llvm dev - Jun 2013 - [LLVMdev] Proposal: type uniquing of debug info for LTO

More details please :]
What do you mean by "name the metadata"? Are you referring to the name
field of the MDNode?

Thanks,
Manman

On Jun 21, 2013, at 7:19 AM, David Blaikie wrote:
> 
> On Jun 21, 2013 1:19 AM, "Jeremy Lakeman" <Jeremy.Lakeman at
gmail.com> wrote:
> >
> > Wouldn't it be simpler to name the metadata based on the hash of
the content? Then you could use a normal reference to that metadata without
needing to create a new type or teach the rest of llvm how to use it...
> 
> Sounds reasonable (probably with some nice prefix to get these things a
namespace). Though this applies to both (a) and (b) equally.
> 
> >
> >
> > On Fri, Jun 21, 2013 at 3:22 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.
> >> ---------------------
> >> Option b) I am going to expand David's description with more
details, correct me if I am wrong.
> >>    A special Value called MDHash in parallel with MDString, MDHash
needs to specify whether it is a use of the hash value, or a definition of the
hash value
> >>    The updated class hierarchy:
> >>    Value -- MDNode
> >>                -- MDHash
> >>                -- MDString
> >>                -- User -- Instruction -- BinaryOperator
> >>
> >>    In the following example:
> >> !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]
> >>    915398439 in !4 is the definition of a hash value, 915398439 in
!7 or !9 is the use of a hash value.
> >>
> >> A separate map "HashToNodeMap" is added to LLVMContext.
> >> MDNode will handle MDHash correctly in the sense that it will
update HashToNodeMap whenever it creates a MDNode with a MDHash operand.
> >> MDNode::getOperand will perform a lookup if the actual operand is
MDHash and return the corresponding MDNode.
> >>
> >> I am not sure whether a named metadata to describe the map is
necessary, assuming we already have the retained type list for each CU.
> >> Also at IR linking, the named metadata from all CUs will be
combined, leading to a long list.
> >>
> >> ---------------------
> >> David's description is pasted here for convenience:
> >> a blessed named MD node root that would contain string+node pairs
> >> (alternating elements probably, for space conservation)
> >>
> >> a means to add an operand to an MD node with an
"indirect"
> >> reference - this would cause the string to be used as the value
(with
> >> some flag that it's a 'special' string) but any access
of the node
> >> through the operand list would return the underlying MDNode from
the
> >> list (lookup via a map in LLVMContext, essentially).
> >>
> >> ---------------------
> >> Options that are vetoed:
> >> 1> Specialize Profile function for DI nodes to use MDNodeSet to
support lookup. No need for a separate map.
> >> 2> Extend the class hierarchy to have a class for DI nodes:
> >>  Value -- MDNode -- DINode
> >>              -- MDString
> >>              -- User -- Instruction -- BinaryOperator
> >>
> >> I personally prefer option b over a since it is much cleaner.
> >>
> >> 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
> >> >>
> >> >>
> >> >>
> >> >> _______________________________________________
> >> >> LLVM Developers mailing list
> >> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >>
> >> >>
> >> >>
> >> >> _______________________________________________
> >> >> llvm-commits mailing list
> >> >> llvm-commits at cs.uiuc.edu
> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> >> >>
> >> >>
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
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On Fri, Jun 21, 2013 at 10:13 AM, Manman Ren <mren at apple.com>
wrote:
>
> More details please :]
> What do you mean by "name the metadata"? Are you referring to the
name field
> of the MDNode?
Using named metadata rather than unnamed metadata.

rather than having:

!llvm.hardref = !{metadata !"foo.h::myClass", !3, metadata
!"bar.h:myEnum", !4} ...
!3 = ...;
!4 = ...;

we could simply have:

!llvm.hardref.foo.h.myClass = ...;
!llvm.hardref.bar.h.myClass = ...;

or something like that.

