[llvm-dev] distinct DISubprograms hindering sharing inlined subprogram descriptions

[David Blaikie via llvm-dev]

On Fri, Dec 23, 2016 at 11:47 AM Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

> A few disjoint thoughts; sorry they're so delayed (I skimmed the responses
> below, and I think these are still relevant/not covered elsewhere).
>
> Firstly, why *should* DISubprogram definitions be distinct?  There were
> two reasons this was valuable (this was from before there was a cu: link).
>
> - It helped to fix long-standing bugs in the IRLinker, where
> uniqued-DISubprograms in different compile units would coalesce.
>

Any idea why that ^ was a problem/bugs?


>  Now that the Function <-> DISubprogram connection has been reversed, I'm
> not entirely sure it's still a necessary precaution.
>
> - It broke a several uniquing cycles involving DISubprogram.  The
> existence of the uniquing cycles meant that affected DISubprograms weren't
> really uniqued, anyway (since we don't bother to solve graph isomorphism);
> and breaking the cycles sped up the IRLinker and reduced memory waste.
>

Ah, fair point - my trivial example didn't have any local variables, so
didn't hit this problem. They do indeed form cycles and I haven't tested,
but seems totally reasonable/likely that that would break my simple example
because cycles break uniquing and all that.


>  Making DISubprogram uniqued again would either have no real effect
> (because distinct nodes they reference aren't uniqued anymore either) or
> bring back uniquing cycles (which would have effects... but not the desired
> one).
>
> But there ought to be a way to get coalescing in the right places.  We
> just need to ensure that the uniqued/coalesced parts of the graph:
> - are acyclic, and
> - reference only coalesced nodes (note that leaves can happily include the
> strangely-coalesced distinct DICompositeType beast I created).
>

Perhaps before I dive into your suggestion (which, it seems, is to separate
out the definition but let the declaration/locals form cycles - that would
at least be dropped because the definition would only be kept due to it
being referenced from the llvm::Function), what about something simpler:

What if scope chains didn't go all the way to the top (the subprogram) but
stopped short of that?

Now, granted - this was part of the great assertion I added several years
back that caught all kinds of silliness - mostly around inlined call sites
not having debugloc and leading to scope chains that lead to distinct
roots. But the alternative to having and maintaining an invariant - is just
to make things true by construction. All debug info scope chains within a
function with an associated DISubprogram have an implicit last element of
that DISubprogram?

That would mean it'd be harder to catch the bugs I found - but I think most
of them came from that specific inlining situation (I can go back and do
some archaeology and see if there's other exposure where we could sure up
some defenses as well) which can be caught more directly (I forget if the
inliner now catches it itself, or if we enhanced the debug info verifier to
catch it after the inliner runs - if it's the verifier, then that wouldn't
suffice if we made the change I'm suggesting - and we'd have to hook it up
in the inliner itself)

Thoughts?


