[llvm-dev] DWARF Generator

Greg Clayton via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 17 17:00:41 PST 2016

> 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
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