llvm dev - Dec 2016 - [ThinLTO] Reducing imported debug metadata

A couple weeks ago I sat down with David Blaikie to figure out what we
could trim when importing debug metadata during function importing. I have
a prototype that reduces the resulting .o sizes significantly with ThinLTO
-g. However, I ran into some issues when cleaning up part of this in
preparation for a patch. Since Mehdi indicated that he and Adrian were
going to be looking at this as well shortly, I wanted to send out a summary
of what David Blaikie and I discussed, what I have right now, and the
issues I am hitting along with possible solutions.

There was a related older patch where I did some of the same things
(D16440: [ThinLTO] Link in only necessary DICompileUnit operands), but that
was made obsolete by a number of changes, such as the reversal of
GlobalVariable/DIGlobalVariable edges, and a number of debug metadata
changes done by Duncan (and Adrian I think?). So I started from scratch
this time.

Here's what I came up with after discussing what to import with David.
Specifically, how to handle fields when importing a DICompileUnit:

a) List of enum types
 - Not needed in the importing module: change to nullptr

b) List of macros
 - Not needed in the importing module: change to nullptr

c) List of global variables
- Only needed if we have imported the corresponding global variable
definition. Since we currently don't import any global variable definitions
(we should assert that there aren't any in the import list so this
doesn't
get stale), we can change this to nullptr.

d) List of imported entities
- For now need to import those that have a DILocalScope (and drop others
from the imported entities list on the imported CU). David had some ideas
on restructuring the way these are referenced, but for now we
conservatively need to keep those that may be from functions, any that are
from functions not actually imported will not be emitted into the object
file anyway.

e) List of retained types
- Leave as-is but import DICompositeTypes as type declarations. This one
David thought would need to be under an option because of lldb's assumption
that the DWARF match the Clang AST (I think I am stating that right,
correct me if not!).


Implementation:

a)-d) (enums/macros/global variables/imported entities):

For a-d I have a simple solution in the IRLinker. At the very start of
IRLinking in a module, if it is being linked in for function importing, I
invoke a function that does the handling (i.e. from the IRLinker
constructor).

This routine handles a-c by pre-populating the ValueMap's MDMap entry for
those Metadata* (e.g. the RawEnumTypes Metadata*) with nullptr, so metadata
mapping automatically makes them nullptr on the imported DICompileUnit.

For d, this routine walks the imported entities on the source CU and builds
a SmallVector of those with local scopes. It then invokes
replaceImportedEntities on the source CU to replace it with the new list
(essentially dropping those with non-local scopes), and again the
subsequent metadata mapping just works.

e) (retained types):

For e, changing the imported DICompisiteType to type declarations, I am
having some issues. I prototyped this by doing it in the BitcodeReader. I
passed in a new flag indicating that we are parsing a module for function
importing. Then when parsing a METADATA_COMPOSITE_TYPE, if we are importing
a module I change the calls to buildODRType and GET_OR_DISTINCT to instead
create a type declaration:
 - pass in flags Flags|DINode::FlagFwdDecl
 - pass in nullptr for BaseType, Elements, VtableHolder, TemplateParams
 - pass in 0 for OffsetInBits

Note that buildODRType will not mutate any existing DICompisiteType for
that identifier found in the DITypeMap to a type declaration (FlagFwdDecl)
though. This is good since we are sharing the DITypeMap with the original
(destination) module, and we don't want to change any type definitions on
the existing destination (importing) module to be type declarations when
the same type is also in the source module we are about to import.

When cleaning this up, I initially tried mutating the types on the source
module at the start of IRLinking of imported modules. I did this by adding
a new DICompositeType function to force convert an existing type definition
to a type declaration (since the bitcode reader already would have created
a type definition when parsing the imported source module, and as I noted
above it wasn't allowed to be mutated to a declaration after that point).
However, this is wrong since it ended up changing type definitions that
were also used by the original destination/importing module to type
declarations if they were also used in the source module (and therefore
shared a DITypeMap entry). To get the forced mutation to a type decl to
work here, I would somehow need to detect when a composite type on the
source module was *not* also used by the original destination module. The
only way I came up with off the top of my head was to also do a
DebugInfoFinder::processModule on the dest module, and subtract the
resulting types() from those I found when finding types on the source
module I'm about to import, and only force-convert the remaining ones to
type decls.


