[LLVMdev] Proposal: type uniquing of debug info for LTO
Manman Ren
mren at apple.com
Fri Jun 21 11:50:09 PDT 2013
On Jun 21, 2013, at 11:35 AM, Eric Christopher wrote:
> On Thu, Jun 20, 2013 at 10:52 PM, Manman Ren <mren at apple.com> wrote:
>>
>> A summary of options for issue #3:
>> 3> To actually access the MDNode referenced via the hash value, we need to perform a lookup from the hash value to find the corresponding MDNode.
>> The questions are where to store this map and how to keep it up-to-date when a MDNode is replaced.
>> ---------------------
>> Option a) a map in DwarfDebug, AsmWriter and DIBuilder, modify access functions in DI classes to pass in the map.
>> May need to modify printing functions in AsmWriter.
>
> You may not need the map in DIBuilder if you piggy back on the
> existing map in CGDebugInfo.cpp. The AsmWriter stuff is unfortunate
> for just debug dumps of the IR. A way around this might be nice, but
> I've not looked into it.
>
>> ---------------------
>> Option b) I am going to expand David's description with more details, correct me if I am wrong.
>
> I'll let David continue this part of the thread with you.
>
>>
>> I personally prefer option b over a since it is much cleaner.
>>
>
> Option b is more general and should probably work. I suggested a
> originally because I know it would work and the impact is limited to
> debug info. Also it doesn't involve touching the generic metadata
> interface or code and could be done quickly and incrementally. b may
> also require further changes to the debug info infrastructure to deal
> with lookup, etc so may end up being as much churn as a. b is more
> useful if we think other users of metadata may want/need similar
> functionality in the future.
>
> I'm fine with b as a choice, but it's going to involve more planning
> and code review and involve people outside of you, me and Dave :)
Got it. David, are you okay with the details I added about option b?
Let's start a new thread with title "Proposal: extend Metadata (MDNode) to support MDHash" or something like that.
We can add a weight to MDHash (other names are fine with me), to support multiple MDNodes with the same hash, but different weight.
Thanks,
Manman
>
> -eric
>
>
>> Thanks,
>> Manman
>>
>> On Jun 20, 2013, at 5:39 PM, David Blaikie wrote:
>>
>>> On Thu, Jun 20, 2013 at 5:25 PM, Manman Ren <mren at apple.com> wrote:
>>>>
>>>> On Jun 20, 2013, at 5:18 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>>>
>>>> On Thu, Jun 20, 2013 at 5:13 PM, Manman Ren <mren at apple.com> wrote:
>>>>
>>>>
>>>> On Jun 20, 2013, at 4:52 PM, David Blaikie wrote:
>>>>
>>>> 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.
>>>>
>>>>
>>>> We also need the map during llvm linking since linking will create new
>>>> MDNodes.
>>>>
>>>>
>>>> I don't understand what you mean by this. IR linking shouldn't need to
>>>> do anything debug-info-specific, it should just be the normal IR
>>>> approach.
>>>>
>>>> The declaration-v-definition resolution can be done at codegen-time.
>>>> We'll have to walk all the lists of retained types anyway to build the
>>>> map, so we can do declaration-v-definition (keep definitions over
>>>> declarations when we see both) at that point.
>>>>
>>>>
>>>> Yes, you are right that at codegen-time, we can generate the map from the
>>>> lists
>>>> of retained types.
>>>>
>>>> But dumping the linked ll file requires the map when outputting comments of
>>>> the MDNode :]
>>>
>>> Depending on which things we print out, but yes, in some cases
>>> (derived types) we do print out the type referenced. I assume the
>>> AsmPrinter can build such a map, then. (in fact, with a few clients
>>> like this, it might be nice to build a bit of an abstraction around it
>>> rather than just using a raw map - something that has a ctor (or I
>>> suppose it could be a factory function) that reads in the right
>>> metadata, walks the compile units, etc, and builds the mapping)
>>>
>>>>
>>>> Thanks,
>>>> Manman
>>>>
>>>>
>>>> So any other opinion on putting it in LLVMContext other than it being
>>>> central?
>>>>
>>>> Thanks,
>>>> Manman
>>>>
>>>>
>>>>
>>>> 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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>>>>
>>>>
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>>>>
>>
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