llvm dev - Nov 2016 - DWARF Generator

> On Nov 17, 2016, at 3:40 PM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> 
> 
> On Thu, Nov 17, 2016 at 3:12 PM Greg Clayton via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> I have recently been modifying the DWARF parser and have more patches
planned and I want to be able to add unit tests that test the internal llvm
DWARF APIs to ensure they continue to work and also validate the changes that I
am making. There are not many DWARF unit tests other than very simple ones that
test DWARF forms currently. I would like to expand this to include many more
tests.
> 
> I had submitted a patch that I aborted as it was too large. One of the
issues with the patch was a stand alone DWARF generator that can turn a few API
calls into the section data required for the DWARFContextInMemory class to be
able to load DWARF from. The idea is to generate a small blurb of DWARF, parse
it using our built in DWARF parser and validate that the API calls we do when
consuming the DWARF match what we expect. The original stand along DWARF
generator class is in unittests/DebugInfo/DWARF/DWARFGenerator2.{h,cpp} in the
patch attached. The original review suggested that I try to use the AsmPrinter
and many of its associated classes to generate the DWARF. I attempted to do so
and the AsmPrinter version is in lib/CodeGen/DwarfGenerator.{h,cpp} in the patch
attached. This AsmPrinter based code steals code from the DwarfLinker.cpp.
> 
> 
> 
> I am having trouble getting things to work with the AsmPrinter. I was able
to get simple DWARF to be emitted with the AsmPrinter version of the DWARF
generator with code like:
> 
> 
>    initLLVM();
>    DwarfGen DG;
>    Triple Triple("x86_64--");
>    StringRef Path("/tmp/test.elf");
>    bool DwarfInitSuccess = DG.init(Triple, Path);
>    EXPECT_TRUE(DwarfInitSuccess);
>    uint16_t Version = 4;
>    uint8_t AddrSize = 8;
>    DwarfGenCU &CU = DG.appendCompileUnit(Version, AddrSize);
>    DwarfGenDIE CUDie = CU.getUnitDIE();
> 
>    CUDie.addAttribute(DW_AT_name, DW_FORM_strp, "/tmp/main.c");
>    CUDie.addAttribute(DW_AT_language, DW_FORM_data2, DW_LANG_C);
> 
>    DwarfGenDIE SubprogramDie = CUDie.addChild(DW_TAG_subprogram);
>    SubprogramDie.addAttribute(DW_AT_name, DW_FORM_strp, "main");
>    SubprogramDie.addAttribute(DW_AT_low_pc, DW_FORM_addr, 0x1000U);
>    SubprogramDie.addAttribute(DW_AT_high_pc, DW_FORM_addr, 0x2000U);
> 
>    DwarfGenDIE IntDie = CUDie.addChild(DW_TAG_base_type);
>    IntDie.addAttribute(DW_AT_name, DW_FORM_strp, "int");
>    IntDie.addAttribute(DW_AT_encoding, DW_FORM_data1, DW_ATE_signed);
>    IntDie.addAttribute(DW_AT_byte_size, DW_FORM_data1, 4);
> 
>    DwarfGenDIE ArgcDie = SubprogramDie.addChild(DW_TAG_formal_parameter);
>    ArgcDie.addAttribute(DW_AT_name, DW_FORM_strp, "argc");
>    //ArgcDie.addAttribute(DW_AT_type, DW_FORM_ref_addr, IntDie); // Crashes
here...
> 
>    DG.generate();
> 
>    auto Obj = object::ObjectFile::createObjectFile(Path);
>    if (Obj) {
>      DWARFContextInMemory DwarfContext(*Obj.get().getBinary());
>      uint32_t NumCUs = DwarfContext.getNumCompileUnits();
>      for (uint32_t i=0; i<NumCUs; ++i) {
>        DWARFCompileUnit *U = DwarfContext.getCompileUnitAtIndex(i);
>        if (U)
>          U->getUnitDIE(false)->dump(llvm::outs(), U, -1u);
>      }
>    }
> 
> 
> But things fall down if I try to uncomment the DW_FORM_ref_addr line above.
The problem is that AsmPrinter really expects a full stack of stuff to be there
and expects people to use the DwarfDebug class and all of its associated
classes. These associated classes really want to use the "DI" objects
(DICompileUnit, etc) so to create a compile unit we would need to create
DICompileUnit object and then make a AsmPrinter/DwarfCompileUnit. That stack is
pretty heavy and requires the code shown above to create many many classes just
to represent the simple output we wish to emit. Another downside of the
AsmPrinter method is we don't know which targets people are going to build
into their binaries and thus we don't know which triples we will be able to
use when generating DWARF info. Adrian Prantl attempted to help me get things
working over here and we kept running into roadblocks.
> 
> It'd be great to have more detail about the roadblocks you hit to
better understand how bad/what the issues are.
A few blocks:

- DIEString doesn't support DW_FORM_string. DW_FORM_string support might
have been pulled so that we never emit it from clang, but we would want to have
a unit test that covers being able to read an inlined C string from a DIE.
Support won't be that hard to add, but we might not want it so that people
can't use it by accident and make less efficient DWARF.
- Asserts, asserts, asserts. As we tried to emit DWARF, we got an asserts in
bool AsmPrinter::doInitialization(Module &M). On the first line:

  MMI = getAnalysisIfAvailable<MachineModuleInfo>();

This asserts if you use the AsmPrinter the way the DwarfLinker and the
AsmPrinter based DwarfGen does if you call this. You must call this to generate
the DebugDwarf. If you get past this by installing a Pass then we assert at:

  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
  assert(MI && "AsmPrinter didn't require GCModuleInfo?");

If we don't have this, we don't get a DwarfDebug.
> 
> Even if we end up adding another set of code to generate DWARF (which
I'd really like to avoid) we'd want to, at some point, coalesce them
back together. Given the goal is to try to coalesce the DWARF parsing code in
LLDB and LLVM, it'd seem unfortunate if that effort just created another
similar (or larger) amount of work for DWARF generation.
This DWARF generator could just live in the unittests/DebugInfo/DWARF directory
so it wouldn't pollute anything in LLVM it we do choose to use
it.
>  
> I wanted to pass this patch along in case someone wants to take a look at
how we can possibly fix the lib/CodeGen/DwarfGenerator.cpp and
lib/CodeGen/DwarfGenerator.h. The code that sets up all the required classes for
the AsmPrinter method is in the DwarfGen class from
lib/CodeGen/DwarfGenerator.cpp in the following function:
> 
> bool DwarfGen::init(Triple TheTriple, StringRef OutputFilename);
> 
> The code in this function was looted from existing DwarfLinker.cpp code.
This functions requires a valid triple and that triple is used to create a lot
of the classes required to make the AsmPrinter. I am not sure if any other code
uses the AsmPrinter like this besides the DwarfLinker.cpp code and that code
uses its own magic to actually link the DWARF. It does reuse some of the
functions as I did, but the DwarfLinker doesn't use any of the DwarfDebug,
DwarfCompileUnit or any of the classes that the compiler/assembler uses when
making DWARF.
> 
> What's the DwarfLinker code missing that you need? If that code is
generating essentially arbitrary DWARF, what's blocking using the same
technique for generating DWARF for parsing tests?
They don't use any of the DwarfDebug, DwarfCompileUnit classes. They also
don't use any of the DI classes when making up the debug info. So both the
DWARF linker and the generator have similar needs: make DWARF that isn't
tied too closely to the clang internal classes and DI
classes.
>  
> The amount of work required for refactoring the AsmPrinter exceeds the time
I am going to have, but I would still like to have DWARF API testing in the unit
tests.
> 
> So my question is if anyone would have objections to using the stand along
DWARF generator in unittests/DebugInfo/DWARF until we can later get the YAML
tools to be able to produce DWARF and we can switch to testing the DWARF data
that way? Chris Bieneman has expressed interest in getting a DWARF/YAML layer
going.
> 
> Those tools would still want to use pretty similar (conceptually)
abstractions to LLVM's codegen and llvm-dsymutil. I'd still strongly
prefer to generalize/keep common APIs here - or better understand why it's
not practical now (& what it will take/how we make sure we have a plan and
resources to get there eventually).
>  
> My reasoning is:
> - I want to be able to test DWARF APIs we have to ensure they work
correctly as there are no Dwarf API tests right now. I will be adding code that
changes many things in the DWARF parser and it will be essential to verify that
there are no regressions in the DWARF APIs.
> - Not sure which targets would be built into LLVM so it might be hard to
write tests that cover 32/64 bit addresses and all the variants if we have to do
things legally via AsmPrinter and valid targets
> 
> Seems like it might be plausible to refactor out whatever features of the
AsmPrinter these APIs require (so we just harvest that data out of AsmPrinter
and pass it down in a struct, say - so that other users can pass their own
struct without needing an AsmPrinter). Though, again, interested to know how
dsymutil is working in these situations.
I can try that method if indeed the only places that use the DwarfDebug are the
DW_FORM_ref_addr and location lists. I'll let you know how that
goes.
>  
> - Not enough time to modify AsmPrinter to not require the full DebugInfo
stack and the classes that it uses (llvm::DwarfCompileUnit which must use
llvm::DICompileUnit, llvm::DIE class which uses many local classes that all
depend on  the full DwarfDebug stack).
> 
> Will you have time at some later date to come back and revisit this?
It's understandable that we may choose to incur short term technical debt
with an understanding that it will be paid off in some timely manner. It'd
be less desirable if there's no such plan/possibility and we incur a fairly
clear case of technical debt (redundant DWARF generation libraries - especially
when this effort is to remove a redundant DWARF parser).
Not sure anyone else will need to generate DWARF manually. The two clients
currently are the DWARF unittests and the DwarfLinker. The DwarfLinker worked
around these issues. If the AsmPrinter wasn't such an integral part of the
entire compiler stack, I could take a stab at refactoring it, but I don't
believe I am the right person to do this at this point as I have no experience
or knowledge of the various ways that this class is used, or how it interacts
with other support classes (DwarfDebug, and many many other classes).