Interested in feedback on any of the above, and in particular on the
cleanest way to create type declarations for imported types.

Thanks!
Teresa

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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> On 2016-Dec-07, at 10:14, Teresa Johnson <tejohnson at google.com>
wrote:
> 
> A couple weeks ago I sat down with David Blaikie to figure out what we
could trim when importing debug metadata during function importing. I have a
prototype that reduces the resulting .o sizes significantly with ThinLTO -g.
However, I ran into some issues when cleaning up part of this in preparation for
a patch. Since Mehdi indicated that he and Adrian were going to be looking at
this as well shortly, I wanted to send out a summary of what David Blaikie and I
discussed, what I have right now, and the issues I am hitting along with
possible solutions.
> 
> There was a related older patch where I did some of the same things
(D16440: [ThinLTO] Link in only necessary DICompileUnit operands), but that was
made obsolete by a number of changes, such as the reversal of
GlobalVariable/DIGlobalVariable edges, and a number of debug metadata changes
done by Duncan (and Adrian I think?). So I started from scratch this time.
> 
> Here's what I came up with after discussing what to import with David.
Specifically, how to handle fields when importing a DICompileUnit:
Do you have a profile of which (if any) of these is causing the debug info
bloat?

I worry/suspect that much of the bloat could be in the DILocation chains
(line-table), which are generally non-mergeable.
> a) List of enum types
>  - Not needed in the importing module: change to nullptr
Seems reasonable for -flto=thin (incorrect for -flto=full, though).
> b) List of macros
>  - Not needed in the importing module: change to nullptr
We don't have macros by default, right?  You need some -gmacros flag I hope?
I remember objecting to the massive debug info bloat when they were introduced,
but was convinced by an argument along the lines of
"it's-never-going-to-be-on-by-default".
> c) List of global variables
> - Only needed if we have imported the corresponding global variable
definition. Since we currently don't import any global variable definitions
(we should assert that there aren't any in the import list so this
doesn't get stale), we can change this to nullptr.
Why does this still exist?  Can't we delete this list in all modes, since
GlobalValues reference their own DIGlobalVariables directly?
> d) List of imported entities
> - For now need to import those that have a DILocalScope (and drop others
from the imported entities list on the imported CU). David had some ideas on
restructuring the way these are referenced, but for now we conservatively need
to keep those that may be from functions, any that are from functions not
actually imported will not be emitted into the object file anyway.
I'd rather just restructure these if we can.
> e) List of retained types
> - Leave as-is but import DICompositeTypes as type declarations. This one
David thought would need to be under an option because of lldb's assumption
that the DWARF match the Clang AST (I think I am stating that right, correct me
if not!).
How many of these exist?  I expect this list to be almost always empty.  If
it's not, why not?  (Did I fail to push the commit to Clang that stopped
adding all C++ types here??)  Can we somehow prune the list?

I'm concerned that this is not a particularly fast thing to do.

I'm also worried that this would cause major debug info quality degradation
on Darwin (defer to Adrian on that...).  I'd rather leave it unoptimized
unless we're sure we need to do something here.

