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

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.

---------------------------
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.

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.

2> Make sure the streamers (bit code or ll writer) will output all required
MDNodes.
For MDNodes that are only used through hash values, we have to make sure they
are dumped.
The solution currently is to add these MDNodes to retained types of the compile
unit.
Thanks David for the suggestion.

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.
Each hash value should have at most one corresponding definition and at most one
declaration.

We need the map when building DI for verification purposes, when writing the ll
file to dump the comments, at llvm linking time to update the map,
at backend when generating the actual dwarf.

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.

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.

---------------------------
Preliminary Results:
SPEC xalancbmk: ld time down from 20+ mins to 4+ mins on my Mac Air

-----------------------------
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.

-----------------------------
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
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
All changes should be local to debug info classes except patch #4.

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.
>
> 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?
> 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.
> 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.
> 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).
> -----------------------------
> 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.
> 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.
> All changes should be local to debug info classes except patch #4.
What's patch #4?

-eric

+1 to roughly everything Eric said.

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.
>
> ---------------------------
> 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.
>
> 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.
>
> 2> Make sure the streamers (bit code or ll writer) will output all
required MDNodes.
> For MDNodes that are only used through hash values, we have to make sure
they are dumped.
should read "to make sure they are not dumped" - just to clarify for
other readers: unnamed metadata is kept alive by being reference
(directly or indirectly) by named metadata. If we break the reference
by using a string identifier pair, the type metadata wouldn't exist as
it would be unreferenced in the IR sense.
> The solution currently is to add these MDNodes to retained types of the
compile unit.
So we can just keep real metadata (rather than string/hash/thingy)
references to these nodes in a flat list to ensure they don't get
dropped.
> Thanks David for the suggestion.
>
> 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.
> Each hash value should have at most one corresponding definition and at
most one declaration.
>
> We need the map when building DI for verification purposes, when writing
the ll file to dump the comments, at llvm linking time to update the map,
> at backend when generating the actual dwarf.
>
> 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 we've already discussed off list, both Eric & I believe this
isn't
an appropriate layering & that LLVM's core IR metadata APIs cannot
reasonably have special hooks for this debug info feature.
> 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.
I'm not sure what you mean by "in sync with the MDNodeSet".
> b> Generalize MDNode to be aware of hash (David can provide more
details)
My suggestion here, which I think is the only reason to really discuss
this broadly (because it would be an LLVM metadata API feature) is
roughly as follows:

An LLVM IR feature, completely independent from debug info that would
work something like:
a) a blessed named MD node root that would contain string+node pairs
(alternating elements probably, for space conservation)
b) 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).

I'm not at all deeply familiar with the LLVM IR metadata APIs - so
this is a strawman at best but an idea about how we could provide the
functionality that debug info could benefit from in an isolated, not
debug-specific, feature. If we're going to touch metadata APIs I think
something like this should be how we do it.

If anyone more familiar with these APIs & the direction we should and
shouldn't take them could clarify if this is
possible/reasonable/acceptable/the right tradeoff compared to (a) that
would be much appreciated. That's what I'd like to get out of this
thread.

(for some more context: (a) will be isolated to debug info code, but
will require (in my naive estimation) a fair amount of mechanical work
to perform manual lookups on types whenever we access them - at some
point we may have a patch that demonstrates just how much work that
is, but until then we can only make vague statements, and by the time
we do this by any substantial amount it will've been a fair bit of
work only to throw it away in favor of (b) if it looks too unweildy.)
> 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.
As discussed offline, Eric & I really don't think this is acceptable.
Again, those who own/maintain/have opinions on LLVM metadata APIs can
weigh in if you think we're off-base in that assessment.
> Other suggestions are welcome.
>
> ---------------------------
> Preliminary Results:
> SPEC xalancbmk: ld time down from 20+ mins to 4+ mins on my Mac Air
>
> -----------------------------
> 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.
>
> -----------------------------
> Patches:
> Expect the following patches:
> 1> add flag -gtype-hashing
> 2> add hash field to DI types
If we can avoid changing the schema of DI types it's really best that
way - especially adding (or removing) fields, moreso while that's
under a flag. It'll be a real pain to update test cases & diverge the
schema making it harder to maintain good test coverage.