Things that still worry me:
- not being able to generate DWARF for 32/64 if targets are missing
- DIEString not supporting DW_FORM_string. I can add support, but I don't
know if we want it as if we add it people might start using it.
- hacking around asserts by constructing classes and copying code from places
that properly use the AsmPrinter that way it is supposed to be used so that we
can use it in a way that it wasn't designed to be used.
>  
> I made a large effort to try and get things working with the AsmPrinter, so
I wanted everyone to know that I tried to get that solution working. Let me know
what you anyone thinks.
> 
> Greg Clayton
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> -----Original Message-----
> From: Greg Clayton [mailto:gclayton at apple.com]
> Sent: Thursday, November 17, 2016 5:01 PM
> To: David Blaikie
> Cc: llvm-dev at lists.llvm.org; Robinson, Paul; Eric Christopher; Adrian
> Prantl
> Subject: Re: [llvm-dev] DWARF Generator
> 
> 
> > On Nov 17, 2016, at 3:40 PM, David Blaikie <dblaikie at
gmail.com> wrote:
> >
> >
> >
> > On Thu, Nov 17, 2016 at 3:12 PM Greg Clayton via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> > I have recently been modifying the DWARF parser and have more patches
> planned and I want to be able to add unit tests that test the internal
> llvm DWARF APIs to ensure they continue to work and also validate the
> changes that I am making. There are not many DWARF unit tests other than
> very simple ones that test DWARF forms currently. I would like to expand
> this to include many more tests.
> >
> > I had submitted a patch that I aborted as it was too large. One of the
> issues with the patch was a stand alone DWARF generator that can turn a
> few API calls into the section data required for the DWARFContextInMemory
> class to be able to load DWARF from. The idea is to generate a small blurb
> of DWARF, parse it using our built in DWARF parser and validate that the
> API calls we do when consuming the DWARF match what we expect. The
> original stand along DWARF generator class is in
> unittests/DebugInfo/DWARF/DWARFGenerator2.{h,cpp} in the patch attached.
> The original review suggested that I try to use the AsmPrinter and many of
> its associated classes to generate the DWARF. I attempted to do so and the
> AsmPrinter version is in lib/CodeGen/DwarfGenerator.{h,cpp} in the patch
> attached. This AsmPrinter based code steals code from the DwarfLinker.cpp.
> >
> >
> >
> > I am having trouble getting things to work with the AsmPrinter. I was
> able to get simple DWARF to be emitted with the AsmPrinter version of the
> DWARF generator with code like:
> >
> >
> >    initLLVM();
> >    DwarfGen DG;
> >    Triple Triple("x86_64--");
> >    StringRef Path("/tmp/test.elf");
> >    bool DwarfInitSuccess = DG.init(Triple, Path);
> >    EXPECT_TRUE(DwarfInitSuccess);
> >    uint16_t Version = 4;
> >    uint8_t AddrSize = 8;
> >    DwarfGenCU &CU = DG.appendCompileUnit(Version, AddrSize);
> >    DwarfGenDIE CUDie = CU.getUnitDIE();
> >
> >    CUDie.addAttribute(DW_AT_name, DW_FORM_strp,
"/tmp/main.c");
> >    CUDie.addAttribute(DW_AT_language, DW_FORM_data2, DW_LANG_C);
> >
> >    DwarfGenDIE SubprogramDie = CUDie.addChild(DW_TAG_subprogram);
> >    SubprogramDie.addAttribute(DW_AT_name, DW_FORM_strp,
"main");
> >    SubprogramDie.addAttribute(DW_AT_low_pc, DW_FORM_addr, 0x1000U);
> >    SubprogramDie.addAttribute(DW_AT_high_pc, DW_FORM_addr, 0x2000U);
> >
> >    DwarfGenDIE IntDie = CUDie.addChild(DW_TAG_base_type);
> >    IntDie.addAttribute(DW_AT_name, DW_FORM_strp, "int");
> >    IntDie.addAttribute(DW_AT_encoding, DW_FORM_data1, DW_ATE_signed);
> >    IntDie.addAttribute(DW_AT_byte_size, DW_FORM_data1, 4);
> >
> >    DwarfGenDIE ArgcDie >
SubprogramDie.addChild(DW_TAG_formal_parameter);
> >    ArgcDie.addAttribute(DW_AT_name, DW_FORM_strp, "argc");
> >    //ArgcDie.addAttribute(DW_AT_type, DW_FORM_ref_addr, IntDie); //
> Crashes here...
> >
> >    DG.generate();
> >
> >    auto Obj = object::ObjectFile::createObjectFile(Path);
> >    if (Obj) {
> >      DWARFContextInMemory DwarfContext(*Obj.get().getBinary());
> >      uint32_t NumCUs = DwarfContext.getNumCompileUnits();
> >      for (uint32_t i=0; i<NumCUs; ++i) {
> >        DWARFCompileUnit *U = DwarfContext.getCompileUnitAtIndex(i);
> >        if (U)
> >          U->getUnitDIE(false)->dump(llvm::outs(), U, -1u);
> >      }
> >    }
> >
> >
> > But things fall down if I try to uncomment the DW_FORM_ref_addr line
> above. The problem is that AsmPrinter really expects a full stack of stuff
> to be there and expects people to use the DwarfDebug class and all of its
> associated classes. These associated classes really want to use the
"DI"
> objects (DICompileUnit, etc) so to create a compile unit we would need to
> create DICompileUnit object and then make a AsmPrinter/DwarfCompileUnit.
> That stack is pretty heavy and requires the code shown above to create
> many many classes just to represent the simple output we wish to emit.
> Another downside of the AsmPrinter method is we don't know which
targets
> people are going to build into their binaries and thus we don't know
which
> triples we will be able to use when generating DWARF info. Adrian Prantl
> attempted to help me get things working over here and we kept running into
> roadblocks.
> >
> > It'd be great to have more detail about the roadblocks you hit to
better
> understand how bad/what the issues are.
> 
> A few blocks:
> 
> - DIEString doesn't support DW_FORM_string. DW_FORM_string support
might
> have been pulled so that we never emit it from clang, but we would want to
> have a unit test that covers being able to read an inlined C string from a
> DIE. Support won't be that hard to add, but we might not want it so
that
> people can't use it by accident and make less efficient DWARF.
Seems to me we originally supported only DW_FORM_string, and then at some
point it was tossed in favor of DW_FORM_strp in order to get space savings
from string pooling.  In fact using DW_FORM_string for small strings would
save some more space (admittedly not much) and a bunch of relocations.
(I found data from an old experiment, in a debug build of Clang it saved 
~0.7MB out of a total 340MB of debug-info size, and >360K ELF relocations.)