Also, why do you need this for -flto=thin?  Why not just nullptr?
> Implementation:
Generally, I'd rather see the direction of explicitly building new
DICompileUnits and linking in the correct/necessary arguments, like the IRLinker
explicitly builds new llvm::Functions.  Ideally, we'd only have to
explicitly deal with DICompileUnit (one reason that I'm concerned about (e)
above), which we can find through !llvm.dbg.cu.
> a)-d) (enums/macros/global variables/imported entities):
> 
> For a-d I have a simple solution in the IRLinker. At the very start of
IRLinking in a module, if it is being linked in for function importing, I invoke
a function that does the handling (i.e. from the IRLinker constructor).
> 
> This routine handles a-c by pre-populating the ValueMap's MDMap entry
for those Metadata* (e.g. the RawEnumTypes Metadata*) with nullptr, so metadata
mapping automatically makes them nullptr on the imported DICompileUnit.
> 
> For d, this routine walks the imported entities on the source CU and builds
a SmallVector of those with local scopes. It then invokes
replaceImportedEntities on the source CU to replace it with the new list
(essentially dropping those with non-local scopes), and again the subsequent
metadata mapping just works.
> 
> e) (retained types):
> 
> For e, changing the imported DICompisiteType to type declarations, I am
having some issues. I prototyped this by doing it in the BitcodeReader. I passed
in a new flag indicating that we are parsing a module for function importing.
Then when parsing a METADATA_COMPOSITE_TYPE, if we are importing a module I
change the calls to buildODRType and GET_OR_DISTINCT to instead create a type
declaration:
>  - pass in flags Flags|DINode::FlagFwdDecl
>  - pass in nullptr for BaseType, Elements, VtableHolder, TemplateParams
>  - pass in 0 for OffsetInBits
> 
> Note that buildODRType will not mutate any existing DICompisiteType for
that identifier found in the DITypeMap to a type declaration (FlagFwdDecl)
though. This is good since we are sharing the DITypeMap with the original
(destination) module, and we don't want to change any type definitions on
the existing destination (importing) module to be type declarations when the
same type is also in the source module we are about to import.
> 
> When cleaning this up, I initially tried mutating the types on the source
module at the start of IRLinking of imported modules. I did this by adding a new
DICompositeType function to force convert an existing type definition to a type
declaration (since the bitcode reader already would have created a type
definition when parsing the imported source module, and as I noted above it
wasn't allowed to be mutated to a declaration after that point). However,
this is wrong since it ended up changing type definitions that were also used by
the original destination/importing module to type declarations if they were also
used in the source module (and therefore shared a DITypeMap entry). To get the
forced mutation to a type decl to work here, I would somehow need to detect when
a composite type on the source module was *not* also used by the original
destination module. The only way I came up with off the top of my head was to
also do a DebugInfoFinder::processModule on the dest module, and subtract the
resulting types() from those I found when finding types on the source module
I'm about to import, and only force-convert the remaining ones to type
decls.
> 
> 
> Interested in feedback on any of the above, and in particular on the
cleanest way to create type declarations for imported types.
> 
> Thanks!
> Teresa
> 
> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com |	
408-460-2413

On Wed, Dec 7, 2016 at 10:36 AM Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
>
> > On 2016-Dec-07, at 10:14, Teresa Johnson <tejohnson at
google.com> wrote:
> >
> > A couple weeks ago I sat down with David Blaikie to figure out what we
> could trim when importing debug metadata during function importing. I have
> a prototype that reduces the resulting .o sizes significantly with ThinLTO
> -g. However, I ran into some issues when cleaning up part of this in
> preparation for a patch. Since Mehdi indicated that he and Adrian were
> going to be looking at this as well shortly, I wanted to send out a summary
> of what David Blaikie and I discussed, what I have right now, and the
> issues I am hitting along with possible solutions.
> >
> > There was a related older patch where I did some of the same things
> (D16440: [ThinLTO] Link in only necessary DICompileUnit operands), but that
> was made obsolete by a number of changes, such as the reversal of
> GlobalVariable/DIGlobalVariable edges, and a number of debug metadata
> changes done by Duncan (and Adrian I think?). So I started from scratch
> this time.
> >
> > Here's what I came up with after discussing what to import with
David.
> Specifically, how to handle fields when importing a DICompileUnit:
>
> Do you have a profile of which (if any) of these is causing the debug info
> bloat?
>
> I worry/suspect that much of the bloat could be in the DILocation chains
> (line-table), which are generally non-mergeable.
>
> > a) List of enum types
> >  - Not needed in the importing module: change to nullptr
>
> Seems reasonable for -flto=thin (incorrect for -flto=full, though).
>
> > b) List of macros
> >  - Not needed in the importing module: change to nullptr
>
> We don't have macros by default, right?  You need some -gmacros flag I
> hope?  I remember objecting to the massive debug info bloat when they were
> introduced, but was convinced by an argument along the lines of
> "it's-never-going-to-be-on-by-default".
>
Yes, it's off by default, but still good to think about whether we could do
something similar for it than other things here.

