[LLVMdev] Proposal: type uniquing of debug info for LTO

[David Blaikie]

On Thu, Jun 20, 2013 at 4:45 PM, Manman Ren <mren at apple.com> wrote:
>
> On Jun 20, 2013, at 3:55 PM, Manman Ren <mren at apple.com> wrote:
>
>
> On Jun 20, 2013, at 2:58 PM, Eric Christopher wrote:
>
> Hi Manman,
>
> On Thu, Jun 20, 2013 at 2:51 PM, Manman Ren <mren at apple.com> wrote:
>
>
> The intent of this proposal is to speedup compilation of "-flto -g" for c++
> programs.
> This is based on discussions with Adrian, David and Eric.
>
>
> Thanks for bringing this back to the list. The original thread was
> getting quite long.
>
> ---------------------------
> Problem:
> A single class can be used in multiple source files and the DI (Debug Info)
> class is included in multiple bc files. The duplication of
> class definitions causes blow-up in # of MDNodes, # of DIEs, leading to
> large memory requirement.
>
> As an example, SPEC xalancbmk requires 7GB of memory when compiled with
> -flto -g.
> With a preliminary implementation of type uniquing, the memory usage will be
> down to 2GB.
>
> In order to unique types, we have to break cycles in the MDNodes.
>
> A simple struct definition
> struct Base {
> int a;
> };
> can cause cycles in MDNodes:
> !12 = metadata !{i32 786451, metadata !13, null, metadata !"Base", i32 1,
> i64 32, i64 32, i32 0, i32 0, null, metadata !14, i32 0, null, null} ; [
> DW_TAG_structure_type ] [Base] [line 1, size 32, align 32, offset 0] [from ]
> !14 = metadata !{metadata !15, metadata !16}
> !15 = metadata !{i32 786445, metadata !13, metadata !12, metadata !"a", i32
> 2, i64 32, i64 32, i64 0, i32 0, metadata !8} ; [ DW_TAG_member ] [a] [line
> 2, size 32, align 32, offset 0] [from int]
> !16 = metadata !{i32 786478, metadata !13, metadata !12, metadata !"Base",
> metadata !"Base", metadata !"", i32 1, metadata !17, i1 false, i1 false, i32
> 0, i32 0, null, i32 320, i1 false, null, null, i32 0, metadata !20, i32 1} ;
> [ DW_TAG_subprogram ] [line 1] [Base]
>
> Cycles: !12 -- !14 -- !15 -- !12
>           !12 -- !14 -- !16 -- !12
>
> These cycles make it hard to unique the same struct used in two bc files.
>
> ---------------------------
> How to fix:
>
> We attach a hash value to types to help type uniquing and we also replace
> references to types with their hash values.
> For the above struct "Base", we now have the following MDNodes:
> !4 = metadata !{i32 786451, metadata !5, null, metadata !"Base", i32 1, i64
> 32, i64 32, i32 0, i32 0, null, metadata !6, i32 0, i32 0, null, i32
> 915398439} ; [ DW_TAG_structure_type ] [Base] [line 1, size 32, align 32,
> offset 0] [from ]
> !6 = metadata !{metadata !7, metadata !9}
> !7 = metadata !{i32 786445, metadata !5, i32 915398439, metadata !"a", i32
> 2, i64 32, i64 32, i64 0, i32 0, metadata !8} ; [ DW_TAG_member ] [a] [line
> 2, size 32, align 32, offset 0] [from int]
> !9 = metadata !{i32 786478, metadata !5, i32 915398439, metadata !"Base",
> metadata !"Base", metadata !"", i32 1, metadata !10, i1 false, i1 false, i32
> 0, i32 0, null, i32 320, i1 false, null, null, i32 0, metadata !13, i32 1} ;
> [ DW_TAG_subprogram ] [line 1] [Base]
>
> Note that the cycles are gone and !4 has a hash value of 915398439, and the
> references to !4 are replaced with 915398439.
> Thanks Eric for suggesting replacing MD reference with a hash value.
>
>
> In particular I recommended this:
>
> a) For C++ odr replace it with the "hash" that's just a string
> representing the type name.
> b) For Internal C++ types and all C types replace it with a string
> that's a concatenation of the type name and the name of the compile
> unit.
>
> Yes, that is what we agreed on over email.
>
>
>
> There are a few issues:
> 1> How to generate the hash for a given type?
> With C++'s ODR, it should be enough by using the context and the name for
> non-internal c++ types.
> For internal c++ types and types of other languages, hash will not be used.
>
>
> Explain this?
>
>
> For a while, I am going to support both hash and MD reference, once
> everything is working with hash,
> I will update all debug info testing cases, turn -gtype-uniquing on, and
> remove the other path.
>
> For internal c++ types, initially they will follow the path of using MD
> references without a hash.
>
>
> My current implementation is to add a few static member functions in MDNode
> to profile DI nodes differently.
> + /// If the array of Vals is for debug info, profile it specially and
> return true.
> + /// If the DI node has a hash value, generate the profile using only the
> hash value and the declaration flag.
> +  static bool profileDebugInfoNode(ArrayRef<Value*> Vals, FoldingSetNodeID
> &ID);
>
> + /// If the MDNode is for debug info, profile it specially and return true.
> + /// If the DI node has a hash value, generate the profile using only the
> hash value and the declaration flag.
> +  static bool profileDebugInfoNode(const MDNode *M, FoldingSetNodeID &ID);
>
> + /// Given a hash value and a flag, generate the profile for later lookup.
> +  static bool profileDebugInfoNode(unsigned Hash, bool Declaration,
> FoldingSetNodeID &ID);
>
> These static functions are called in Metadata.cpp:
> void MDNode::Profile(FoldingSetNodeID &ID) const {
> +  if (profileDebugInfoNode(this, ID))
> +    return;
> +
>
> There are other examples of these in MDNode for handling of specific
> metadata.
> /// Methods for metadata merging.
> static MDNode *getMostGenericTBAA(MDNode *A, MDNode *B);
> static MDNode *getMostGenericFPMath(MDNode *A, MDNode *B);
> static MDNode *getMostGenericRange(MDNode *A, MDNode *B);
>
> Comments are welcome on whether this violates any layering rule.
>
>
> As I've said many times in email, I don't think this is a good idea
> and would prefer either a or b below. a) is a much simpler solution.
>
> Any reason that why it is not a good idea?
>
>
> Other choices are:
> a> Keep a map in DwarfDebug
>   Keep in mind that the map is used at many stages, and it has to be in sync
> with MDNodeSet.
> b> Generalize MDNode to be aware of hash (David can provide more details)
> c> Extend MDNode to DINode and modify streamers (bitcode reader|writer, ll
> reader|writer) to be aware of DINode
>   We can provide DINode::get(…) to create a DINode. DINode can have its own
> Profile function.
> Other suggestions are welcome.
>
>
> a or b please.
>
> Option a> will require a DwarfDebug pointer in every stage of the compiler,
> and passing the map to the DI classes.
> A rough estimation is around 100 places.
> Is it reasonable to pass a DwarfDebug pointer to DIBuilder and llvm linker?
> Also the map needs to be in sync with MDNodeSet, maybe using ValueHandle can
> solve the problem.
>
>
> What about putting the map in LLVMContextImpl?
> It already has a few things specifically for debug info:
> std::vector<DebugRecVH> ScopeRecords;
> DenseMap<std::pair<MDNode*, MDNode*>, int> ScopeInlinedAt;
> …
>
> I remember David mentioned it once and I forgot about the conclusion.

