace, e.g.:

$ ./testrun.sh ./tst-no-asynchronous-unwind-tables 3
backtrace() returned 1 addresses
./tst-no-asynchronous-unwind-tables(+0xb48) [0x7fff99a00b48]

8 addresses were expected in the previous test.

With that said, I don't think the powerpc files are just an optimization and
removing them may cause old binaries to stop getting the right backtrace.

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=0359465c703a
[2] https://www.man7.org/linux/man-pages/man3/backtrace.3.html

-- 
Tulio Magno