>
> > c) List of global variables
> > - Only needed if we have imported the corresponding global variable
> definition. Since we currently don't import any global variable
definitions
> (we should assert that there aren't any in the import list so this
doesn't
> get stale), we can change this to nullptr.
>
> Why does this still exist?  Can't we delete this list in all modes,
since
> GlobalValues reference their own DIGlobalVariables directly?
>
Yeah, I forget why - I think there's a reason, though.

>
> > d) List of imported entities
> > - For now need to import those that have a DILocalScope (and drop
others
> from the imported entities list on the imported CU). David had some ideas
> on restructuring the way these are referenced, but for now we
> conservatively need to keep those that may be from functions, any that are
> from functions not actually imported will not be emitted into the object
> file anyway.
>
> I'd rather just restructure these if we can.
>
In the simplest form, it probably means adding a new list to DISubprograms
(of all the imported entities in that function) - that way at least the
function local imported entities would be dropped naturally when the
subprogram was dropped (or not imported).

>
> > e) List of retained types
> > - Leave as-is but import DICompositeTypes as type declarations. This
one
> David thought would need to be under an option because of lldb's
assumption
> that the DWARF match the Clang AST (I think I am stating that right,
> correct me if not!).
>
> How many of these exist?  I expect this list to be almost always empty.
> If it's not, why not?  (Did I fail to push the commit to Clang that
stopped
> adding all C++ types here??)  Can we somehow prune the list?
>
Ah, you're right - I was probably operating under the old model that
everything was in retained types. It's good to know/see that's not the
case. I wonder what, if anything, we put in retained types these days.
(there certainly used to be ways - like cases where a type is used without
any variable: "((foo*)void_ptr)->x = 3;" - we could probably help
in those
cases by pushing a retained types list down onto subprograms too, so the
uses were attached to subprograms - but that means potentially a lot of
duplication (if a type is 'retained' in many functions))

> I'm concerned that this is not a particularly fast thing to do.
>
> I'm also worried that this would cause major debug info quality
> degradation on Darwin (defer to Adrian on that...).  I'd rather leave
it
> unoptimized unless we're sure we need to do something here.
>
Turning definitions into declarations like that is incompatible with lldb -
it'd need to be turned off (Teresa mentioned putting it under a flag for
this reason) for any lldb platform.

> Also, why do you need this for -flto=thin?  Why not just nullptr?
>
Legit point - same as enums, etc, they'll be emitted somewhere, no need to
retain them everywhere, even as declarations.

The proper definition->declaration demotion would have to happen by walking
all the debug info looking for types... :/

>
> > Implementation:
>
> Generally, I'd rather see the direction of explicitly building new
> DICompileUnits and linking in the correct/necessary arguments, like the
> IRLinker explicitly builds new llvm::Functions.  Ideally, we'd only
have to
> explicitly deal with DICompileUnit (one reason that I'm concerned about
(e)
> above), which we can find through !llvm.dbg.cu.
>
> > a)-d) (enums/macros/global variables/imported entities):
> >
> > For a-d I have a simple solution in the IRLinker. At the very start of
> IRLinking in a module, if it is being linked in for function importing, I
> invoke a function that does the handling (i.e. from the IRLinker
> constructor).
> >
> > This routine handles a-c by pre-populating the ValueMap's MDMap
entry
> for those Metadata* (e.g. the RawEnumTypes Metadata*) with nullptr, so
> metadata mapping automatically makes them nullptr on the imported
> DICompileUnit.
> >
> > For d, this routine walks the imported entities on the source CU and
> builds a SmallVector of those with local scopes. It then invokes
> replaceImportedEntities on the source CU to replace it with the new list
> (essentially dropping those with non-local scopes), and again the
> subsequent metadata mapping just works.
> >
> > e) (retained types):
> >
> > For e, changing the imported DICompisiteType to type declarations, I
am
> having some issues. I prototyped this by doing it in the BitcodeReader. I
> passed in a new flag indicating that we are parsing a module for function
> importing. Then when parsing a METADATA_COMPOSITE_TYPE, if we are importing
> a module I change the calls to buildODRType and GET_OR_DISTINCT to instead
> create a type declaration:
> >  - pass in flags Flags|DINode::FlagFwdDecl
> >  - pass in nullptr for BaseType, Elements, VtableHolder,
TemplateParams
> >  - pass in 0 for OffsetInBits
> >
> > Note that buildODRType will not mutate any existing DICompisiteType
for
> that identifier found in the DITypeMap to a type declaration (FlagFwdDecl)
> though. This is good since we are sharing the DITypeMap with the original
> (destination) module, and we don't want to change any type definitions
on
> the existing destination (importing) module to be type declarations when
> the same type is also in the source module we are about to import.
> >
> > When cleaning this up, I initially tried mutating the types on the
> source module at the start of IRLinking of imported modules. I did this by
> adding a new DICompositeType function to force convert an existing type
> definition to a type declaration (since the bitcode reader already would
> have created a type definition when parsing the imported source module, and
> as I noted above it wasn't allowed to be mutated to a declaration after
> that point). However, this is wrong since it ended up changing type
> definitions that were also used by the original destination/importing
> module to type declarations if they were also used in the source module
> (and therefore shared a DITypeMap entry). To get the forced mutation to a
> type decl to work here, I would somehow need to detect when a composite
> type on the source module was *not* also used by the original destination
> module. The only way I came up with off the top of my head was to also do a
> DebugInfoFinder::processModule on the dest module, and subtract the
> resulting types() from those I found when finding types on the source
> module I'm about to import, and only force-convert the remaining ones
to
> type decls.
> >
> >
> > Interested in feedback on any of the above, and in particular on the
> cleanest way to create type declarations for imported types.
> >
> > Thanks!
> > Teresa
> >
> > --
> > Teresa Johnson |       Software Engineer |     tejohnson at google.com
|
> 408-460-2413 <(408)%20460-2413>
>
>
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+pcc to answer question about global variable list on CU

