[Driver][SYCL] Do not imply defaultlib msvcrt for Linux based driver on Windows

[mdtoguchi]

When compiling for Windows target, we should only be adding the
defaultlib msvcrt directive when performing MSVC compatible compilations.
If adding msvcrt for Linux style compilations, we do not know if
the library should be the regular library or the debug library, as that
is determined by the use of /MD or /MDd.

[AGindinson]

The patch LGTM; I double @bader's request for @v-klochkov's input here (could be some context that I'm missing).

[mdtoguchi]

Always pulling in msvcrt is problematic as it can potentially conflict with msvcrtd when attempting to build a debug configured project. Maybe it can be tweaked to add msvcrtd if -D_DEBUG is also passed?

[v-klochkov]

Why the description says "do not imply msvcrt for non-msvc Windows", while the change removes the msvcrt for msvc-windows? The change is done under 'if (AuxT.isWindowsMSVCEnvironment())' condition.

[v-klochkov]

It is hard to tell, why the new code would work properly. It does not seem correct to me now.
The idea of the line that is removed here was to enforce dynamic MSVC RT with -fsycl switch for clang++ driver.

For CL mode the MSVC RT is set in addClangCLArgs() and CL allows either /MD (msvcrt) or /MDd (msvcrtd).
The line that was removed in this patch is for 'clang++' driver, which does not allow setting /MDd, and thus the only option for -fsycl is msvcrt.

[v-klochkov]

What RT is used in the patched compiler (i.e. clang++ -fsycl test.cpp)? If that is libcmt (which is by default), then there may be errors caused by using msvcrt in SYCL and libcmt in test.obj

[mdtoguchi]

The default library (msvcrt) used is passed to the link step via -defaultlib:

> clang++ -fsycl --target=x86_64-pc-windows-msvc ~/a.cpp -### 2>&1 | grep msvcrt "link.exe" "-out:a.exe" "-defaultlib:msvcrt" "-defaultlib:oldnames" "-defaultlib:sycl.lib" "/IGNORE:4078" "-libpath:lib/amd64" "-libpath:atlmfc/lib/amd64" "-nologo" "/tmp/a-ca1b1c.o" "/tmp/a-028002.o"

The use of the -defaultlib option is used for clang++ and the embedded default directives are used for clang-cl.

[v-klochkov]

I still do not understand the motivation for the change.
The description says:

"If adding msvcrt for Linux style compilations, we do not know if
the library should be the regular library or the debug library, as that
is determined by the use of /MD or /MDd."

clang++ does NOT accept /MDd switch and thus there is no way to use debug msvcrtd RT, right? I would think the RT library is always regular (even with -g).

My other concern is that with the patch there is no even a warning telling user that something may go wrong if user compiles sources as below:

NOW (without patch):
ksh-3.2$ clang++ -fsycl test.cpp -c
ksh-3.2$ clang++ test.o ../builds/xmainefi2win_prod/llvm/lib/sycl.lib
LINK : warning LNK4098: defaultlib 'msvcrt.lib' conflicts with use of other libs; use /NODEFAULTLIB:library
test.o : warning LNK4217: locally defined symbol _invalid_parameter_noinfo_noreturn imported in function "void __cdecl std::_Adjust_manually_vector_aligned(void * &,unsigned __int64 &)" (?_Adjust_manually_vector_aligned@std@@YAXAEAPEAXAEA_K@Z)

With the patch:
ksh-3.2$ clang++ -fsycl test.cpp -c
ksh-3.2$ clang++ test.o ../builds/xmainefi2win_prod/llvm/lib/sycl.lib
test.o : warning LNK4217: locally defined symbol _invalid_parameter_noinfo_noreturn imported in function "void __cdecl std::_Adjust_manually_vector_aligned(void * &,unsigned __int64 &)" (?_Adjust_manually_vector_aligned@std@@YAXAEAPEAXAEA_K@Z)

ksh-3.2$ clang++ test.o ../builds/xmainefi2win_prod/llvm/lib/sycl.lib -### 2>&1 | grep defaultlib
"c:/Program files (x86)/Microsoft Visual Studio/2017/Professional/VC/Tools/MSVC/14.16.27023\bin\Hostx64\x64\link.exe" "-out:a.exe" "-defaultlib:libcmt" "-defaultlib:libmmt" "-defaultlib:oldnames" "-nologo" "test.o" "../builds/xmainefi2win_prod/llvm/lib/sycl.lib"

[v-klochkov]

clang++ does NOT accept /MDd switch and thus there is no way to use debug msvcrtd RT, right? I would think the RT library is always regular (even with -g).

Ok, I forgot about using "-Xclang --dependent-lib=msvcrtd", which is also correct usage and interferes with the implied msvcrt.
I am not blocking the patch anymore.
Perhaps @alexbatashev would also take a look and comment.

[bader]

Perhaps @alexbatashev would also take a look and comment.

@alexbatashev, ping.