Re: genmatch infinite loop during bootstrap on AIX

[David Edelsohn <dje.gcc@gmail.com>]

On Thu, Oct 30, 2014 at 4:51 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On Wed, Oct 29, 2014 at 6:13 PM, David Edelsohn <dje.gcc@gmail.com> wrote:
>> On Wed, Oct 29, 2014 at 9:24 AM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>
>>> Because only genmatch calls functions from libstdc++.  Btw, why
>>> would genmatch miscompile an empty function or the call to it?
>>
>> I tried bootstrapping with libstdc++ built without the AIX ld "-G"
>> flag and that is succeeding.
>>
>> "-G" produces a shared object for use with SVR4-style runtime linking,
>> so this version of libstdc++ no longer allows runtime function
>> interposition, e.g., operator new, although it is not used frequently.
>> Something about the GCC-produced tail calls is interacting badly with
>> that feature.
>>
>> Note that this makes GCC bootstrap on AIX very fragile at the moment
>> because it depends on how libstdc++ was built in previous releases.  I
>> can bootstrap with GCC 4.6.3 and 4.8.1 but not with 4.7.3, 4.8.0, nor
>> 4.9.0.  A problematic libstdc++ from earlier releases causes genmatch
>> to loop in stage 1.
>
> I see.  A bootstrap with IPA ICF disabled did not succeed but runs into
> the same issue in stage2.  So I wonder if the issue is latent for much
> longer and genmatch just exposes it now.
>
> I'll see if I can remove the use of std::map from genmatch as a
> workaround.

I assume that this is a pervasive issue in the interaction between GCC
optimizations and AIX -G linker option. Something expects the second
call to point to a different implementation. I don't know if the
address in the TOC (GOT) is wrong or something expects the second call
to use a different TOC, but there is a circular reference.

I am going to apply a patch to GCC to not build libstdc++ with -G.  I
also will apply it to GCC 4.9 branch.  I was hoping that it could be
included in GCC 4.9.2, but I needed to test it thoroughly and Jakub
beat me with the release.

- David