Thanks for the comments, responses below.

Teresa

On Wed, Dec 7, 2016 at 10:36 AM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
>
> > On 2016-Dec-07, at 10:14, Teresa Johnson <tejohnson at
google.com> wrote:
> >
> > A couple weeks ago I sat down with David Blaikie to figure out what we
> could trim when importing debug metadata during function importing. I have
> a prototype that reduces the resulting .o sizes significantly with ThinLTO
> -g. However, I ran into some issues when cleaning up part of this in
> preparation for a patch. Since Mehdi indicated that he and Adrian were
> going to be looking at this as well shortly, I wanted to send out a summary
> of what David Blaikie and I discussed, what I have right now, and the
> issues I am hitting along with possible solutions.
> >
> > There was a related older patch where I did some of the same things
> (D16440: [ThinLTO] Link in only necessary DICompileUnit operands), but that
> was made obsolete by a number of changes, such as the reversal of
> GlobalVariable/DIGlobalVariable edges, and a number of debug metadata
> changes done by Duncan (and Adrian I think?). So I started from scratch
> this time.
> >
> > Here's what I came up with after discussing what to import with
David.
> Specifically, how to handle fields when importing a DICompileUnit:
>
> Do you have a profile of which (if any) of these is causing the debug info
> bloat?
>
> I worry/suspect that much of the bloat could be in the DILocation chains
> (line-table), which are generally non-mergeable.
>
Here is a breakdown of the debug section sizes aggregated across all .o
files for a build of Chromium with -g2. These are both with ThinLTO, but
the first column is with all function importing disabled:

  No importing Normal importing (aka "Head" in the below graph)
.debug_abbrev 32013702 60570915
.debug_info 1714741154 8724603593
.debug_line 108033535 440090737
.debug_loc 178426363 264907421
.debug_macinfo 17820     17820
.debug_pubnames 251081673 661630495
.debug_pubtypes 284086653 1867377375
.debug_ranges 65470016 106560864
.debug_str 3304122338 19013084756



I'm not sure if this answers your question above about DILocation chains
though - would that be directly translated into .debug_loc size?

> > a) List of enum types
> >  - Not needed in the importing module: change to nullptr
>
> Seems reasonable for -flto=thin (incorrect for -flto=full, though).
>
Right, all this is for ThinLTO - I haven't thought about Full LTO as I am
concerned about ThinLTO importing-specific bloat.