I mentioned it only as speculation as to how you were implementing it
already (but you were doing the profile-changing stuff).

I don't think it should be necessary to have the map (in option (a))
in such a central location as LLVMContext. It should be usable just
from DwarfDebug for generation, and DIBuilder can have its own,
separate map to do similar things during DI building.

>
> Thanks,
> Manman
>
>
> More details for option b from David
>
> < The alternative I have in mind is a more complete version of what
> < you're proposing - a full MD feature, not an MD feature that's just
> < barely enough to support the needs of debug info. What we could do is
> < allow the insertion of these MDHash things you spoke about but take it
> < a step further and have MDNode::getOperand walk through the hash &
> < give the value (in this way, DebugInfo wouldn't have to change at all
> < to handle hashes - if the Metadata APIs are going to be aware of the
> < hashes anyway, they might as well provide this convenience
> < functionality) the metadata feature would also have to have some
> < blessed top-level named metadata that would have a list of hash+MDNode
> < to keep those MDNodes alive (so you wouldn't have to stuff all the
> < types in the retained types list - metadata would provide the full
> < support, not just half of it).
>
>
>
> Transition from current DI Metadata:
> To have a smooth transition, I will add a flag "-gtype-hashing" for the type
> uniquing work and turn it on by default when we are ready.
>
>
> I'd prefer just make the change to have the front end emit the "hash"
> (it's not really a hash, it's just a string).
>
> Are you saying no transition period? A single patch to have correct handling
> of "hash" and to update all existing testing cases?
>
>
> -----------------------------
> Patches:
> Expect the following patches:
> 1> add flag -gtype-hashing
> 2> add hash field to DI types
> 3> modify DIBuilder to use hash instead of MD reference
> 4> related to issue 3
>
>
> These can all be a single patch since it shouldn't be very large if we
> go with a) above. If we go with b) then the MDNode work should be done
> in isolation first and then the debug info on top of it.
>
> What is wrong with smaller patches?
> My estimation for all the above with a) is about 30K + testing cases.
>
>
> 5> backend change (in DwarfDebug|CompileUnit) to support types shared among
> compile units
>   requires gdwarf-2 gdwarf-3 gdwarf-4 support for issues related to ref_addr
>
>
> #5 can and should be done before the rest of them.
>
> I prefer to submit patches according to the flow of the compiler, starting
> from the frontend, then IR, then backend.
> The testing cases will be added for front end, llvm-link and backend.
> Any reason why #5 should be done first?
>
>
> All changes should be local to debug info classes except patch #4.
>
>
> What's patch #4?
>
> Patch #4 above: related to issue 3 (changes corresponding to how to solve
> issue #3)
>
> -Manman
>
>
> -eric
>
>
>
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