I'd favor an API that passed the string down and let the DIE generator
(as opposed to the DWARF generator) pick the form.
> - Asserts, asserts, asserts. As we tried to emit DWARF, we got an asserts
> in bool AsmPrinter::doInitialization(Module &M). On the first line:
> 
>   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
> 
> This asserts if you use the AsmPrinter the way the DwarfLinker and the
> AsmPrinter based DwarfGen does if you call this. You must call this to
> generate the DebugDwarf. If you get past this by installing a Pass then we
> assert at:
> 
>   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
>   assert(MI && "AsmPrinter didn't require
GCModuleInfo?");
> 
> If we don't have this, we don't get a DwarfDebug.
> 
> >
> > Even if we end up adding another set of code to generate DWARF (which
> I'd really like to avoid) we'd want to, at some point, coalesce
them back
> together. Given the goal is to try to coalesce the DWARF parsing code in
> LLDB and LLVM, it'd seem unfortunate if that effort just created
another
> similar (or larger) amount of work for DWARF generation.
> 
> This DWARF generator could just live in the unittests/DebugInfo/DWARF
> directory so it wouldn't pollute anything in LLVM it we do choose to
use
> it.
> >
> > I wanted to pass this patch along in case someone wants to take a look
> at how we can possibly fix the lib/CodeGen/DwarfGenerator.cpp and
> lib/CodeGen/DwarfGenerator.h. The code that sets up all the required
> classes for the AsmPrinter method is in the DwarfGen class from
> lib/CodeGen/DwarfGenerator.cpp in the following function:
> >
> > bool DwarfGen::init(Triple TheTriple, StringRef OutputFilename);
> >
> > The code in this function was looted from existing DwarfLinker.cpp
code.
> This functions requires a valid triple and that triple is used to create a
> lot of the classes required to make the AsmPrinter. I am not sure if any
> other code uses the AsmPrinter like this besides the DwarfLinker.cpp code
> and that code uses its own magic to actually link the DWARF. It does reuse
> some of the functions as I did, but the DwarfLinker doesn't use any of
the
> DwarfDebug, DwarfCompileUnit or any of the classes that the
> compiler/assembler uses when making DWARF.
> >
> > What's the DwarfLinker code missing that you need? If that code is
> generating essentially arbitrary DWARF, what's blocking using the same
> technique for generating DWARF for parsing tests?
> 
> They don't use any of the DwarfDebug, DwarfCompileUnit classes. They
also
> don't use any of the DI classes when making up the debug info. So both
the
> DWARF linker and the generator have similar needs: make DWARF that
isn't
> tied too closely to the clang internal classes and DI classes.
> >
> > The amount of work required for refactoring the AsmPrinter exceeds the
> time I am going to have, but I would still like to have DWARF API testing
> in the unit tests.
> >
> > So my question is if anyone would have objections to using the stand
> along DWARF generator in unittests/DebugInfo/DWARF until we can later get
> the YAML tools to be able to produce DWARF and we can switch to testing
> the DWARF data that way? Chris Bieneman has expressed interest in getting
> a DWARF/YAML layer going.
> >
> > Those tools would still want to use pretty similar (conceptually)
> abstractions to LLVM's codegen and llvm-dsymutil. I'd still
strongly
> prefer to generalize/keep common APIs here - or better understand why
it's
> not practical now (& what it will take/how we make sure we have a plan
and
> resources to get there eventually).
> >
> > My reasoning is:
> > - I want to be able to test DWARF APIs we have to ensure they work
> correctly as there are no Dwarf API tests right now. I will be adding code
> that changes many things in the DWARF parser and it will be essential to
> verify that there are no regressions in the DWARF APIs.
> > - Not sure which targets would be built into LLVM so it might be hard
to
> write tests that cover 32/64 bit addresses and all the variants if we have
> to do things legally via AsmPrinter and valid targets
> >
> > Seems like it might be plausible to refactor out whatever features of
> the AsmPrinter these APIs require (so we just harvest that data out of
> AsmPrinter and pass it down in a struct, say - so that other users can
> pass their own struct without needing an AsmPrinter). Though, again,
> interested to know how dsymutil is working in these situations.
> 
> I can try that method if indeed the only places that use the DwarfDebug
> are the DW_FORM_ref_addr and location lists. I'll let you know how that
> goes.
> >
> > - Not enough time to modify AsmPrinter to not require the full
DebugInfo
> stack and the classes that it uses (llvm::DwarfCompileUnit which must use
> llvm::DICompileUnit, llvm::DIE class which uses many local classes that
> all depend on  the full DwarfDebug stack).
> >
> > Will you have time at some later date to come back and revisit this?
> It's understandable that we may choose to incur short term technical
debt
> with an understanding that it will be paid off in some timely manner.
It'd
> be less desirable if there's no such plan/possibility and we incur a
> fairly clear case of technical debt (redundant DWARF generation libraries
> - especially when this effort is to remove a redundant DWARF parser).
> 
> Not sure anyone else will need to generate DWARF manually. The two clients
> currently are the DWARF unittests and the DwarfLinker. The DwarfLinker
> worked around these issues. If the AsmPrinter wasn't such an integral
part
> of the entire compiler stack, I could take a stab at refactoring it, but I
> don't believe I am the right person to do this at this point as I have
no
> experience or knowledge of the various ways that this class is used, or
> how it interacts with other support classes (DwarfDebug, and many many
> other classes).
> 
> Things that still worry me:
> - not being able to generate DWARF for 32/64 if targets are missing
You mean DWARF-32 and DWARF-64 formats?  LLVM doesn't do DWARF-64.
If you mean 64-bit target-machine addresses, I guess I don't understand
the problem.  If you have target-dependent tests, then they only work
when the right targets are there.  This is extremely common and I'm 
not clear why it would be a problem for the DWARF tests.
> - DIEString not supporting DW_FORM_string. I can add support, but I
don't
> know if we want it as if we add it people might start using it.
See above. If the API picked the form this would not be a concern.
> - hacking around asserts by constructing classes and copying code from
> places that properly use the AsmPrinter that way it is supposed to be used
> so that we can use it in a way that it wasn't designed to be used.
> 
> >
> > I made a large effort to try and get things working with the
AsmPrinter,
> so I wanted everyone to know that I tried to get that solution working.
> Let me know what you anyone thinks.
> >
> > Greg Clayton
--paulr
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> On Nov 17, 2016, at 5:40 PM, Robinson, Paul <paul.robinson at
sony.com> wrote:
> 
> 
> 
>> -----Original Message-----
>> From: Greg Clayton [mailto:gclayton at apple.com]
>> Sent: Thursday, November 17, 2016 5:01 PM
>> To: David Blaikie
>> Cc: llvm-dev at lists.llvm.org; Robinson, Paul; Eric Christopher;
Adrian
>> Prantl
>> Subject: Re: [llvm-dev] DWARF Generator
>> 
>> 
>>> On Nov 17, 2016, at 3:40 PM, David Blaikie <dblaikie at
gmail.com> wrote:
>>> 
>>> 
>>> 
>>> On Thu, Nov 17, 2016 at 3:12 PM Greg Clayton via llvm-dev <llvm-
>> dev at lists.llvm.org> wrote:
>>> I have recently been modifying the DWARF parser and have more
patches
>> planned and I want to be able to add unit tests that test the internal
>> llvm DWARF APIs to ensure they continue to work and also validate the
>> changes that I am making. There are not many DWARF unit tests other
than
>> very simple ones that test DWARF forms currently. I would like to
expand
>> this to include many more tests.
>>> 
>>> I had submitted a patch that I aborted as it was too large. One of
the
>> issues with the patch was a stand alone DWARF generator that can turn a
>> few API calls into the section data required for the
DWARFContextInMemory
>> class to be able to load DWARF from. The idea is to generate a small
blurb
>> of DWARF, parse it using our built in DWARF parser and validate that
the
>> API calls we do when consuming the DWARF match what we expect. The
>> original stand along DWARF generator class is in
>> unittests/DebugInfo/DWARF/DWARFGenerator2.{h,cpp} in the patch
attached.
