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

[João Matos]

Hi Timur,

There's also a pending patch adding CodeView support in Phab:
http://llvm-reviews.chandlerc.com/D165

Does your patch provide just a subset of the CodeView debug info provided
in the other patch?

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.





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
>
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>


-- 
João Matos
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