[LLVMdev] Adding line table debug information to LLVM on Windows

Tue Nov 19 10:42:59 PST 2013

Attached is a slightly updated patch.
(it doesn't include D2222 yet).

2013/11/19 Eric Christopher <echristo at gmail.com>:
> In general I do think we're going to need to abstract it out as much
> as possible. I'm not sure what the previous patch looks like, but
> abstracting the interface out would be general goodness for this. We
> can talk about designs for that as we move on.

How about http://llvm-reviews.chandlerc.com/D2222 ?
// Ha! A lucky number!

> As far as how to
> migrate the decision down we can have it both as an option to code gen
> maybe or, for now, make it dependent upon triple. The former is, I
> think, the best option there.

You mean LLVM CodeGen or Clang CodeGen?

Can you suggest a few places in the code where I can find the clues for that?
I'm not yet familiar with this part of the project...

> I'm not sure if you're going to want to support both debug info at the
> same time, but it's a readonly format at debug emission so I don't see
> it as being a problem.

Well, if two debug formats share the same section name, they might
conflict with each other.
I don't think this is the case for Dwarf&CodeView [yet?].

> -Zmlt makes sense as the clang-cl name, or just
> make it whatever the debug mode flag is for cl.exe - this is at least
> a start down that path.

2013/11/19 Reid Kleckner <rnk at google.com>:
> I'd just use /Z7, since that's the cl.exe flag for the compatible format.
> This will need a long-form clang -cc1 flag name, though.

Z7 implies a much more complete debug info than what we'll emit short-term.
Since Z7 is not widely used, I don't think threading Z7 is any simpler
than Zmlt.
I don't have a strong opinion though, WDYT?

> I think the best way for the tests will be to write a set of parsers,
> etc that can dump the info. I realize this is likely a very large
> undertaking as well, but it seems like the only way to ensure we're
> getting some decent testing out.

As you can see from my patch, I actually succeeded in writing *some*
tests without a dumper - just by using the MCAsmStreamer.
Do you think we should really write a dumper too?
That's kinda hard and we don't plan to fully support the CodeView format yet...

> Any other questions I missed?

Please see the TODOs in the attached patch.
You are very likely to come up with a better design/ideas given I'm
new to this part of the codebase.

One particular question I'd like to emphasize is getting a full
filepath for a given MDNode.
As far as I can tell, the metadata for scopes holds pairs of
<filename, directory>, which reflects how DWARF stores them.
However, CodeView stores full paths as entire strings (I admit that
ain't efficient).
Currently, I concat the directory and filename together, but it
a) requires some extra memory management
b) requires special tricks to handle filenames starting from "./", "../", etc.
c) the slashes in the directory name and filename are not consistent on Windows
and is ugly in general.

Do you think it's appropriate to change the scope metadata format to
store <filename, directory, fullpath> instead?
That'd require changing Clang, right?

