[llvm-dev] LLVM_DYLIB and CLANG_DYLIB with MSVC

Tom Stellard via llvm-dev llvm-dev at lists.llvm.org
Mon Jun 14 10:49:20 PDT 2021

On 6/14/21 10:33 AM, Joachim Meyer via llvm-dev wrote:
> Hi all,
> I'm new to this mailing list so, a short intro from my side :)
> ======
> I am a computer engineering student from Heidelberg University, currently
> working on my master's thesis where I extend the hipSYCL Clang plugin to
> enable optimized support for SYCL's nd_range parallel_for on CPUs.
> In hipSYCL we happen to be able to support Windows with our Clang plugin and
> would obviously love to keep it that way to bring my current work to Windows
> at some point as well.
> The pain point starts when using the new PM with the currently only
> optimization pass in the plugin, as the static Key members of Analyses are not
> correctly ex-/imported on Windows. As mentioned by Martin this could be worked
> around by moving them into function local static variables, but this would be
> just another workaround and would not solve the general issue with symbol
> exports on Windows, which also includes some symbols being filtered out, which
> makes working with the system pretty flakey.
> (e.g. llvm::DominatorTreeBase<class llvm::BasicBlock, 1> seems to be exported
> correctly but llvm::DominatorTreeBase<class llvm::BasicBlock, 0> is not for no
> obvious reason.. ref: https://github.com/fodinabor/hipSYCL/runs/2736333414?
> check_suite_focus=true#step:10:3489)
> As far as I understand, marking required symbols as exported, can help in this
> situation, especially as proper support for using LLVM as shared library in
> the Clang driver and so on could drastically reduce the symbol pressure in
> each of the binaries.
> ======
> I am thus coming from a slightly different angle to the problem, but see the
> same ideal solution as Reid and Fang-rui, therefore I'd like to ask *if there
> are objections against starting to explicitly mark symbols as being exported*.

This would be a huge improvement, if you want to work on it, go for it.

> The proposal from last weeks' Windows/COFF call, was to start by adding a
> header defining `LLVM_EXPORT` macros for explicit marking of exported symbols,
> which can be used to mark data symbols as `__declspec(dllimport)` as well.
> With this in the codebase, the symbols required when using shared libraries,
> can be marked successively to enable more API boundaries to rely on the
> explicit exports.

There is already the LLVM_EXTERNAL_VISIBILITY macro defined in
llvm/Support/Compiler.h macro which is used in llvm/lib/Target.
I would start using this one instead of creating a new LLVM_EXPORT macro.
We can always rename the macro later if people like the name LLVM_EXPORT