>> The original review suggested that I try to use the AsmPrinter and many
of
>> its associated classes to generate the DWARF. I attempted to do so and
the
>> AsmPrinter version is in lib/CodeGen/DwarfGenerator.{h,cpp} in the
patch
>> attached. This AsmPrinter based code steals code from the
DwarfLinker.cpp.
>>> 
>>> 
>>> 
>>> I am having trouble getting things to work with the AsmPrinter. I
was
>> able to get simple DWARF to be emitted with the AsmPrinter version of
the
>> DWARF generator with code like:
>>> 
>>> 
>>>   initLLVM();
>>>   DwarfGen DG;
>>>   Triple Triple("x86_64--");
>>>   StringRef Path("/tmp/test.elf");
>>>   bool DwarfInitSuccess = DG.init(Triple, Path);
>>>   EXPECT_TRUE(DwarfInitSuccess);
>>>   uint16_t Version = 4;
>>>   uint8_t AddrSize = 8;
>>>   DwarfGenCU &CU = DG.appendCompileUnit(Version, AddrSize);
>>>   DwarfGenDIE CUDie = CU.getUnitDIE();
>>> 
>>>   CUDie.addAttribute(DW_AT_name, DW_FORM_strp,
"/tmp/main.c");
>>>   CUDie.addAttribute(DW_AT_language, DW_FORM_data2, DW_LANG_C);
>>> 
>>>   DwarfGenDIE SubprogramDie = CUDie.addChild(DW_TAG_subprogram);
>>>   SubprogramDie.addAttribute(DW_AT_name, DW_FORM_strp,
"main");
>>>   SubprogramDie.addAttribute(DW_AT_low_pc, DW_FORM_addr, 0x1000U);
>>>   SubprogramDie.addAttribute(DW_AT_high_pc, DW_FORM_addr, 0x2000U);
>>> 
>>>   DwarfGenDIE IntDie = CUDie.addChild(DW_TAG_base_type);
>>>   IntDie.addAttribute(DW_AT_name, DW_FORM_strp, "int");
>>>   IntDie.addAttribute(DW_AT_encoding, DW_FORM_data1,
DW_ATE_signed);
>>>   IntDie.addAttribute(DW_AT_byte_size, DW_FORM_data1, 4);
>>> 
>>>   DwarfGenDIE ArgcDie >>
SubprogramDie.addChild(DW_TAG_formal_parameter);
>>>   ArgcDie.addAttribute(DW_AT_name, DW_FORM_strp, "argc");
>>>   //ArgcDie.addAttribute(DW_AT_type, DW_FORM_ref_addr, IntDie); //
>> Crashes here...
>>> 
>>>   DG.generate();
>>> 
>>>   auto Obj = object::ObjectFile::createObjectFile(Path);
>>>   if (Obj) {
>>>     DWARFContextInMemory DwarfContext(*Obj.get().getBinary());
>>>     uint32_t NumCUs = DwarfContext.getNumCompileUnits();
>>>     for (uint32_t i=0; i<NumCUs; ++i) {
>>>       DWARFCompileUnit *U = DwarfContext.getCompileUnitAtIndex(i);
>>>       if (U)
>>>         U->getUnitDIE(false)->dump(llvm::outs(), U, -1u);
>>>     }
>>>   }
>>> 
>>> 
>>> But things fall down if I try to uncomment the DW_FORM_ref_addr
line
>> above. The problem is that AsmPrinter really expects a full stack of
stuff
>> to be there and expects people to use the DwarfDebug class and all of
its
>> associated classes. These associated classes really want to use the
"DI"
>> objects (DICompileUnit, etc) so to create a compile unit we would need
to
>> create DICompileUnit object and then make a
AsmPrinter/DwarfCompileUnit.
>> That stack is pretty heavy and requires the code shown above to create
>> many many classes just to represent the simple output we wish to emit.
>> Another downside of the AsmPrinter method is we don't know which
targets
>> people are going to build into their binaries and thus we don't
know which
>> triples we will be able to use when generating DWARF info. Adrian
Prantl
>> attempted to help me get things working over here and we kept running
into
>> roadblocks.
>>> 
>>> It'd be great to have more detail about the roadblocks you hit
to better
>> understand how bad/what the issues are.
>> 
>> A few blocks:
>> 
>> - DIEString doesn't support DW_FORM_string. DW_FORM_string support
might
>> have been pulled so that we never emit it from clang, but we would want
to
>> have a unit test that covers being able to read an inlined C string
from a
>> DIE. Support won't be that hard to add, but we might not want it so
that
>> people can't use it by accident and make less efficient DWARF.
> 
> Seems to me we originally supported only DW_FORM_string, and then at some
> point it was tossed in favor of DW_FORM_strp in order to get space savings
> from string pooling.  In fact using DW_FORM_string for small strings would
> save some more space (admittedly not much) and a bunch of relocations.
> (I found data from an old experiment, in a debug build of Clang it saved 
> ~0.7MB out of a total 340MB of debug-info size, and >360K ELF
relocations.)
This is true, but it also adversely affects DWARF parsing speed as you will need
to manually skip each C string when parsing the DIEs.
> 
> I'd favor an API that passed the string down and let the DIE generator
> (as opposed to the DWARF generator) pick the form.
I have currently added a DIEInlinedString class that can be used for
DW_FORM_string attributes.
> 
>> - Asserts, asserts, asserts. As we tried to emit DWARF, we got an
asserts
>> in bool AsmPrinter::doInitialization(Module &M). On the first line:
>> 
>>  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
>> 
>> This asserts if you use the AsmPrinter the way the DwarfLinker and the
>> AsmPrinter based DwarfGen does if you call this. You must call this to
>> generate the DebugDwarf. If you get past this by installing a Pass then
we
>> assert at:
>> 
>>  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
>>  assert(MI && "AsmPrinter didn't require
GCModuleInfo?");
>> 
>> If we don't have this, we don't get a DwarfDebug.
>> 
>>> 
>>> Even if we end up adding another set of code to generate DWARF
(which
>> I'd really like to avoid) we'd want to, at some point, coalesce
them back
>> together. Given the goal is to try to coalesce the DWARF parsing code
in
>> LLDB and LLVM, it'd seem unfortunate if that effort just created
another
>> similar (or larger) amount of work for DWARF generation.
>> 
>> This DWARF generator could just live in the unittests/DebugInfo/DWARF
>> directory so it wouldn't pollute anything in LLVM it we do choose
to use
>> it.
>>> 
>>> I wanted to pass this patch along in case someone wants to take a
look
>> at how we can possibly fix the lib/CodeGen/DwarfGenerator.cpp and
>> lib/CodeGen/DwarfGenerator.h. The code that sets up all the required
>> classes for the AsmPrinter method is in the DwarfGen class from
>> lib/CodeGen/DwarfGenerator.cpp in the following function:
>>> 
>>> bool DwarfGen::init(Triple TheTriple, StringRef OutputFilename);
>>> 
>>> The code in this function was looted from existing DwarfLinker.cpp
code.
>> This functions requires a valid triple and that triple is used to
create a
>> lot of the classes required to make the AsmPrinter. I am not sure if
any
>> other code uses the AsmPrinter like this besides the DwarfLinker.cpp
code
>> and that code uses its own magic to actually link the DWARF. It does
reuse
>> some of the functions as I did, but the DwarfLinker doesn't use any
of the
>> DwarfDebug, DwarfCompileUnit or any of the classes that the
>> compiler/assembler uses when making DWARF.
>>> 
>>> What's the DwarfLinker code missing that you need? If that code
is
>> generating essentially arbitrary DWARF, what's blocking using the
same
>> technique for generating DWARF for parsing tests?
>> 
>> They don't use any of the DwarfDebug, DwarfCompileUnit classes.
They also
>> don't use any of the DI classes when making up the debug info. So
both the
>> DWARF linker and the generator have similar needs: make DWARF that
isn't
>> tied too closely to the clang internal classes and DI classes.
>>> 
>>> The amount of work required for refactoring the AsmPrinter exceeds
the
>> time I am going to have, but I would still like to have DWARF API
testing
>> in the unit tests.
>>> 
>>> So my question is if anyone would have objections to using the
stand
>> along DWARF generator in unittests/DebugInfo/DWARF until we can later
get
>> the YAML tools to be able to produce DWARF and we can switch to testing
>> the DWARF data that way? Chris Bieneman has expressed interest in
getting
>> a DWARF/YAML layer going.
>>> 
>>> Those tools would still want to use pretty similar (conceptually)
>> abstractions to LLVM's codegen and llvm-dsymutil. I'd still
strongly
>> prefer to generalize/keep common APIs here - or better understand why
it's
>> not practical now (& what it will take/how we make sure we have a
plan and
>> resources to get there eventually).
>>> 
>>> My reasoning is:
>>> - I want to be able to test DWARF APIs we have to ensure they work
>> correctly as there are no Dwarf API tests right now. I will be adding
code
>> that changes many things in the DWARF parser and it will be essential
to
>> verify that there are no regressions in the DWARF APIs.
>>> - Not sure which targets would be built into LLVM so it might be
hard to
>> write tests that cover 32/64 bit addresses and all the variants if we
have
>> to do things legally via AsmPrinter and valid targets
>>> 
>>> Seems like it might be plausible to refactor out whatever features
of
