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

Wed Mar 10 12:48:13 PST 2021

llvm-dva (not yet in tree but see
https://reviews.llvm.org/D88661 ) might also be useful here depending on
how mature the codeview support is (cc Carlos - please correct me if
anything I said is wrong!).

I think, in theory it should allow us to do a direct comparison between the
debug info generated by clang-cl, and that generated by cl.exe for the same
code (and also with clang's DWARF output if that's possible to generate
too) so we can see exactly what the differences are and what's missing.

When I previously looked into some of this stuff using dexter I found a few
examples like
https://bugs.llvm.org/show_bug.cgi?id=37682 where clang was comparing
unfavorably with MSVC because it was doing more aggressive loop
optimizations. I don't know how widespread it is or whether it's still the
case in more recent versions, but I certainly ran into it on more than one
occasion where MSVC was doing things like leaving stores in loops whereas
clang would hoist it instead for better codegen but an unfortunately worse
debugging experience.

-Greg

On Wed, 10 Mar 2021 at 20:02, Alexandre Ganea via llvm-dev <
llvm-dev at lists.llvm.org> 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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