llvm dev - Mar 2021 - Debug information with clang-cl on Windows

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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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