[llvm-dev] Debug information with clang-cl on Windows

[James Henderson via llvm-dev]

I'm not the person to ask DExTer questions (I just fixed a few tests that
were causing me bother with regards to the cross-project-tests proposal).
You'll want someone like Jeremy Morse (CC'ed).

James

On Wed, 10 Mar 2021 at 20:02, Alexandre Ganea <alexandre.ganea at ubisoft.com>
wrote:

>
>
>
>
> *De :* Alexandre Ganea
> *Envoyé :* March 10, 2021 3:02 PM
> *À :* 'Tobias Hieta' <tobias at plexapp.com>; Adrian McCarthy <
> amccarth at google.com>; 'James Henderson' <jh7370.2008 at my.bristol.ac.uk>
> *Objet :* RE: [llvm-dev] Debug information with clang-cl on Windows
>
>
>
> DExTer can probably help bisecting to the specific optimization pass(es)
> that cause the debug infos to be omitted?
> https://lists.llvm.org/pipermail/llvm-dev/2018-August/125780.html
>
>
>
> +James.
>
>
>
> *De :* llvm-dev <llvm-dev-bounces at lists.llvm.org> *De la part de* Tobias
> Hieta via llvm-dev
> *Envoyé :* March 10, 2021 2:26 PM
> *À :* Adrian McCarthy <amccarth at google.com>
> *Cc :* llvm-dev <llvm-dev at lists.llvm.org>
> *Objet :* Re: [llvm-dev] Debug information with clang-cl on Windows
>
>
>
> Hello,
>
>
>
> Thanks for your reply!
>
>
>
> Is it possible to disable optimisations with clang-cl and see if that
> makes a difference?
>
>
>
> I will also try to build with clang 12 instead and see if there is any
> improvement.
>
>
>
> Happy to try to reproduce or fix the issue - but I am unfamiliar with this
> debug info on windows and how to even start looking into it, any pointers
> are appreciated.
>
>
>
> Thanks,
>
> Tobias.
>
>
>
> On Wed, 10 Mar 2021 at 00:45, Adrian McCarthy <amccarth at google.com> wrote:
>
> I believe that, even with `/Od` (that's Microsoft for "disable
> optimizations"), clang-cl applies some optimizations.
>
>
>
> People on my team have been fixing debug info quality bugs for Windows for
> a while.  I don't have specific knowledge of these issues, but it's
> possible newer versions (like 12 or head) may have solved this.  We're
> somewhat more focused on 64-bit than 32-bit builds, so you might encounter
> some differences there.
>
>
>
> If you can reproduce the problem with a reduced code sample, please file a
> bug with all the deets.  If you do some debugging yourself, please let us
> know what you find.
>
>
>
> On Tue, Mar 9, 2021 at 2:46 PM David Blaikie via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I would guess there's no one bug here - debug info quality
> (specifically/especially the location of variables when compiling with
> optimizations enabled) is a long tail/sliding scale of issues. LLVM is, on
> the whole, not spectacular at this - a matter of bugs to fix, some
> small/simple, others more systemic/representational.
>
> If you're interested in contributing to this work - generally the first
> step is to isolate the problem - first thing you can do is if you have some
> experience dumping/examining debug info in object files, you can
> dump/examine the relevant object files then try to reduce/remove code from
> the relevant objects, etc, to see what the minimal example is that'll make
> it easier to figure out how to fix.
>
>
>
> On Tue, Mar 9, 2021 at 12:37 AM Tobias Hieta via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hello,
>
> We recently switched our windows builds from the CL compiler to
> clang-cl. This was a pretty big jump in terms of code generation and
> having a uniform compiler across all the major platforms.
>
> Our setup is that our Windows developer use MSVC as the IDE and
> compile with our own build of clang-cl and link with lld-link.
>
> Some of the developers noticed that some of the debug information are
> not as good as with CL. Or rather some variables seems to be optimized
> out, this is what the IDE says, see attached screenshot. I have tried
> to figure out if there is a rhyme and reason to what variables are
> gone - but I have not been able to see any pattern.
>
> Our command line is:
>
> clang-cl.exe /nologo -DUNICODE -DUSE_BREAKPAD -DWIN32
> -DWIN32_LEAN_AND_MEAN -DWINVER=0x0601 -D_CRT_NONSTDC_NO_DEPRECATE
> -D_CRT_SECURE_NO_WARNINGS -D_LEGACY_NAT_TRAVERSAL_ -D_REENTRANT
> -D_SCL_SECURE_NO_WARNINGS -D_SILENCE_ALL_CXX17_DEPRECATION_WARNINGS
> -D_UNICODE -D_WIN32_WINNT=0x0601 -D_WINSOCK_DEPRECATED_NO_WARNINGS
> -Iincludes -m32 -fdiagnostics-absolute-paths
> --target=i686-windows-msvc -m32 /DWIN32 /D_WINDOWS /GR /EHsc /Zi /Ob0
> /Od /RTC1 -MDd /Zm512 /EHsc /bigobj /W3 -Werror -Wsign-compare
> -Wno-reorder-ctor -Wno-delete-non-virtual-dtor -Wunused-variable
> -Wuninitialized -std:c++14
>
> Is there any good way to debug this or is it a known limitation with
> clang-cl or 32 bit (we don't do 64 yet - but with clang-cl we hope to
> move towards that soon). Or am I missing some flags?
>
> Thanks,
> Tobias
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