[llvm-dev] LLVM_DYLIB and CLANG_DYLIB with MSVC

[Reid Kleckner via llvm-dev]

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:
>
>    - LLVM_BUILD_LLVM_DYLIB=ON
>    - LLVM_LINK_LLVM_DYLIB=ON
>    - CLANG_LINK_CLANG_DYLIB=ON
>
> 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_TARGETS_TO_BUILD=X86
>    - 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/3a3b8a7df17a49ba7c0153b0c9a7ee25705ede46>
> 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:78:0
> #5 0x00007ff689e26d04 __scrt_common_main_seh
> D:\agent\_work\10\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288:0
> #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.
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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