llvm dev - Dec 2016 - distinct DISubprograms hindering sharing inlined subprogram descriptions

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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> On Dec 23, 2016, at 18:36, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> 
> 
>> 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
No (new) cycles; in fact this breaks some. 
- the Function itself references the defn and the locals;
- the locals and the defn reference (transitively) the decl; and
- the decl doesn't reference any of those.

The only cycle is the decl <-> composite type one that already exists (and
that odr magic (mostly) fixes).  Maybe that one could be removed somehow too,
but it isn't directly relevant to the problem you're looking at.

(Note that this is somewhat inspired by Reid's point in his recent debug
info talk, that CodeView avoids type cycles by breaking types into two records.)
> - 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 seems reasonable, I think, now that the function points directly at its
subprogram.

I'm not sure it's simpler to get right though.  Dropping the scope runs
the risk of coalescing things incorrectly.  I imagine the backend relies on
pointer distinctness of two local variables called "a" that are in (or
are parameters of) unrelated subprograms.  I think in some situations they could
be coalesced incorrectly if they had no scope.
> That would mean it'd be harder to catch the bugs I found
A shame if so, but if we can find another way to catch it or design it away, as
you mention, it doesn't much matter.
> - 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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On Fri, Dec 23, 2016 at 7:25 PM Duncan Exon Smith <dexonsmith at
apple.com>
wrote:
>
>
> On Dec 23, 2016, at 18:36, David Blaikie <dblaikie at gmail.com>
wrote:
>
>
>
> 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
>
>
> No (new) cycles; in fact this breaks some.
> - the Function itself references the defn and the locals;
> - the locals and the defn reference (transitively) the decl; and
> - the decl doesn't reference any of those.
>
> The only cycle is the decl <-> composite type one that already exists
(and
> that odr magic (mostly) fixes).  Maybe that one could be removed somehow
> too, but it isn't directly relevant to the problem you're looking
at.
>
Oh, yeah - I've an aside there too, maybe for another thread. (punchy
summary: anonymous enums in headers used for constants are technically ODR
violations (well, if they're ever used in an ODR entity - like an inline
function) - they're not public types, so we don't put the mangled name
on
them & don't deduplicate them in DWARF type units - and probably
similarly
don't deduplicate them in the IR... - not sure what the right answer is
there)

> (Note that this is somewhat inspired by Reid's point in his recent
debug
> info talk, that CodeView avoids type cycles by breaking types into two
> records.)
>
Ah, thanks, that helps with me picture it!

>
> - 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 seems reasonable, I think, now that the function points directly at
> its subprogram.
>
> I'm not sure it's simpler to get right though.  Dropping the scope
runs
> the risk of coalescing things incorrectly.  I imagine the backend relies on
> pointer distinctness of two local variables called "a" that are
in (or are
> parameters of) unrelated subprograms.  I think in some situations they
> could be coalesced incorrectly if they had no scope.
>
Just no top level scope - so it'd only happen if you had two variables with
the same name in the same scope. It might break in debug-having inline
functions inlined into non-debug-having functions (which then might in turn
be inlined into a debug-having function) - would have to check.

Also, not sure if inlined situations might introduce circularity. Yep. The
scope for inlinedAt DILocations point to the DISubprogram - so that'd
create a cycle. Ah well.

>
> That would mean it'd be harder to catch the bugs I found
>
>
> A shame if so, but if we can find another way to catch it or design it
> away, as you mention, it doesn't much matter.
>
> - 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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