(assuming solution (b), in solution (a) it'd look more like
"llvm.dbg.type.foo.h.myClass" - but the same idea (then we could skip
using the retained types list, avoid adding a new 'name' member to
DITypes or changing the schema of the retained types list to be
string/mdnode pairs))
>
> Thanks,
> Manman
>
> On Jun 21, 2013, at 7:19 AM, David Blaikie wrote:
>
>
> On Jun 21, 2013 1:19 AM, "Jeremy Lakeman" <Jeremy.Lakeman at
gmail.com> wrote:
>>
>> Wouldn't it be simpler to name the metadata based on the hash of
the
>> content? Then you could use a normal reference to that metadata without
>> needing to create a new type or teach the rest of llvm how to use it...
>
> Sounds reasonable (probably with some nice prefix to get these things a
> namespace). Though this applies to both (a) and (b) equally.
>
>>
>>
>> On Fri, Jun 21, 2013 at 3:22 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.
>>> ---------------------
>>> Option b) I am going to expand David's description with more
details,
>>> correct me if I am wrong.
>>>    A special Value called MDHash in parallel with MDString, MDHash
needs
>>> to specify whether it is a use of the hash value, or a definition
of the
>>> hash value
>>>    The updated class hierarchy:
>>>    Value -- MDNode
>>>                -- MDHash
>>>                -- MDString
>>>                -- User -- Instruction -- BinaryOperator
>>>
>>>    In the following example:
>>> !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]
>>>    915398439 in !4 is the definition of a hash value, 915398439 in
!7 or
>>> !9 is the use of a hash value.
>>>
>>> A separate map "HashToNodeMap" is added to LLVMContext.
>>> MDNode will handle MDHash correctly in the sense that it will
update
>>> HashToNodeMap whenever it creates a MDNode with a MDHash operand.
>>> MDNode::getOperand will perform a lookup if the actual operand is
MDHash
>>> and return the corresponding MDNode.
>>>
>>> I am not sure whether a named metadata to describe the map is
necessary,
>>> assuming we already have the retained type list for each CU.
>>> Also at IR linking, the named metadata from all CUs will be
combined,
>>> leading to a long list.
>>>
>>> ---------------------
>>> David's description is pasted here for convenience:
>>> a blessed named MD node root that would contain string+node pairs
>>> (alternating elements probably, for space conservation)
>>>
>>> a means to add an operand to an MD node with an
"indirect"
>>> reference - this would cause the string to be used as the value
(with
>>> some flag that it's a 'special' string) but any access
of the node
>>> through the operand list would return the underlying MDNode from
the
>>> list (lookup via a map in LLVMContext, essentially).
>>>
>>> ---------------------
>>> Options that are vetoed:
>>> 1> Specialize Profile function for DI nodes to use MDNodeSet to
support
>>> lookup. No need for a separate map.
>>> 2> Extend the class hierarchy to have a class for DI nodes:
>>>  Value -- MDNode -- DINode
>>>              -- MDString
>>>              -- User -- Instruction -- BinaryOperator
>>>
>>> I personally prefer option b over a since it is much cleaner.
>>>
>>> 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
>>> >>
>>> >>
>>> >>
>>> >> _______________________________________________
>>> >> LLVM Developers mailing list
>>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> >>
>>> >>
>>> >>
>>> >> _______________________________________________
>>> >> llvm-commits mailing list
>>> >> llvm-commits at cs.uiuc.edu
>>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>> >>
>>> >>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>
>

On 21 June 2013 18:18, David Blaikie <dblaikie at gmail.com> wrote:
> !llvm.hardref.foo.h.myClass = ...;
> !llvm.hardref.bar.h.myClass = ...;
>
I like this idea! It's much easier to read than metadata fields, but will
it need to have mangled names that today, are quoted?

Do we still have the notion that named metadata cannot be deleted?

cheers,
--renato
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On Jun 21, 2013, at 10:18 AM, David Blaikie wrote:
> On Fri, Jun 21, 2013 at 10:13 AM, Manman Ren <mren at apple.com>
wrote:
>> 
>> More details please :]
>> What do you mean by "name the metadata"? Are you referring to
the name field
>> of the MDNode?
> 
> Using named metadata rather than unnamed metadata.
> 
> rather than having:
> 
> !llvm.hardref = !{metadata !"foo.h::myClass", !3, metadata
> !"bar.h:myEnum", !4} ...
> !3 = ...;
> !4 = ...;
> 
> we could simply have:
> 
> !llvm.hardref.foo.h.myClass = ...;
> !llvm.hardref.bar.h.myClass = ...;
> 
> or something like that.
From the documentation:
NamedMDNode - a tuple of MDNodes. Despite its name, a NamedMDNode isn't
itself an MDNode. NamedMDNodes belong to modules, have names, and contain lists
of MDNodes.

