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

[Adrian McCarthy via llvm-dev]

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
>>
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