- Tom

> As I see it, this is also in-line with the comment by Tom Stellard regarding
> the LLD-as-a-library design, where they wish for a single shared library that
> only exports symbols if explicitly marked.
> Another suggestion in the call was to provide a single shared library for each
> of the projects (LLVM, Clang, ..), instead of many small shared libraries as
> it is the case at the moment when using shared lib builds. The obligatory
> exception to the rule might be the Support lib, which could be a useful
> candidate to keep separate.
> To conclude, I'd love feedback on whether this is generally something the
> wider community sees as something worth to pursue and I am open to more
> suggestions on the best way forward.
> -- Joachim Meyer
> Am Donnerstag, 3. Juni 2021, 19:11:40 CEST schrieb Reid Kleckner:
>> I agree, for the reasons you outline, LLVM needs to do more to support
>> producing DLLs on Windows. However, I don't really have time to dig into
>> your proposal and the issues you are running into.
>> Personally, I think LLVM should move away from that .def file generation
>> python script, and towards source-level annotations. It would allow us to
>> use fvisibility=hidden in the shared library build on other platforms. That
>> would greatly reduce the number of dynamic symbols in LLVM, which I
>> understand is desirable. Today we used Bsymbolic-functions, so we may have
>> already captured most of the benefits of fvisibility=hidden, but hidden
>> visibility is always better for performance and binary size.
>> I put out this alternative proposal mainly to see if there is any
>> enthusiasm for it. If not, you are welcome to continue forward with our
>> existing solutions. But if there is broad interest in adding API
>> annotations to LLVM, I think that would be a better way to go long term.
>> On Sun, May 30, 2021 at 7:17 AM Cristian Adam via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Hi,
>>> Clang 12 can be configured on Windows with MinGW (either GNU or LLVM) with
>>> the following CMake parameters:
>>> which has some effect on the binary size of the build.
>>> I configured the llvm-project with the following parameters:
>>>     - CMAKE_BUILD_TYPE=Release
>>>     - LLVM_ENABLE_PROJECTS=clang;clang-tools-extra
>>> The installed (stripped) build of Clang 12 with llvm-mingw release 12.0.0
>>> <https://github.com/mstorsjo/llvm-mingw/releases/tag/20210423> resulted
>>> in:
>>>     - Normal build: 1,76 GB
>>>     - shlib build: 481 MB
>>> Due to the nature of MSVC regarding default visibility of symbols (hidden
>>> by default, whereas MinGW has visible by default), one needs to generate a
>>> .def file with the symbols needed to be exported.
>>> This is done already in two cases for LLVM_BUILD_LLVM_C_DYLIB
>>> (llvm/tools/llvm-shlib/gen-msvc-exports.py) and for
>>> LLVM_EXPORT_SYMBOLS_FOR_PLUGINS (llvm/utils/extract_symbols.py).
>>> I've put together a patch
>>> <https://github.com/cristianadam/llvm-project/commit/3a3b8a7df17a49ba7c015
>>> 3b0c9a7ee25705ede46> that enables LLVM_DYLIB and CLANG_DYLIB for MSVC.
>>> I tested with clang-cl from the official Clang 12 x64 Windows binary
>>> release:
>>>     - Normal build: 1,42 GB
>>>     - shlib build: 536 MB
>>> The shlib release build compiled and linked fine with LLVM.dll and
>>> clang-cpp.dll, unfortunately it crashes at runtime. For example llvm-nm:
>>> $ llvm-nm
>>> PLEASE submit a bug report to https://bugs.llvm.org/ and include the
>>> crash backtrace.
>>> Stack dump:
>>> 0.      Program arguments: llvm-nm
>>> #0 0x00007ffd32807d43 llvm::StringMap<llvm::cl::Option
>>> *,llvm::MallocAllocator>::begin
>>> C:\Projects\llvm-project\repo\llvm\include\llvm\ADT\StringMap.h:204:0
>>> #1 0x00007ffd32807d43 llvm::cl::HideUnrelatedOptions(class
>>> llvm::cl::OptionCategory &, class llvm::cl::SubCommand &)
>>> C:\Projects\llvm-project\repo\llvm\lib\Support\CommandLine.cpp:2589:0
>>> #2 0x00007ff689df2b13 llvm::StringRef::StringRef
>>> C:\Projects\llvm-project\repo\llvm\include\llvm\ADT\StringRef.h:107:0
>>> #3 0x00007ff689df2b13 main
>>> C:\Projects\llvm-project\repo\llvm\tools\llvm-nm\llvm-nm.cpp:2232:0
>>> #4 0x00007ff689e26d04 invoke_main
>>> D:\agent\_work\10\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:7
>>> 8:0 #5 0x00007ff689e26d04 __scrt_common_main_seh
>>> D:\agent\_work\10\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:2
>>> 88:0 #6 0x00007ffd9a7f7034 (C:\Windows\System32\KERNEL32.DLL+0x17034)
>>> #7 0x00007ffd9b742651 (C:\Windows\SYSTEM32\ntdll.dll+0x52651)
>>> This crash is due to llvm::cl::HideUnrelatedOptions which accesses
>>> TopLevelSubCommand, which is defined as:
>>> extern ManagedStatic<SubCommand> TopLevelSubCommand;
>>> The MSVC 2019 build behaves in the same way as the clang-cl build.
>>> I have tried building without LLVM_ENABLE_THREADS, or by linking to the
>>> CRT statically LLVM_USE_CRT_RELEASE=MT, didn't help.
>>> The MSVC 2019 build sizes were:
>>>     - Normal build: 1,74 GB
>>>     - shlib build: 949 MB
>>> I would appreciate any help in getting the shlib build running. It works
>>> fine with llvm-mingw, I think it should also work with clang-cl / cl.
>>> Cheers,
>>> Cristian.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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