Our DI classes all wrap around a MDNode. Also during IR linking, NamedMDNode is
handled differently.
Did I miss something?

Thanks,
Manman
> 
> 
> (assuming solution (b), in solution (a) it'd look more like
> "llvm.dbg.type.foo.h.myClass" - but the same idea (then we could
skip
> using the retained types list, avoid adding a new 'name' member to
> DITypes or changing the schema of the retained types list to be
> string/mdnode pairs))
> 
>> 
>> Thanks,
>> Manman
>> 
>> On Jun 21, 2013, at 7:19 AM, David Blaikie wrote:
>> 
>> 
>> On Jun 21, 2013 1:19 AM, "Jeremy Lakeman" <Jeremy.Lakeman
at gmail.com> wrote:
>>> 
>>> Wouldn't it be simpler to name the metadata based on the hash
of the
>>> content? Then you could use a normal reference to that metadata
without
>>> needing to create a new type or teach the rest of llvm how to use
it...
>> 
>> Sounds reasonable (probably with some nice prefix to get these things a
>> namespace). Though this applies to both (a) and (b) equally.
>> 
>>> 
>>> 
>>> On Fri, Jun 21, 2013 at 3:22 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.
>>>> ---------------------
>>>> Option b) I am going to expand David's description with
more details,
>>>> correct me if I am wrong.
>>>>   A special Value called MDHash in parallel with MDString,
MDHash needs
>>>> to specify whether it is a use of the hash value, or a
definition of the
>>>> hash value
>>>>   The updated class hierarchy:
>>>>   Value -- MDNode
>>>>               -- MDHash
>>>>               -- MDString
>>>>               -- User -- Instruction -- BinaryOperator
>>>> 
>>>>   In the following example:
>>>> !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]
>>>>   915398439 in !4 is the definition of a hash value, 915398439
in !7 or
>>>> !9 is the use of a hash value.
>>>> 
>>>> A separate map "HashToNodeMap" is added to
LLVMContext.
>>>> MDNode will handle MDHash correctly in the sense that it will
update
>>>> HashToNodeMap whenever it creates a MDNode with a MDHash
operand.
>>>> MDNode::getOperand will perform a lookup if the actual operand
is MDHash
>>>> and return the corresponding MDNode.
>>>> 
>>>> I am not sure whether a named metadata to describe the map is
necessary,
>>>> assuming we already have the retained type list for each CU.
>>>> Also at IR linking, the named metadata from all CUs will be
combined,
>>>> leading to a long list.
>>>> 
>>>> ---------------------
>>>> David's description is pasted here for convenience:
>>>> a blessed named MD node root that would contain string+node
pairs
>>>> (alternating elements probably, for space conservation)
>>>> 
>>>> a means to add an operand to an MD node with an
"indirect"
>>>> reference - this would cause the string to be used as the value
(with
>>>> some flag that it's a 'special' string) but any
access of the node
>>>> through the operand list would return the underlying MDNode
from the
>>>> list (lookup via a map in LLVMContext, essentially).
>>>> 
>>>> ---------------------
>>>> Options that are vetoed:
>>>> 1> Specialize Profile function for DI nodes to use MDNodeSet
to support
>>>> lookup. No need for a separate map.
>>>> 2> Extend the class hierarchy to have a class for DI nodes:
>>>> Value -- MDNode -- DINode
>>>>             -- MDString
>>>>             -- User -- Instruction -- BinaryOperator
>>>> 
>>>> I personally prefer option b over a since it is much cleaner.
>>>> 
>>>> 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
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> _______________________________________________
>>>>>> llvm-commits mailing list
>>>>>> llvm-commits at cs.uiuc.edu
>>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>>>>> 
>>>>>> 
>>>> 
>>>> 
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> 
>>> 
>> 
>> 
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