>
> > b) List of macros
> >  - Not needed in the importing module: change to nullptr
>
> We don't have macros by default, right?  You need some -gmacros flag I
> hope?  I remember objecting to the massive debug info bloat when they were
> introduced, but was convinced by an argument along the lines of
> "it's-never-going-to-be-on-by-default".
>
No I haven't seen a case where this is non-null in my ThinLTO testing, but
it seemed safe to do for completeness.

> > c) List of global variables
> > - Only needed if we have imported the corresponding global variable
> definition. Since we currently don't import any global variable
definitions
> (we should assert that there aren't any in the import list so this
doesn't
> get stale), we can change this to nullptr.
>
> Why does this still exist?  Can't we delete this list in all modes,
since
> GlobalValues reference their own DIGlobalVariables directly?
>
Peter, can you answer this question? I looked back at the old thread and
ISTR that there were some cases where the reference from the CU was needed
but don't remember the details.

>
> > d) List of imported entities
> > - For now need to import those that have a DILocalScope (and drop
others
> from the imported entities list on the imported CU). David had some ideas
> on restructuring the way these are referenced, but for now we
> conservatively need to keep those that may be from functions, any that are
> from functions not actually imported will not be emitted into the object
> file anyway.
>
> I'd rather just restructure these if we can.
>
That sounds like a good long-term solution. For for the short term what I
did was incredibly simple. If it has a significant impact (haven't measured
this one independently), I think the short-term fix would be good in the
meantime. David can probably provide more details on what needs to be done
to fix this longer term.

> > e) List of retained types
> > - Leave as-is but import DICompositeTypes as type declarations. This
one
> David thought would need to be under an option because of lldb's
assumption
> that the DWARF match the Clang AST (I think I am stating that right,
> correct me if not!).
>
> How many of these exist?  I expect this list to be almost always empty.
> If it's not, why not?  (Did I fail to push the commit to Clang that
stopped
> adding all C++ types here??)  Can we somehow prune the list?
>
It seems that there are quite a lot. Just doing this caused a huge
reduction on sizes. E.g. on Chromium, importing DICompositeType as type
decls alone dropped the aggregate .o size increase with -g2 over plain -O2
from 5.2x (current ThinLTO) to 1.7x.

> I'm concerned that this is not a particularly fast thing to do.
>
In what way?

>
> I'm also worried that this would cause major debug info quality
> degradation on Darwin (defer to Adrian on that...).  I'd rather leave
it
> unoptimized unless we're sure we need to do something here.
>
Maybe this is the issue David was referring to about potentially needing to
put this under an option to disable for lldb. For us apparently it is not
an issue.

> Also, why do you need this for -flto=thin?  Why not just nullptr?
>
Meaning never import retained types? I think we need the type decls at
least - David?

