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Re: VN_INFO might be NULL in PRE


On 23 October 2012 18:02, Richard Biener <richard.guenther@gmail.com> wrote:
> On Tue, Oct 23, 2012 at 7:36 AM, Zhenqiang Chen
> <zhenqiang.chen@linaro.org> wrote:
>> Hi,
>>
>> PRE bases on the result of value numbering (run_scc_vn). At the end,
>> it free_scc_vn. But before free_scc_vn, it might call cleanup_tree_cfg
>> ();
>>
>>   if (do_eh_cleanup || do_ab_cleanup)
>>     cleanup_tree_cfg ();
>>
>> cleanup_tree_cfg might call make_ssa_name which might reuse some
>> "name" from the FREE_SSANAMES list. If the VN_INFO of the "name" is
>> NULL, free_scc_vn will "Segmentation fault". PR 54902
>> (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54902) shows a real case.
>> The attached log shows the gdb backtrace to create a new ssa name.
>>
>> Here is function VN_INFO:
>>
>> vn_ssa_aux_t
>> VN_INFO (tree name)
>> {
>>   vn_ssa_aux_t res = VEC_index (vn_ssa_aux_t, vn_ssa_aux_table,
>>                                 SSA_NAME_VERSION (name));
>>   gcc_checking_assert (res);
>>   return res;
>> }
>>
>> Can we make sure "res" is not NULL for the new ssa name?
>>
>> In trunk, Richard's "Make gsi_remove return whether EH cleanup is
>> required" patches in r186159 and r186164 make "do_eh_cleanup" to
>> false. So cleanup_tree_cfg is not called in PRE. Then no new ssa name
>> will be created.
>>
>> Does the Richard's patch fix the root cause?
>
> I don't think so.  Where does cfgcleanup call make_ssa_name?  The solution
> would be to post-pone cleaning up the CFG until after free_ssc_vn.  To
> be able to compute dominators we only have to remove unreachable blocks
> (delete_unreachable_blocks)

The attachment (bt.log) in previous mail shows the trace to call
make_ssa_name. PR 54902 includes a test case to reproduce it. The
bt.log bases on 4.7.

I have no case to reproduce it for trunk. But cleanup_tree_cfg is
still called before free_ssc_vn.

Thanks!
-Zhenqiang


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