>> the AsmPrinter these APIs require (so we just harvest that data out of
>> AsmPrinter and pass it down in a struct, say - so that other users can
>> pass their own struct without needing an AsmPrinter). Though, again,
>> interested to know how dsymutil is working in these situations.
>> 
>> I can try that method if indeed the only places that use the DwarfDebug
>> are the DW_FORM_ref_addr and location lists. I'll let you know how
that
>> goes.
>>> 
>>> - Not enough time to modify AsmPrinter to not require the full
DebugInfo
>> stack and the classes that it uses (llvm::DwarfCompileUnit which must
use
>> llvm::DICompileUnit, llvm::DIE class which uses many local classes that
>> all depend on  the full DwarfDebug stack).
>>> 
>>> Will you have time at some later date to come back and revisit
this?
>> It's understandable that we may choose to incur short term
technical debt
>> with an understanding that it will be paid off in some timely manner.
It'd
>> be less desirable if there's no such plan/possibility and we incur
a
>> fairly clear case of technical debt (redundant DWARF generation
libraries
>> - especially when this effort is to remove a redundant DWARF parser).
>> 
>> Not sure anyone else will need to generate DWARF manually. The two
clients
>> currently are the DWARF unittests and the DwarfLinker. The DwarfLinker
>> worked around these issues. If the AsmPrinter wasn't such an
integral part
>> of the entire compiler stack, I could take a stab at refactoring it,
but I
>> don't believe I am the right person to do this at this point as I
have no
>> experience or knowledge of the various ways that this class is used, or
>> how it interacts with other support classes (DwarfDebug, and many many
>> other classes).
>> 
>> Things that still worry me:
>> - not being able to generate DWARF for 32/64 if targets are missing
> 
> You mean DWARF-32 and DWARF-64 formats?  LLVM doesn't do DWARF-64.
> If you mean 64-bit target-machine addresses, I guess I don't understand
> the problem.  If you have target-dependent tests, then they only work
> when the right targets are there.  This is extremely common and I'm 
> not clear why it would be a problem for the DWARF tests.
I wasn't aware that there were target-dependent tests. Do you know of one in
the unittest directory you can point me to? I did mean 32 bit address target,
versus 64 bit address targets. I am not sure how I can test 4 and 8 byte
addresses reliably. What triple to I use in the unittest? I can't assume
x86_64 as we may have been built on a 32 bit ARM system with only the 32 bit ARM
targets. 
> 
>> - DIEString not supporting DW_FORM_string. I can add support, but I
don't
>> know if we want it as if we add it people might start using it.
> 
> See above. If the API picked the form this would not be a concern.
For DWARF parsing speed I still like the DW_FORM_strp.
> 
>> - hacking around asserts by constructing classes and copying code from
>> places that properly use the AsmPrinter that way it is supposed to be
used
>> so that we can use it in a way that it wasn't designed to be used.
>> 
>>> 
>>> I made a large effort to try and get things working with the
AsmPrinter,
>> so I wanted everyone to know that I tried to get that solution working.
>> Let me know what you anyone thinks.
>>> 
>>> Greg Clayton
> 
> --paulr
> 
>>> 
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On Thu, Nov 17, 2016 at 5:00 PM Greg Clayton <gclayton at apple.com>
wrote:
>
> > On Nov 17, 2016, at 3:40 PM, David Blaikie <dblaikie at
gmail.com> wrote:
> >
> >
> >
> > On Thu, Nov 17, 2016 at 3:12 PM Greg Clayton via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > I have recently been modifying the DWARF parser and have more patches
> planned and I want to be able to add unit tests that test the internal llvm
> DWARF APIs to ensure they continue to work and also validate the changes
> that I am making. There are not many DWARF unit tests other than very
> simple ones that test DWARF forms currently. I would like to expand this to
> include many more tests.
> >
> > I had submitted a patch that I aborted as it was too large. One of the
> issues with the patch was a stand alone DWARF generator that can turn a few
> API calls into the section data required for the DWARFContextInMemory class
> to be able to load DWARF from. The idea is to generate a small blurb of
> DWARF, parse it using our built in DWARF parser and validate that the API
> calls we do when consuming the DWARF match what we expect. The original
> stand along DWARF generator class is in
> unittests/DebugInfo/DWARF/DWARFGenerator2.{h,cpp} in the patch attached.
> The original review suggested that I try to use the AsmPrinter and many of
> its associated classes to generate the DWARF. I attempted to do so and the
> AsmPrinter version is in lib/CodeGen/DwarfGenerator.{h,cpp} in the patch
> attached. This AsmPrinter based code steals code from the DwarfLinker.cpp.
> >
> >
> >
> > I am having trouble getting things to work with the AsmPrinter. I was
> able to get simple DWARF to be emitted with the AsmPrinter version of the
> DWARF generator with code like:
> >
> >
> >    initLLVM();
> >    DwarfGen DG;
> >    Triple Triple("x86_64--");
> >    StringRef Path("/tmp/test.elf");
> >    bool DwarfInitSuccess = DG.init(Triple, Path);
> >    EXPECT_TRUE(DwarfInitSuccess);
> >    uint16_t Version = 4;
> >    uint8_t AddrSize = 8;
> >    DwarfGenCU &CU = DG.appendCompileUnit(Version, AddrSize);
> >    DwarfGenDIE CUDie = CU.getUnitDIE();
> >
> >    CUDie.addAttribute(DW_AT_name, DW_FORM_strp,
"/tmp/main.c");
> >    CUDie.addAttribute(DW_AT_language, DW_FORM_data2, DW_LANG_C);
> >
> >    DwarfGenDIE SubprogramDie = CUDie.addChild(DW_TAG_subprogram);
> >    SubprogramDie.addAttribute(DW_AT_name, DW_FORM_strp,
"main");
> >    SubprogramDie.addAttribute(DW_AT_low_pc, DW_FORM_addr, 0x1000U);
> >    SubprogramDie.addAttribute(DW_AT_high_pc, DW_FORM_addr, 0x2000U);
> >
> >    DwarfGenDIE IntDie = CUDie.addChild(DW_TAG_base_type);
> >    IntDie.addAttribute(DW_AT_name, DW_FORM_strp, "int");
> >    IntDie.addAttribute(DW_AT_encoding, DW_FORM_data1, DW_ATE_signed);
> >    IntDie.addAttribute(DW_AT_byte_size, DW_FORM_data1, 4);
> >
> >    DwarfGenDIE ArgcDie =
SubprogramDie.addChild(DW_TAG_formal_parameter);
> >    ArgcDie.addAttribute(DW_AT_name, DW_FORM_strp, "argc");
> >    //ArgcDie.addAttribute(DW_AT_type, DW_FORM_ref_addr, IntDie); //
> Crashes here...
> >
> >    DG.generate();
> >
> >    auto Obj = object::ObjectFile::createObjectFile(Path);
> >    if (Obj) {
> >      DWARFContextInMemory DwarfContext(*Obj.get().getBinary());
> >      uint32_t NumCUs = DwarfContext.getNumCompileUnits();
> >      for (uint32_t i=0; i<NumCUs; ++i) {
> >        DWARFCompileUnit *U = DwarfContext.getCompileUnitAtIndex(i);
> >        if (U)
> >          U->getUnitDIE(false)->dump(llvm::outs(), U, -1u);
> >      }
> >    }
> >
> >
> > But things fall down if I try to uncomment the DW_FORM_ref_addr line
> above. The problem is that AsmPrinter really expects a full stack of stuff
> to be there and expects people to use the DwarfDebug class and all of its
> associated classes. These associated classes really want to use the
"DI"
> objects (DICompileUnit, etc) so to create a compile unit we would need to
> create DICompileUnit object and then make a AsmPrinter/DwarfCompileUnit.
> That stack is pretty heavy and requires the code shown above to create many
> many classes just to represent the simple output we wish to emit. Another
> downside of the AsmPrinter method is we don't know which targets people
are
> going to build into their binaries and thus we don't know which triples
we
> will be able to use when generating DWARF info. Adrian Prantl attempted to
> help me get things working over here and we kept running into roadblocks.
> >
> > It'd be great to have more detail about the roadblocks you hit to
better
> understand how bad/what the issues are.
>
> A few blocks:
>
> - DIEString doesn't support DW_FORM_string. DW_FORM_string support
might
> have been pulled so that we never emit it from clang, but we would want to
> have a unit test that covers being able to read an inlined C string from a
> DIE. Support won't be that hard to add, but we might not want it so
that
> people can't use it by accident and make less efficient DWARF.
>
The codepaths for generating strings in DWARF in LLVM are pretty
streamlined (see DwarfUnit::addString) - it's not likely the presence of
support for DW_FORM_string will suddenly cause anyone to mistakenly create
such an attribute because they all go through that API because it's
convenient to do so & handles all the cases.