>
> > Implementation:
>
> Generally, I'd rather see the direction of explicitly building new
> DICompileUnits and linking in the correct/necessary arguments, like the
> IRLinker explicitly builds new llvm::Functions.  Ideally, we'd only
have to
> explicitly deal with DICompileUnit (one reason that I'm concerned about
(e)
> above), which we can find through !llvm.dbg.cu.
>
> > a)-d) (enums/macros/global variables/imported entities):
> >
> > For a-d I have a simple solution in the IRLinker. At the very start of
> IRLinking in a module, if it is being linked in for function importing, I
> invoke a function that does the handling (i.e. from the IRLinker
> constructor).
> >
> > This routine handles a-c by pre-populating the ValueMap's MDMap
entry
> for those Metadata* (e.g. the RawEnumTypes Metadata*) with nullptr, so
> metadata mapping automatically makes them nullptr on the imported
> DICompileUnit.
> >
> > For d, this routine walks the imported entities on the source CU and
> builds a SmallVector of those with local scopes. It then invokes
> replaceImportedEntities on the source CU to replace it with the new list
> (essentially dropping those with non-local scopes), and again the
> subsequent metadata mapping just works.
> >
> > e) (retained types):
> >
> > For e, changing the imported DICompisiteType to type declarations, I
am
> having some issues. I prototyped this by doing it in the BitcodeReader. I
> passed in a new flag indicating that we are parsing a module for function
> importing. Then when parsing a METADATA_COMPOSITE_TYPE, if we are importing
> a module I change the calls to buildODRType and GET_OR_DISTINCT to instead
> create a type declaration:
> >  - pass in flags Flags|DINode::FlagFwdDecl
> >  - pass in nullptr for BaseType, Elements, VtableHolder,
TemplateParams
> >  - pass in 0 for OffsetInBits
> >
> > Note that buildODRType will not mutate any existing DICompisiteType
for
> that identifier found in the DITypeMap to a type declaration (FlagFwdDecl)
> though. This is good since we are sharing the DITypeMap with the original
> (destination) module, and we don't want to change any type definitions
on
> the existing destination (importing) module to be type declarations when
> the same type is also in the source module we are about to import.
> >
> > When cleaning this up, I initially tried mutating the types on the
> source module at the start of IRLinking of imported modules. I did this by
> adding a new DICompositeType function to force convert an existing type
> definition to a type declaration (since the bitcode reader already would
> have created a type definition when parsing the imported source module, and
> as I noted above it wasn't allowed to be mutated to a declaration after
> that point). However, this is wrong since it ended up changing type
> definitions that were also used by the original destination/importing
> module to type declarations if they were also used in the source module
> (and therefore shared a DITypeMap entry). To get the forced mutation to a
> type decl to work here, I would somehow need to detect when a composite
> type on the source module was *not* also used by the original destination
> module. The only way I came up with off the top of my head was to also do a
> DebugInfoFinder::processModule on the dest module, and subtract the
> resulting types() from those I found when finding types on the source
> module I'm about to import, and only force-convert the remaining ones
to
> type decls.
> >
> >
> > Interested in feedback on any of the above, and in particular on the
> cleanest way to create type declarations for imported types.
> >
> > Thanks!
> > Teresa
> >
> > --
> > Teresa Johnson |       Software Engineer |     tejohnson at google.com
|
> 408-460-2413
>
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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> On Dec 7, 2016, at 10:36 AM, Duncan P. N. Exon Smith <dexonsmith at
apple.com> wrote:
> 
> 
>> On 2016-Dec-07, at 10:14, Teresa Johnson <tejohnson at
google.com> wrote:
>> 
>> A couple weeks ago I sat down with David Blaikie to figure out what we
could trim when importing debug metadata during function importing. I have a
prototype that reduces the resulting .o sizes significantly with ThinLTO -g.
However, I ran into some issues when cleaning up part of this in preparation for
a patch. Since Mehdi indicated that he and Adrian were going to be looking at
this as well shortly, I wanted to send out a summary of what David Blaikie and I
discussed, what I have right now, and the issues I am hitting along with
possible solutions.
>> 
>> There was a related older patch where I did some of the same things
(D16440: [ThinLTO] Link in only necessary DICompileUnit operands), but that was
made obsolete by a number of changes, such as the reversal of
GlobalVariable/DIGlobalVariable edges, and a number of debug metadata changes
done by Duncan (and Adrian I think?). So I started from scratch this time.
>> 
>> Here's what I came up with after discussing what to import with
David. Specifically, how to handle fields when importing a DICompileUnit:
> 
> Do you have a profile of which (if any) of these is causing the debug info
bloat?
> 
> I worry/suspect that much of the bloat could be in the DILocation chains
(line-table), which are generally non-mergeable.
> 
>> a) List of enum types
>> - Not needed in the importing module: change to nullptr
> 
> Seems reasonable for -flto=thin (incorrect for -flto=full, though).
> 
>> b) List of macros
>> - Not needed in the importing module: change to nullptr
> 
> We don't have macros by default, right?  You need some -gmacros flag I
hope?  I remember objecting to the massive debug info bloat when they were
introduced, but was convinced by an argument along the lines of
"it's-never-going-to-be-on-by-default".
> 
>> c) List of global variables
>> - Only needed if we have imported the corresponding global variable
definition. Since we currently don't import any global variable definitions
(we should assert that there aren't any in the import list so this
doesn't get stale), we can change this to nullptr.
> 
> Why does this still exist?  Can't we delete this list in all modes,
since GlobalValues reference their own DIGlobalVariables directly?
Global constants may not necessarily have a GlobalValue backing them up (could
have been optimized away) and thus are need to be referenced by the CU's
list of global variables (or via some other mechanism).