> -eric
>
>
> On Mon, Nov 18, 2013 at 9:14 AM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
>> I wrote some more lit tests for my patch and realized I was generating
>> some redundant info. This is fixed now. Attached is a new version
>> of the prototype patch with some more tests.
>>
>> 2013/11/15 João Matos <ripzonetriton at gmail.com>:
>>> Hi Timur,
>>>
>>> There's also a pending patch adding CodeView support in Phab:
>>> http://llvm-reviews.chandlerc.com/D165
>>
>> I haven't looked at your patch yet, but based on the very low phab
>> review number, I'm pretty sure there are good reasons this wasn't
>> committed.
>>
>>> Does your patch provide just a subset of the CodeView debug info provided in
>>> the other patch?
>>
>> Yes. I prefer small incremental changes.
>> Also, the file:line debug info is much much more important for me atm
>> than the other types of debug info.
>>
>>> Looking at the patch, I think the approach the other patch took of
>>> abstracting the emission of debug information is a bit cleaner and it will
>>> probably make life easier when adding more debug formats in the future.
>>
>> Why wasn't the "abstract the emission of DI" part of that patch
>> reviewed/committed separately?
>>
>>> On Fri, Nov 15, 2013 at 4:39 PM, Timur Iskhodzhanov <timurrrr at google.com>
>>> wrote:
>>>>
>>>> 2013/11/14 Timur Iskhodzhanov <timurrrr at google.com>:
>>>> > Hi David, Eric, LLVM devs,
>>>> >
>>>> > You've probably heard about AddressSanitizer (ASan) and other
>>>> > sanitizers based on LLVM. One of the things that makes ASan
>>>> > not as awesome on Windows as it is on Linux
>>>> > is the symbolization of the stacks.
>>>> >
>>>> > Currently, ASan runtime on Windows uses
>>>> > CaptureStackBackTrace/SymFromAddr/SymGetLineFromAddr64
>>>> > to unwind and symbolize stacks.  This works like a charm
>>>> > in-process for stack frames built with CL, but yields
>>>> > "function+0x0ff537" for frames built with Clang.
>>>> >
>>>> > I came up with a prototype which emits "old-style debug info" COFF
>>>> > sections that are sufficient to get function name / filename /
>>>> > linenumber information.  That's pretty much everything that's required
>>>> > for ASan to work beautifully in terms of the completeness of error
>>>> > reports.
>>>> >
>>>> > Attached is a prototype patch which I've tried on some simple tests,
>>>> > including some more complex ones with weird #line constructions.
>>>> > It also works just great on ASan/Win tests without any link/run-time
>>>> > warnings (I had a bunch of those during development, so I can tell it
>>>> > works rather than fails silently).
>>>> >
>>>> > I didn't have time to work on threading the command-line flags into
>>>> > the AsmPrinter yet, so currently it just replaces DWARF entirely.
>>>> > Of course, this should be fixed before this lands into trunk.
>>>> > Currently, one can try this patch by using "clang-cl -Xclang -g ...".
>>>> > Eventually we should have some dedicated flag for clang-cl.
>>>> >
>>>> > Can you please take a look at the patch and suggest a good path forward?
>>>> >
>>>> > I'm very unfamiliar with LLVM CodeGen/MC, so I'm pretty sure I made a
>>>> > few weird things in the code...  I've also put a few TODOs with
>>>> > questions and suggestions.
>>>> >
>>>> > Some general questions:
>>>> > 1) Threading flags from the driver down to CodeGen.
>>>> >   How do we do that? Should we support all 4 combinations
>>>> >   of no-info/DWARF/CVLT/both?
>>>> >   How about "-Zmlt" as the clang-cl flag name? ("minimal line tables")
>>>> >
>>>> > 2) Am I right that DWARF is pretty much the only debug info format
>>>> >   supported by LLVM/AsmPrinter right now?  Do we want to take
>>>> >   an effort to come up with a generic debuginfogenerator interface
>>>> >   to share between DwarfDebug and WinCodeViewLineTables?
>>>> >   Then AsmPrinter should just hold a SmallVector<DebugInfoEmitter*>
>>>> >   rather than a pair of DD/DE pointers.
>>>> >
>>>> > 3) How would you suggest to write WinCodeViewLineTables tests
>>>> >   given that dumpbin is not available everywhere except Windows?
>>>> >   // Yeah, I should have looked at the DwarfDebug LIT tests and
>>>> >   // written some; but the prototype development went faster
>>>> >   // than I expected...
>>>>
>>>> I found the MCAsmStreamer being used by llc which gives a decent text
>>>> format.
>>>> I wrote a simple x86+x86_64 .ll test and FileCheck expectations for
>>>> the llc asm output.
>>>> Is this the right approach to write tests?
>>>> If so, I'll convert my remaining C program test cases into such an
>>>> .ll+llc tests.
>>>>
>>>> > Can you suggest ways to split this patch so it's easier
>>>> > to review part-by-part before this hits trunk?
>>>>
>>>> Attached is an updated patch with a new test and a few minor things
>>>> improved.
>>>> I also removed the "TODO: test on X64" as I did try it on x64 and no
>>>> changes were required.
>>>>
>>>> Looking forward to your feedback!
>>>>
>>>> > Thanks!
>>>> > --
>>>> > Timur Iskhodzhanov,
>>>> > Google
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>>
>>>
>>>
>>>
>>> --
>>> João Matos
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