If we ever wanted to support DW_FORM_string (perhaps for users who care a
lot more about size than the performance characteristics you're concerned
with (I don't know if GDB does the same optimizations - so perhaps we're
paying the size for little/no benefit there, for example)) we would do it
in that one place, quite deliberately/explicitly.

> - Asserts, asserts, asserts. As we tried to emit DWARF, we got an asserts
> in bool AsmPrinter::doInitialization(Module &M). On the first line:
>
>   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
>
> This asserts if you use the AsmPrinter the way the DwarfLinker and the
> AsmPrinter based DwarfGen does if you call this. You must call this to
> generate the DebugDwarf. If you get past this by installing a Pass then we
> assert at:
>
>   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
>   assert(MI && "AsmPrinter didn't require
GCModuleInfo?");
>
> If we don't have this, we don't get a DwarfDebug.
>
Right - my general thinking is: DwarfLinker uses these APIs somehow so I
expect you could use them similarly. If there are differences in your use
case that make DwarfLinker's usage not applicable to yours, it'd be good
to
understand that. If it's just that DwarfLinker's putting up with pain
you'd
rather not - well, seems like it'd be good to fix/ease that pain for both
users.

> > Even if we end up adding another set of code to generate DWARF (which
> I'd really like to avoid) we'd want to, at some point, coalesce
them back
> together. Given the goal is to try to coalesce the DWARF parsing code in
> LLDB and LLVM, it'd seem unfortunate if that effort just created
another
> similar (or larger) amount of work for DWARF generation.
>
> This DWARF generator could just live in the unittests/DebugInfo/DWARF
> directory so it wouldn't pollute anything in LLVM it we do choose to
use it.
>
It's not so much about pollution - much like the DWARF parser in LLDB and
LLVM didn't pollute each other - it's still two systems that end up
growing
their own features, bugs, fixes, etc, which is best avoided.

> > I wanted to pass this patch along in case someone wants to take a look
> at how we can possibly fix the lib/CodeGen/DwarfGenerator.cpp and
> lib/CodeGen/DwarfGenerator.h. The code that sets up all the required
> classes for the AsmPrinter method is in the DwarfGen class from
> lib/CodeGen/DwarfGenerator.cpp in the following function:
> >
> > bool DwarfGen::init(Triple TheTriple, StringRef OutputFilename);
> >
> > The code in this function was looted from existing DwarfLinker.cpp
code.
> This functions requires a valid triple and that triple is used to create a
> lot of the classes required to make the AsmPrinter. I am not sure if any
> other code uses the AsmPrinter like this besides the DwarfLinker.cpp code
> and that code uses its own magic to actually link the DWARF. It does reuse
> some of the functions as I did, but the DwarfLinker doesn't use any of
the
> DwarfDebug, DwarfCompileUnit or any of the classes that the
> compiler/assembler uses when making DWARF.
> >
> > What's the DwarfLinker code missing that you need? If that code is
> generating essentially arbitrary DWARF, what's blocking using the same
> technique for generating DWARF for parsing tests?
>
> They don't use any of the DwarfDebug, DwarfCompileUnit classes. They
also
> don't use any of the DI classes when making up the debug info. So both
the
> DWARF linker and the generator have similar needs: make DWARF that
isn't
> tied too closely to the clang internal classes and DI classes.
>
Right - I don't think either of these use cases should use the DI classes.
So how is the DwarfLinker working to generate DWARF in ways that don't work
for your needs?

> > The amount of work required for refactoring the AsmPrinter exceeds the
> time I am going to have, but I would still like to have DWARF API testing
> in the unit tests.
> >
> > So my question is if anyone would have objections to using the stand
> along DWARF generator in unittests/DebugInfo/DWARF until we can later get
> the YAML tools to be able to produce DWARF and we can switch to testing the
> DWARF data that way? Chris Bieneman has expressed interest in getting a
> DWARF/YAML layer going.
> >
> > Those tools would still want to use pretty similar (conceptually)
> abstractions to LLVM's codegen and llvm-dsymutil. I'd still
strongly prefer
> to generalize/keep common APIs here - or better understand why it's not
> practical now (& what it will take/how we make sure we have a plan and
> resources to get there eventually).
> >
> > My reasoning is:
> > - I want to be able to test DWARF APIs we have to ensure they work
> correctly as there are no Dwarf API tests right now. I will be adding code
> that changes many things in the DWARF parser and it will be essential to
> verify that there are no regressions in the DWARF APIs.
> > - Not sure which targets would be built into LLVM so it might be hard
to
> write tests that cover 32/64 bit addresses and all the variants if we have
> to do things legally via AsmPrinter and valid targets
> >
> > Seems like it might be plausible to refactor out whatever features of
> the AsmPrinter these APIs require (so we just harvest that data out of
> AsmPrinter and pass it down in a struct, say - so that other users can pass
> their own struct without needing an AsmPrinter). Though, again, interested
> to know how dsymutil is working in these situations.
>
> I can try that method if indeed the only places that use the DwarfDebug
> are the DW_FORM_ref_addr and location lists. I'll let you know how that
> goes.
> >
> > - Not enough time to modify AsmPrinter to not require the full
DebugInfo
> stack and the classes that it uses (llvm::DwarfCompileUnit which must use
> llvm::DICompileUnit, llvm::DIE class which uses many local classes that all
> depend on  the full DwarfDebug stack).
> >
> > Will you have time at some later date to come back and revisit this?
> It's understandable that we may choose to incur short term technical
debt
> with an understanding that it will be paid off in some timely manner.
It'd
> be less desirable if there's no such plan/possibility and we incur a
fairly
> clear case of technical debt (redundant DWARF generation libraries -
> especially when this effort is to remove a redundant DWARF parser).
>
> Not sure anyone else will need to generate DWARF manually.

