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

Wed Mar 10 12:02:09 PST 2021

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<mailto: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<mailto:amccarth at google.com>>
Cc : llvm-dev <llvm-dev at lists.llvm.org<mailto: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<mailto: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<mailto: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<mailto: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<mailto: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<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20210310/05563712/attachment.html>