Visual Studio LLVM toolchain clang-cl --coverage option and linker errors

[Neumann-A]

Bugzilla Link	40877
Version	unspecified
OS	Windows NT
CC	@zmodem,@marco-c,@rnk,@yurybura

Extended Description

So I played with the --coverage option for clang-cl and found that my test projects where not linking due to missing symbols.

After a bit of search for the symbols I found out that I need to link against "clang_rt.profile-x86_64.lib". After doing that linking works fine. The problem however is that linking to the file requires a full path. In my CMake file the command looks like:

target_link_libraries(TestObjLib INTERFACE "\$(LLVMInstallDir)\lib\clang\${CMAKE_CXX_COMPILER_VERSION}\lib\windows\clang_rt.profile-x86_64.lib")

Personally I would expect one of the following behaviors

a) --coverage automatically adds "clang_rt.profile-x86_64.lib" to the required dependencies

or

b) I have to manually add "clang_rt.profile-x86_64.lib" as a dependency but only that and not the full path.

target_link_libraries(TestObjLib INTERFACE clang_rt.profile-x86_64.lib) should be enough This obviously requires the LLVM toolset in VS to add \$(LLVMInstallDir)\lib\clang\\lib\windows\ to the library paths.

Other minor details: clang-cl -help does not list the --coverage option

[rnk]

In the long run, I'd like things to work the way I (apparently!) described for asan back in 2014: https://reviews.llvm.org/D4428#56536

What do you think of embedding just the basename into .drectve, and adding clang's resource dir to the LIB environment variable in the build system? That's a far less invasive change than making clang do the link step instead of link.exe, and should solve the problem.

Which, I guess, is midway between what we do for --coverage and ubsan today.

[llvmbot]

I think it's better for clang to works without explicit linker flags just like using -fsanitize=undefined: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#usage

Use clang++ to compile and link your program with -fsanitize=undefined flag. Make sure to use clang++ (not ld) as a linker, so that your executable is linked with proper UBSan runtime libraries.

[llvmbot]

If clang can find its own resource directory, can't we make LLD also be able to find this as a builtin library path?

This would only work when the linker is set to LLD of course.

[marco-c]

Other minor details: clang-cl -help does not list the --coverage option

This is because the option doesn't have any help text.

Marco, how was your experience using clang-cl with -coverage? Would you like to help improve the documentation, including help text for the flag?

Yes, we've been using it for a while now. I'll file a separate bug for adding documentation, but I'm not sure when I'll get to it.

Between the two proposed options, I definitely prefer (a). It'd be nice if the --coverage option worked by itself, without the user having to manually link to the profile lib. BTW, the same problem applies to PGO.

[Neumann-A]

Detail: LLVM toolset in VS defaults to ldd-link.exe

the LLVM Toolset would require a registry key for the installed version. Currently only the Path is in the registry and read out by the toolset via:

$(Registry:HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\LLVM\LLVM)

Something like

$(Registry:HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\LLVM\Version)

Would be necessary to add the dir \$(LLVMInstallDir)\lib\clang\$(LLVMVersion)\lib\windows\

to the property LibraryPath within the LLVM.Common.props file

The question thus is: Who writes the registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\LLVM\LLVM and could add HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\LLVM\Version to it

(but this would be way b) and not way a) )

Way a could be resolved by the above and adding all *.lib within \$(LLVMInstallDir)\lib\clang\$(LLVMVersion)\lib\windows\ to the additional dependencies in the props file

[zmodem]

+mcastelluccio who exposed the -coverage flag in clang-cl in r317709 and +rnk who reviewed it +zturner who maintains the LLVM VS toolset

Personally I would expect one of the following behaviors

a) --coverage automatically adds "clang_rt.profile-x86_64.lib" to the required dependencies

It works automatically if you also use clang-cl to link.

But I see you're using the LLVM toolset, so I suppose it's invoking link.exe directly.

Maybe Zach knows if we could do something in the toolset to make this work better.

Other minor details: clang-cl -help does not list the --coverage option

This is because the option doesn't have any help text.

Marco, how was your experience using clang-cl with -coverage? Would you like to help improve the documentation, including help text for the flag?

[gatkinso]

Is this related to https://gitlab.kitware.com/cmake/cmake/-/issues/24021 ?

Are there any workarounds for this bug?