[llvm-dev] LLVM_DYLIB and CLANG_DYLIB with MSVC

[Cristian Adam via llvm-dev]

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 
accessesTopLevelSubCommand, 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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