Essentially what we want to do is a single pass (as Eric said) - where
before there were MDNodes, there would now be strings. That means
updating DIBuilder and DwarfDebug, etc is one go, basically.
> 3> modify DIBuilder to use hash instead of MD reference
> 4> related to issue 3
> 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
> All changes should be local to debug info classes except patch #4.

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.

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

On Jun 20, 2013, at 3:27 PM, David Blaikie wrote:
> +1 to roughly everything Eric said.
> 
> 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.
>> 
>> ---------------------------
>> 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.
>> 
>> 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.
>> 
>> 2> Make sure the streamers (bit code or ll writer) will output all
required MDNodes.
>> For MDNodes that are only used through hash values, we have to make
sure they are dumped.
> 
> should read "to make sure they are not dumped"
I meant to say make sure the MDNodes are written out to bc or ll file.

> - just to clarify for
> other readers: unnamed metadata is kept alive by being reference
> (directly or indirectly) by named metadata. If we break the reference
> by using a string identifier pair, the type metadata wouldn't exist as
> it would be unreferenced in the IR sense.
Thanks for the clarification.
> 
>> The solution currently is to add these MDNodes to retained types of the
compile unit.
> 
> So we can just keep real metadata (rather than string/hash/thingy)
> references to these nodes in a flat list to ensure they don't get
> dropped.
> 
>> Thanks David for the suggestion.
>> 
>> 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.
>> Each hash value should have at most one corresponding definition and at
most one declaration.
>> 
>> We need the map when building DI for verification purposes, when
writing the ll file to dump the comments, at llvm linking time to update the
map,
>> at backend when generating the actual dwarf.
>> 
>> 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 we've already discussed off list, both Eric & I believe this
isn't
> an appropriate layering & that LLVM's core IR metadata APIs cannot
> reasonably have special hooks for this debug info feature.
Understand, that is why it is an option and comments from other people are
welcome.
 
> 
>> 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.
> 
> I'm not sure what you mean by "in sync with the MDNodeSet".
When a node is created or RAUW, the map has to be updated.
MDNodeSet keeps all live MDNodes and the map keeps part of MDNodeSet.
> 
>> b> Generalize MDNode to be aware of hash (David can provide more
details)
> 
> My suggestion here, which I think is the only reason to really discuss
> this broadly (because it would be an LLVM metadata API feature) is
> roughly as follows:
> 
> An LLVM IR feature, completely independent from debug info that would
> work something like:
> a) a blessed named MD node root that would contain string+node pairs
> (alternating elements probably, for space conservation)
> b) 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).
> 
> I'm not at all deeply familiar with the LLVM IR metadata APIs - so
> this is a strawman at best but an idea about how we could provide the
> functionality that debug info could benefit from in an isolated, not
> debug-specific, feature. If we're going to touch metadata APIs I think
> something like this should be how we do it.
> 
> If anyone more familiar with these APIs & the direction we should and
> shouldn't take them could clarify if this is
> possible/reasonable/acceptable/the right tradeoff compared to (a) that
> would be much appreciated. That's what I'd like to get out of this
> thread.
> 
> (for some more context: (a) will be isolated to debug info code, but
> will require (in my naive estimation) a fair amount of mechanical work
> to perform manual lookups on types whenever we access them - at some
> point we may have a patch that demonstrates just how much work that
> is, but until then we can only make vague statements, and by the time
> we do this by any substantial amount it will've been a fair bit of
> work only to throw it away in favor of (b) if it looks too unweildy.)
> 
>> 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.
> 
> As discussed offline, Eric & I really don't think this is
acceptable.
> Again, those who own/maintain/have opinions on LLVM metadata APIs can
> weigh in if you think we're off-base in that assessment.
It is an option to go in the other direction, either generalize MDNode or have a
special class for DINode.

