On Mon, Jun 2, 2014 at 11:00 AM, Bernd Edlinger
<bernd.edlin...@hotmail.de> wrote:
> Hi,
>
> the test case g++.old-deja/g++.mike/p4736b.C is mis-compiled with with all
> optimization levels, except -O0 and -Og. This probably started with gcc 4.7.x.
>
> The constructor Main::Main() is first inlined and then completely optimized
> away in the dce1 pass. But the thunk is still using the vtable, and therefore
> crashes.
>
> Well, the problem seems to be, that the thunk code is
> not emitted in the normal way using gimple code,
> but instead by the i386 back-end with a callback.
>
> This makes the thunk invisible to the ipa-pure-const pass,
> which assumes that all values just flow straight thu the thunk.
>
> See ipa-pure-const.c (analyze_function):
>
> if (fn->thunk.thunk_p || fn->alias)
> {
> /* Thunk gets propagated through, so nothing interesting happens. */
> gcc_assert (ipa);
> return l;
> }
>
> But this is not true for a virtual base class:
> The thunk itself references the vtable, so if nothing else might need
> the vtable, the optimizer may remove the initalization of the vtable,
> which happened in this example.
>
> The most simple work around would be the attached patch, which
> simply falls back to emitting gimple code, if virtual base classes
> are used.
>
> Boot-Strapped and Regression-Tested on x86_64-linux-gnu.
> Ok for trunk?
Honza should know better. I'd rather skip the above for
thunks going the output_mi_thunk way.
That is, the target hook docs should be updated to reflect which
kind of thunks it is supposed to handle.
Richard.
>
> Thanks
> Bernd.
>