>
> I've been thinking idly whether we can/should break DISubprogram apart
> into uniqued (coalesce-able) and distinct parts.  Still being someone
> ignorant of DWARF itself (and CodeView, for that matter), I'm not entirely
> sure.  But...
>
> Here's my understanding of the current uses of DISubprogram (where I'm
> using DISubprogramDecl for the usually uniqued declarations, and
> DISubprogramDefn for the always distinct definitions):
>
> // !dbg
> Function -> DISubprogram
>
> // Declaration (optional, only for member functions).
> DISubprogramDefn -> DISubprogramDecl
>
> // Variables:
> DISubprogramDefn -> MDNode -> DILocalVariable
>
> // Terminator for scope chains:
> DILocalVariable [-> DILocalScope [-> ...]] -> DISubprogramDefn
> DILocation [-> DILocalScope [-> ...]] -> DISubprogramDefn
> DICompositeType -> DISubprogramDefn // function-local type
>
> // Elements (member functions):
> DICompositeType -> MDNode -> DISubprogramDecl
> DISubprogramDecl -> DICompositeType
> DISubprogramDefn -> DICompositeType
>
>
> And here are the changes I would consider:
>
> 1. Split DISubprogramDecl and DISubprogramDefn into different types.
>
> 2. Remove definition-specific fields from DISubprogramDecl.
> - variables, a "dangerous" source of uniquing cycles.
> - declaration, which it will never have.
> - scopeLine, which it will never have.
> - unit, which the Defn will have when it's relevant.
>
> 3. Make a DISubprogramDecl mandatory for DISubprogramDefn.
>
> 4. Remove redundant fields from DISubprogramDefn.
> - name and linkageName are on the declaration.
> - type is redundant.
> - flags is likely redundant?
>
> Nothing so far has made any real changes, I think.  DISubprogramDecl
> should coalesce nicely between modules though.
>
> 5. Migrate links to point at DISubprogramDecl instead of DISubprogramDefn.
> - DILocalVariable scope chain.
> - DILocation scope chain.
> - DICompositeType scope.
>
> I don't think this will create any uniquing cycles.  DISubprogramDecl
> won't point at any of these.  Also, DILocalVariable should start coalescing
> with each other.
>
> Since DISubprogramDefn is only referenced from the Function !dbg
> attachment, only one should survive function-coalescing.
>
> One open question, since I don't know the backend code, is: can you emit
> the "correct" DWARF after step (5)?  I'm hoping you only need the Defn of
> the Function that has instructions referencing the
> DILocalVariable/DILocation.  In which case, we suddenly get a lot of
> coalescing.
>
> On 2016-Dec-15, at 10:54, David Blaikie <dblaikie at gmail.com> wrote:
>
> Branching off from a discussion of improvements to DIGlobalVariable
> representations that Adrian's working on - got me thinking about related
> changes that have already been made to DISubprogram.
>
> To reduce duplicate debug info when things like linkonce_odr functions
> were deduplicated in LTO linking, the relationship between a CU and
> DISubprogram was inverted (instead of a CU maintaining a list
> of subprograms, subprograms specify which CU they come from - and the
> llvm::Function references the DISubprogram, so if the llvm::Function goes
> away, so does the associated DISubprogram)
>
> I'm not sure if this caused a regression, but at least seems to miss a
> possible improvement:
>
> During IR linking (for LTO, ThinLTO, etc) these distinct DISubprogram
> definitions (& their CU link, even if they weren't marked 'distinct', the
> CU link would cause them to effectively be so) remain separate - this means
> that inlined versions in one CU don't refer to an existing subprogram
> definition in another CU.
>
> To demonstrate:
> inl.h:
> void f1();
> inline __attribute__((always_inline)) void f2() {
>   f1();
> }
> inl1.cpp:
> #include "inl.h"
> void c1() {
>   f2();
> }
> inl2.cpp:
> #include "inl.h"
> void c2() {
>   f2();
> }
>
> Compile to IR, llvm-link the result. The DWARF you get is basically the
> same as the DWARF you'd get without linking:
>
> DW_TAG_compile_unit
>   DW_AT_name "inl1.cpp"
>   DW_TAG_subprogram #0
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c1"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
> DW_TAG_compile_unit
>   DW_AT_name "inl2.cpp"
>   DW_TAG_subprogram #1
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c2"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #1 "f2"
>
> Instead of something more like this:
>
> DW_TAG_compile_unit
>   DW_AT_name "inl1.cpp"
>   DW_TAG_subprogram #0
>     DW_AT_name "f2"
>   DW_TAG_subprogram
>     DW_AT_name "c1"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
> DW_TAG_compile_unit
>   DW_AT_name "inl2.cpp"
>   DW_TAG_subprogram
>     DW_AT_name "c2"
>     DW_TAG_inlined_subroutine
>       DW_TAG_abstract_origin #0 "f2"
>
> (note that only one abstract definition of f2 is produced here)
>
> Any thoughts? I imagine this is probably worth a reasonable amount of
> savings in an optimized build. Not huge, but not nothing. (probably not the
> top of anyone's list though, I realize)
>
> Should we remove the CU link from a non-internal linkage subprogram (&
> this may have an effect on the GV representation issue originally being
> discussed) and just emit it into whichever CU happens to need it first?
>
> This might be slightly sub-optimal, due to, say, the namespace being
> foreign to that CU. But it's how we do types currently, I think? So at
> least it'd be consistent and probably cheap enough/better.
>
>
>
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