- Manman
> 
>> Other suggestions are welcome.
>> 
>> ---------------------------
>> Preliminary Results:
>> SPEC xalancbmk: ld time down from 20+ mins to 4+ mins on my Mac Air
>> 
>> -----------------------------
>> 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.
>> 
>> -----------------------------
>> Patches:
>> Expect the following patches:
>> 1> add flag -gtype-hashing
>> 2> add hash field to DI types
> 
> If we can avoid changing the schema of DI types it's really best that
> way - especially adding (or removing) fields, moreso while that's
> under a flag. It'll be a real pain to update test cases & diverge
the
> schema making it harder to maintain good test coverage.
> 
> Essentially what we want to do is a single pass (as Eric said) - where
> before there were MDNodes, there would now be strings. That means
> updating DIBuilder and DwarfDebug, etc is one go, basically.
> 
>> 3> modify DIBuilder to use hash instead of MD reference
>> 4> related to issue 3
>> 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
>> All changes should be local to debug info classes except patch #4.

Hi Manman,

On 21.06.2013 1:51, Manman Ren 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.
I see that the uniquing patches are already in trunk.  Are there more to 
come or the feature is essentially completed?  It would be great to have 
some announcement on the LLVMdev for this (even better with some 
statistics, if you have it).

Andrey

On 20 June 2013 17:51, 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.
>
> ---------------------------
> 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.
This is awesome! :-)
> In order to unique types, we have to break cycles in the MDNodes.
Sorry I missed this email earlier, but do we really need to break the
cycles? This is not a generic graph isomorphism problem. If we just
add the hashes to the metadata nodes representing types, merging then
becomes

for every two types with the same hashes:
  try merging them recursively. The order is important, so this is
just a dfs that succeeds or not.

Cheers,
Rafael

Hi Andrey,

Yes, there are more patches to come. I will announce it when the feature is
complete.

Manman


On Tue, Oct 1, 2013 at 12:39 AM, Andrey Belevantsev <abel at ispras.ru>
wrote:
> Hi Manman,
>
> On 21.06.2013 1:51, Manman Ren 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.
>>
>
> I see that the uniquing patches are already in trunk.  Are there more to
> come or the feature is essentially completed?  It would be great to have
> some announcement on the LLVMdev for this (even better with some
> statistics, if you have it).
>
> Andrey
>
>
> ______________________________**_________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>
http://lists.cs.uiuc.edu/**mailman/listinfo/llvmdev<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>
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On Tue, Oct 1, 2013 at 5:54 AM, Rafael Espíndola <rafael.espindola at
gmail.com
> wrote:
> On 20 June 2013 17:51, 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.
> >
> > ---------------------------
> > 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.
>
> This is awesome! :-)
>
> > In order to unique types, we have to break cycles in the MDNodes.
>
> Sorry I missed this email earlier, but do we really need to break the
> cycles? This is not a generic graph isomorphism problem. If we just
> add the hashes to the metadata nodes representing types, merging then
> becomes
>
> for every two types with the same hashes:
>   try merging them recursively. The order is important, so this is
> just a dfs that succeeds or not.
>
We considered the approach you mentioned above, to implement that, we need
to have special handling for debug info MDNodes.

For example:
a special implementation of the profiling function to make two types with
the same hash have the same profiling.
special handling at linking time to merge the type nodes.

We chose to localize the changes to debug info itself by using the hash
(type identifier: the current notation) when referring to a type node.

Thanks,
Manman

>
> Cheers,
> Rafael
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
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On Oct 1, 2013 5:57 AM, "Rafael Espíndola" <rafael.espindola at
gmail.com>
wrote:
>
> On 20 June 2013 17:51, 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.
> >
> > ---------------------------
> > 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.
>
> This is awesome! :-)
>
> > In order to unique types, we have to break cycles in the MDNodes.
>
> Sorry I missed this email earlier, but do we really need to break the
> cycles? This is not a generic graph isomorphism problem. If we just
> add the hashes to the metadata nodes representing types, merging then
> becomes
>
> for every two types with the same hashes:
>   try merging them recursively. The order is important, so this is
> just a dfs that succeeds or not.
>
Fwiw it's not really hashing anything, just a way to remove references and
replace it with a name that will be identical between translation units.
For C++ we can abuse the odr in places. For occasions when this isn't going
to work we can fall back to name and scope.

The idea is basically to let the folding set do all the work rather than
merge the graphs ourselves.

I gave a brief description of the problem and solution when I spoke at
eurollvm this last year.

--eric
> Cheers,
> Rafael
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
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