Re: [Patch] LTO: Force externally_visible for offload_vars/funcs (PR97179)

[Tobias Burnus <tobias@codesourcery.com>]

On 9/24/20 10:03 AM, Richard Biener wrote:

>> The symbols are added to offload_vars + offload_funcs.
>> In lto-cgraph.c's output_offload_tables there is the last chance
>> to remove now unused nodes ? as once the tables are streamed
>> for device usage, they cannot be changed. Hence, there one
>> has
>>     node->force_output = 1;
>> [Unrelated: this prevents later optimizations, which still
>> could be done; cf. PR95622]
>>
>>
>> The table itself is written in omp-offload.c's omp_finish_file.
> But this is called at LTRANS time only, in particular we seem
> to stream the offload_funcs/vars array, marking streamed nodes
> as force_output but we do not make the offload table visible
> to the partitioner.  But force_output should make the
> nodes not renamed.  But then output_offload_tables is called at
> the very end and we likely do not stream the altered
> force_output state.
>
> So - can you try, in prune_offload_funcs, in addition to
> setting DECL_PRESERVE_P, mark the cgraph node ->force_output
> so this happens early?  I guess the same is needed for
> variables (there's no prune_offloar_vars ...).

As it accesses global variables, I could do just the same
with the variables – but it did not seems to have an effect.

Following Jakub's suggestion, I also added
   __attribute__((used))
to the tree belonging to both tables in omp-offload.c's omp_finish
but that did not help, either.

I think both the 'used' and 'force_output' are red herrings:
after all, the tables and the referrenced funcs/vars are output;
the problem is 'just' that they end up in different ltrans
while not being public. – Thus, some property ia wrong
during building the cgraph or when it is partitioned into ltrans.

Any additional suggestion to try?

Tobias

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