I think all 3 cases (unit tests, dsymutil, llvm codegen) all generate DWARF
about as 'manually' as each other & would likely benefit from using
the
same APIs/commonality as much as possible.

> The two clients currently are the DWARF unittests and the DwarfLinker. The
> DwarfLinker worked around these issues. If the AsmPrinter wasn't such
an
> integral part of the entire compiler stack, I could take a stab at
> refactoring it, but I don't believe I am the right person to do this at
> this point as I have no experience or knowledge of the various ways that
> this class is used, or how it interacts with other support classes
> (DwarfDebug, and many many other classes).
>
Right - I'd generally just suggest doing whatever DwarfLinker's doing,
as a
baseline - if that situation can be improved (in small or large ways)
that'd be cool too. But I don't think creating a second DWARF generation
library is exactly forward progress.

> Things that still worry me:
> - not being able to generate DWARF for 32/64 if targets are missing
>
How's DwarfLinker deal with this? Or does it punt on it?

In any case, again, hopefully things can be extracted/refactored a bit to
simplify this.

> - DIEString not supporting DW_FORM_string. I can add support, but I
don't
> know if we want it as if we add it people might start using it.
>
I think it'll be fine - we only generate them in one place, if we ever
support DW_FORM_string it'll be a very explicit choice/explicit code to do
so.

> - hacking around asserts by constructing classes and copying code from
> places that properly use the AsmPrinter that way it is supposed to be used
> so that we can use it in a way that it wasn't designed to be used.
>
Right - refactoring is probably required (or at least cribbing whatever
techniques DwarfLinker's already using to do the same thing), which is how
these things tend to go.

>
> >
> > I made a large effort to try and get things working with the
AsmPrinter,
> so I wanted everyone to know that I tried to get that solution working. Let
> me know what you anyone thinks.
> >
> > Greg Clayton
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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After Paul Robinson's comments on how few places were using DwarfDebug in
AsmPrinter, I switched from trying to create a DwarfDebug to a solution that
works with requiring a DebugDebug. There were in fact only 2 places that used
the DD (DwarfDebug) variable in AsmPrinter and they are easy to work around.

I added a "uint16_6 DwarfVersion;" and "bool SplitDwarf;"
member variables into AsmPrinter. DwarfVersion is initialized with zero and set
to a valid version if the AsmPrinter creates a DebugDebug object in
AsmPrinter::doInitialization(...), or if clients manually call "void
AsmPrinter::setDwarfInfo(uint16_t Version, bool UseSplitDwarf);". I then
added accessors in AsmPrinter:

  uint16_t getDwarfVersion() const;
  bool getUseSplitDwarf() const;

The "AsmPrinter::getDwarfVersion()" will assert if the DwarfVersion is
zero and explains that either a DwarfDebug must be created via
AsmPrinter::doInitialization() or users should manally call
AsmPrinter::setDwarfInfo(...)".

I also added DW_FORM_string support. 

So I believe I have a workable solution using the AsmPrinter. 

Thanks for the help. Will post a patch to review soon.