-- adrian
> 
>> d) List of imported entities
>> - For now need to import those that have a DILocalScope (and drop
others from the imported entities list on the imported CU). David had some ideas
on restructuring the way these are referenced, but for now we conservatively
need to keep those that may be from functions, any that are from functions not
actually imported will not be emitted into the object file anyway.
> 
> I'd rather just restructure these if we can.
> 
>> e) List of retained types
>> - Leave as-is but import DICompositeTypes as type declarations. This
one David thought would need to be under an option because of lldb's
assumption that the DWARF match the Clang AST (I think I am stating that right,
correct me if not!).
> 
> How many of these exist?  I expect this list to be almost always empty.  If
it's not, why not?  (Did I fail to push the commit to Clang that stopped
adding all C++ types here??)  Can we somehow prune the list?
> 
> I'm concerned that this is not a particularly fast thing to do.
> 
> I'm also worried that this would cause major debug info quality
degradation on Darwin (defer to Adrian on that...).  I'd rather leave it
unoptimized unless we're sure we need to do something here.
> 
> Also, why do you need this for -flto=thin?  Why not just nullptr?
> 
>> Implementation:
> 
> Generally, I'd rather see the direction of explicitly building new
DICompileUnits and linking in the correct/necessary arguments, like the IRLinker
explicitly builds new llvm::Functions.  Ideally, we'd only have to
explicitly deal with DICompileUnit (one reason that I'm concerned about (e)
above), which we can find through !llvm.dbg.cu.
> 
>> a)-d) (enums/macros/global variables/imported entities):
>> 
>> For a-d I have a simple solution in the IRLinker. At the very start of
IRLinking in a module, if it is being linked in for function importing, I invoke
a function that does the handling (i.e. from the IRLinker constructor).
>> 
>> This routine handles a-c by pre-populating the ValueMap's MDMap
entry for those Metadata* (e.g. the RawEnumTypes Metadata*) with nullptr, so
metadata mapping automatically makes them nullptr on the imported DICompileUnit.
>> 
>> For d, this routine walks the imported entities on the source CU and
builds a SmallVector of those with local scopes. It then invokes
replaceImportedEntities on the source CU to replace it with the new list
(essentially dropping those with non-local scopes), and again the subsequent
metadata mapping just works.
>> 
>> e) (retained types):
>> 
>> For e, changing the imported DICompisiteType to type declarations, I am
having some issues. I prototyped this by doing it in the BitcodeReader. I passed
in a new flag indicating that we are parsing a module for function importing.
Then when parsing a METADATA_COMPOSITE_TYPE, if we are importing a module I
change the calls to buildODRType and GET_OR_DISTINCT to instead create a type
declaration:
>> - pass in flags Flags|DINode::FlagFwdDecl
>> - pass in nullptr for BaseType, Elements, VtableHolder, TemplateParams
>> - pass in 0 for OffsetInBits
>> 
>> Note that buildODRType will not mutate any existing DICompisiteType for
that identifier found in the DITypeMap to a type declaration (FlagFwdDecl)
though. This is good since we are sharing the DITypeMap with the original
(destination) module, and we don't want to change any type definitions on
the existing destination (importing) module to be type declarations when the
same type is also in the source module we are about to import.
>> 
>> When cleaning this up, I initially tried mutating the types on the
source module at the start of IRLinking of imported modules. I did this by
adding a new DICompositeType function to force convert an existing type
definition to a type declaration (since the bitcode reader already would have
created a type definition when parsing the imported source module, and as I
noted above it wasn't allowed to be mutated to a declaration after that
point). However, this is wrong since it ended up changing type definitions that
were also used by the original destination/importing module to type declarations
if they were also used in the source module (and therefore shared a DITypeMap
entry). To get the forced mutation to a type decl to work here, I would somehow
need to detect when a composite type on the source module was *not* also used by
the original destination module. The only way I came up with off the top of my
head was to also do a DebugInfoFinder::processModule on the dest module, and
subtract the resulting types() from those I found when finding types on the
source module I'm about to import, and only force-convert the remaining ones
to type decls.
>> 
>> 
>> Interested in feedback on any of the above, and in particular on the
cleanest way to create type declarations for imported types.
>> 
>> Thanks!
>> Teresa
>> 
>> -- 
>> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com |	
408-460-2413
>