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Re: [debug-early] LTO streaming of on-the-side dwarf data structures


On Tue, Oct 14, 2014 at 2:48 AM, Aldy Hernandez <aldyh@redhat.com> wrote:
> Gentlemen, your feedback would be greatly appreciated!
>
> I was investigating why locals were not being early dumped, and realized
> Michael's patch was skipping decls_for_scope() unless
> DECL_STRUCT_FUNCTION->gimple_df was set.  I assume this was to wait until
> location information was available.  That design caused locals to be dumped
> LATE, which defeats the whole purpose of this exercise.
>
> Since we want the local DECL DIEs to be generated early as well, we'd want
> the location information to be amended in the second dwarf pass. This got me
> thinking about deferred_locations, and all these on-the-side data structures
> that a second dwarf pass would depend on.  Unless I'm misunderstanding
> something, we need a plan...
>
> Basically, any early collected data that dwarf2out_finish() and
> dwarf2out_function_decl() need, would need to be LTO streamed out after
> early dwarf generation and streamed back before the second dwarf pass. For
> instance, I see at least the following that need to be streamed in/out:
>
>         file_table
>         deferred_locations
>         limbo_die_list (and deferred_asm_name)
>         decl_die_table
>         pubname_table
>         pubtype_table

I think that all but decl_die_table should not be needed (which may
need implementation changes in dwarf2out.c of course).  Maybe you
can explain why you think they are needed late.

For decl_die_table the idea was to be able to create references to
the early output DIEs via decl->die_offset (to be added and LTO streamed)
and the translation unit decls symbol of the dwarf tree root.

> We could either stream the hash tables and/or data structures above (and
> merge them from different compilation units upon stream-in), or we could
> come up with some way of annotating existing dwarf (to be read/merged back
> in and annotated).  For instance, deferred_locations, decl_die_table, and
> limbo_die_lists need to associate a DIE with a TREE.  We could tag the DIE
> with an artificial DW_AT_* that has some byte representation of the TREE and
> recreate the hash at LTO stream-in time.  For other data structures (perhaps
> file_table and pub*_table), perhaps we could come up with yet another way o
> representing the data in Dwarf.

Why do we end up with any deferred stuff after early dwarf?  Similarly
nothing should be on the limbo list (instead we should properly construct
early dwarf!).

> However...I don't know if this is worth the trouble, or if we should just
> stream the individual hash tables and data structures, and not bother with
> this dwarf gymnastics.
>
> Did anybody have a plan for this?  Am I misunderstanding something, or do we
> need to stream a lot of these on-the-side structures?

I indeed hope we don't need to stream all this, but it may need more
"structured" generation of early dwarf (so we always have an origin
and thus do not need the limbo list for example).

Richard.

> Aldy


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