Greg
> On Nov 18, 2016, at 9:26 AM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> 
> 
> On Thu, Nov 17, 2016 at 5:00 PM Greg Clayton <gclayton at apple.com>
wrote:
> 
> > On Nov 17, 2016, at 3:40 PM, David Blaikie <dblaikie at
gmail.com> wrote:
> >
> >
> >
> > On Thu, Nov 17, 2016 at 3:12 PM Greg Clayton via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> > I have recently been modifying the DWARF parser and have more patches
planned and I want to be able to add unit tests that test the internal llvm
DWARF APIs to ensure they continue to work and also validate the changes that I
am making. There are not many DWARF unit tests other than very simple ones that
test DWARF forms currently. I would like to expand this to include many more
tests.
> >
> > I had submitted a patch that I aborted as it was too large. One of the
issues with the patch was a stand alone DWARF generator that can turn a few API
calls into the section data required for the DWARFContextInMemory class to be
able to load DWARF from. The idea is to generate a small blurb of DWARF, parse
it using our built in DWARF parser and validate that the API calls we do when
consuming the DWARF match what we expect. The original stand along DWARF
generator class is in unittests/DebugInfo/DWARF/DWARFGenerator2.{h,cpp} in the
patch attached. The original review suggested that I try to use the AsmPrinter
and many of its associated classes to generate the DWARF. I attempted to do so
and the AsmPrinter version is in lib/CodeGen/DwarfGenerator.{h,cpp} in the patch
attached. This AsmPrinter based code steals code from the DwarfLinker.cpp.
> >
> >
> >
> > I am having trouble getting things to work with the AsmPrinter. I was
able to get simple DWARF to be emitted with the AsmPrinter version of the DWARF
generator with code like:
> >
> >
> >    initLLVM();
> >    DwarfGen DG;
> >    Triple Triple("x86_64--");
> >    StringRef Path("/tmp/test.elf");
> >    bool DwarfInitSuccess = DG.init(Triple, Path);
> >    EXPECT_TRUE(DwarfInitSuccess);
> >    uint16_t Version = 4;
> >    uint8_t AddrSize = 8;
> >    DwarfGenCU &CU = DG.appendCompileUnit(Version, AddrSize);
> >    DwarfGenDIE CUDie = CU.getUnitDIE();
> >
> >    CUDie.addAttribute(DW_AT_name, DW_FORM_strp,
"/tmp/main.c");
> >    CUDie.addAttribute(DW_AT_language, DW_FORM_data2, DW_LANG_C);
> >
> >    DwarfGenDIE SubprogramDie = CUDie.addChild(DW_TAG_subprogram);
> >    SubprogramDie.addAttribute(DW_AT_name, DW_FORM_strp,
"main");
> >    SubprogramDie.addAttribute(DW_AT_low_pc, DW_FORM_addr, 0x1000U);
> >    SubprogramDie.addAttribute(DW_AT_high_pc, DW_FORM_addr, 0x2000U);
> >
> >    DwarfGenDIE IntDie = CUDie.addChild(DW_TAG_base_type);
> >    IntDie.addAttribute(DW_AT_name, DW_FORM_strp, "int");
> >    IntDie.addAttribute(DW_AT_encoding, DW_FORM_data1, DW_ATE_signed);
> >    IntDie.addAttribute(DW_AT_byte_size, DW_FORM_data1, 4);
> >
> >    DwarfGenDIE ArgcDie =
SubprogramDie.addChild(DW_TAG_formal_parameter);
> >    ArgcDie.addAttribute(DW_AT_name, DW_FORM_strp, "argc");
> >    //ArgcDie.addAttribute(DW_AT_type, DW_FORM_ref_addr, IntDie); //
Crashes here...
> >
> >    DG.generate();
> >
> >    auto Obj = object::ObjectFile::createObjectFile(Path);
> >    if (Obj) {
> >      DWARFContextInMemory DwarfContext(*Obj.get().getBinary());
> >      uint32_t NumCUs = DwarfContext.getNumCompileUnits();
> >      for (uint32_t i=0; i<NumCUs; ++i) {
> >        DWARFCompileUnit *U = DwarfContext.getCompileUnitAtIndex(i);
> >        if (U)
> >          U->getUnitDIE(false)->dump(llvm::outs(), U, -1u);
> >      }
> >    }
> >
> >
> > But things fall down if I try to uncomment the DW_FORM_ref_addr line
above. The problem is that AsmPrinter really expects a full stack of stuff to be
there and expects people to use the DwarfDebug class and all of its associated
classes. These associated classes really want to use the "DI" objects
(DICompileUnit, etc) so to create a compile unit we would need to create
DICompileUnit object and then make a AsmPrinter/DwarfCompileUnit. That stack is
pretty heavy and requires the code shown above to create many many classes just
to represent the simple output we wish to emit. Another downside of the
AsmPrinter method is we don't know which targets people are going to build
into their binaries and thus we don't know which triples we will be able to
use when generating DWARF info. Adrian Prantl attempted to help me get things
working over here and we kept running into roadblocks.
> >
> > It'd be great to have more detail about the roadblocks you hit to
better understand how bad/what the issues are.
> 
> A few blocks:
> 
> - DIEString doesn't support DW_FORM_string. DW_FORM_string support
might have been pulled so that we never emit it from clang, but we would want to
have a unit test that covers being able to read an inlined C string from a DIE.
Support won't be that hard to add, but we might not want it so that people
can't use it by accident and make less efficient DWARF.
> 
> The codepaths for generating strings in DWARF in LLVM are pretty
streamlined (see DwarfUnit::addString) - it's not likely the presence of
support for DW_FORM_string will suddenly cause anyone to mistakenly create such
an attribute because they all go through that API because it's convenient to
do so & handles all the cases.
> 
> If we ever wanted to support DW_FORM_string (perhaps for users who care a
lot more about size than the performance characteristics you're concerned
with (I don't know if GDB does the same optimizations - so perhaps we're
paying the size for little/no benefit there, for example)) we would do it in
that one place, quite deliberately/explicitly.
>  
> - Asserts, asserts, asserts. As we tried to emit DWARF, we got an asserts
in bool AsmPrinter::doInitialization(Module &M). On the first line:
> 
>   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
> 
> This asserts if you use the AsmPrinter the way the DwarfLinker and the
AsmPrinter based DwarfGen does if you call this. You must call this to generate
the DebugDwarf. If you get past this by installing a Pass then we assert at:
> 
>   GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
>   assert(MI && "AsmPrinter didn't require
GCModuleInfo?");
> 
> If we don't have this, we don't get a DwarfDebug.
> 
> Right - my general thinking is: DwarfLinker uses these APIs somehow so I
expect you could use them similarly. If there are differences in your use case
that make DwarfLinker's usage not applicable to yours, it'd be good to
understand that. If it's just that DwarfLinker's putting up with pain
you'd rather not - well, seems like it'd be good to fix/ease that pain
for both users.
>  
> > Even if we end up adding another set of code to generate DWARF (which
I'd really like to avoid) we'd want to, at some point, coalesce them
back together. Given the goal is to try to coalesce the DWARF parsing code in
LLDB and LLVM, it'd seem unfortunate if that effort just created another
similar (or larger) amount of work for DWARF generation.
> 
> This DWARF generator could just live in the unittests/DebugInfo/DWARF
directory so it wouldn't pollute anything in LLVM it we do choose to use it.
> 
> It's not so much about pollution - much like the DWARF parser in LLDB
and LLVM didn't pollute each other - it's still two systems that end up
growing their own features, bugs, fixes, etc, which is best avoided.
>  
> > I wanted to pass this patch along in case someone wants to take a look
at how we can possibly fix the lib/CodeGen/DwarfGenerator.cpp and
lib/CodeGen/DwarfGenerator.h. The code that sets up all the required classes for
the AsmPrinter method is in the DwarfGen class from
lib/CodeGen/DwarfGenerator.cpp in the following function:
> >
> > bool DwarfGen::init(Triple TheTriple, StringRef OutputFilename);
> >
> > The code in this function was looted from existing DwarfLinker.cpp
code. This functions requires a valid triple and that triple is used to create a
lot of the classes required to make the AsmPrinter. I am not sure if any other
code uses the AsmPrinter like this besides the DwarfLinker.cpp code and that
code uses its own magic to actually link the DWARF. It does reuse some of the
functions as I did, but the DwarfLinker doesn't use any of the DwarfDebug,
DwarfCompileUnit or any of the classes that the compiler/assembler uses when
making DWARF.
> >
> > What's the DwarfLinker code missing that you need? If that code is
generating essentially arbitrary DWARF, what's blocking using the same
technique for generating DWARF for parsing tests?
> 
> They don't use any of the DwarfDebug, DwarfCompileUnit classes. They
also don't use any of the DI classes when making up the debug info. So both
the DWARF linker and the generator have similar needs: make DWARF that isn't
tied too closely to the clang internal classes and DI classes.
> 
> Right - I don't think either of these use cases should use the DI
classes. So how is the DwarfLinker working to generate DWARF in ways that
don't work for your needs?
>  
> > The amount of work required for refactoring the AsmPrinter exceeds the
time I am going to have, but I would still like to have DWARF API testing in the
unit tests.
> >
> > So my question is if anyone would have objections to using the stand
along DWARF generator in unittests/DebugInfo/DWARF until we can later get the
YAML tools to be able to produce DWARF and we can switch to testing the DWARF
data that way? Chris Bieneman has expressed interest in getting a DWARF/YAML
layer going.
> >
> > Those tools would still want to use pretty similar (conceptually)
abstractions to LLVM's codegen and llvm-dsymutil. I'd still strongly
prefer to generalize/keep common APIs here - or better understand why it's
not practical now (& what it will take/how we make sure we have a plan and
resources to get there eventually).
> >
> > My reasoning is:
> > - I want to be able to test DWARF APIs we have to ensure they work
correctly as there are no Dwarf API tests right now. I will be adding code that
changes many things in the DWARF parser and it will be essential to verify that
there are no regressions in the DWARF APIs.
> > - Not sure which targets would be built into LLVM so it might be hard
to write tests that cover 32/64 bit addresses and all the variants if we have to
do things legally via AsmPrinter and valid targets
> >
> > Seems like it might be plausible to refactor out whatever features of
the AsmPrinter these APIs require (so we just harvest that data out of
AsmPrinter and pass it down in a struct, say - so that other users can pass
their own struct without needing an AsmPrinter). Though, again, interested to
know how dsymutil is working in these situations.
> 
> I can try that method if indeed the only places that use the DwarfDebug are
the DW_FORM_ref_addr and location lists. I'll let you know how that goes.
> >
> > - Not enough time to modify AsmPrinter to not require the full
DebugInfo stack and the classes that it uses (llvm::DwarfCompileUnit which must
use llvm::DICompileUnit, llvm::DIE class which uses many local classes that all
depend on  the full DwarfDebug stack).
> >
> > Will you have time at some later date to come back and revisit this?
It's understandable that we may choose to incur short term technical debt
with an understanding that it will be paid off in some timely manner. It'd
be less desirable if there's no such plan/possibility and we incur a fairly
clear case of technical debt (redundant DWARF generation libraries - especially
when this effort is to remove a redundant DWARF parser).
> 
> Not sure anyone else will need to generate DWARF manually.
> 
> I think all 3 cases (unit tests, dsymutil, llvm codegen) all generate DWARF
about as 'manually' as each other & would likely benefit from using
the same APIs/commonality as much as possible.
>  
> The two clients currently are the DWARF unittests and the DwarfLinker. The
DwarfLinker worked around these issues. If the AsmPrinter wasn't such an
integral part of the entire compiler stack, I could take a stab at refactoring
it, but I don't believe I am the right person to do this at this point as I
have no experience or knowledge of the various ways that this class is used, or
how it interacts with other support classes (DwarfDebug, and many many other
classes).
> 
> Right - I'd generally just suggest doing whatever DwarfLinker's
doing, as a baseline - if that situation can be improved (in small or large
ways) that'd be cool too. But I don't think creating a second DWARF
generation library is exactly forward progress.
>  
> Things that still worry me:
> - not being able to generate DWARF for 32/64 if targets are missing
> 
> How's DwarfLinker deal with this? Or does it punt on it?
> 
> In any case, again, hopefully things can be extracted/refactored a bit to
simplify this.
>  
> - DIEString not supporting DW_FORM_string. I can add support, but I
don't know if we want it as if we add it people might start using it.
> 
> I think it'll be fine - we only generate them in one place, if we ever
support DW_FORM_string it'll be a very explicit choice/explicit code to do
so.
>  
> - hacking around asserts by constructing classes and copying code from
places that properly use the AsmPrinter that way it is supposed to be used so
that we can use it in a way that it wasn't designed to be used.
> 
> Right - refactoring is probably required (or at least cribbing whatever
techniques DwarfLinker's already using to do the same thing), which is how
these things tend to go.
>  
> 
> >
> > I made a large effort to try and get things working with the
AsmPrinter, so I wanted everyone to know that I tried to get that solution
working. Let me know what you anyone thinks.
> >
> > Greg Clayton
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev