[clang] [msvc] Automatically link against oldnames just as linking against libcmt

[mstorsjo]

Summary

If invoking clang-cl, both libcmt (or another lib, depending on the /M[TD](|d) options) and oldnames are passed as dependent libs. If compiling and linking via the clang driver, we used to just pass libcmt when linking; include oldnames consistently there.

[rnk]

lgtm

We need to invent spellings for /MD /MT for the GCC-style driver if we want it to be usable in the MSVC environment. I don't think there's a precise translation to the -static-* family of options, unless I'm mistaken.

[amccarth]

It seems weird that we're implicitly adding defaultlib options without checking if the user specified nodefaultlib. But given that's already the case, I don't see a big problem with also adding oldnames.lib.

[mstorsjo]

In D89702#2339716, @rnk wrote:

lgtm

We need to invent spellings for /MD /MT for the GCC-style driver if we want it to be usable in the MSVC environment. I don't think there's a precise translation to the -static-* family of options, unless I'm mistaken.

Yeah I don't think there's any good match in existing GCC-style options. I guess the most appropriate in such a case would be to use -nostdlib or something such, and explicitly mention msvcrt.lib and oldnames.lib. (If compiling with the GCC-style driver but linking by invoking (lld-)link directly, that's essentially what one has to do anyway.)

In D89702#2339719, @amccarth wrote:

It seems weird that we're implicitly adding defaultlib options without checking if the user specified nodefaultlib. But given that's already the case, I don't see a big problem with also adding oldnames.lib.

Well we do check for the -nostdlib and -nostartupfiles options - I guess making it react to -nodefaultlibs also could be considered sensible - as an orthogonal change to this one.

clang/lib/Driver/ToolChains/MSVC.cpp

void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   auto &TC = static_cast<const toolchains::MSVCToolChain &>(getToolChain());
 
   assert((Output.isFilename() || Output.isNothing()) && "invalid output");
   if (Output.isFilename())
     CmdArgs.push_back(
         Args.MakeArgString(std::string("-out:") + Output.getFilename()));
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles) &&
-      !C.getDriver().IsCLMode())
+      !C.getDriver().IsCLMode()) {
     CmdArgs.push_back("-defaultlib:libcmt");
+    CmdArgs.push_back("-defaultlib:oldnames");
+  }
 
   if (!llvm::sys::Process::GetEnv("LIB")) {
     // If the VC environment hasn't been configured (perhaps because the user
     // did not run vcvarsall), try to build a consistent link environment.  If
     // the environment variable is set however, assume the user knows what
     // they're doing.
     CmdArgs.push_back(Args.MakeArgString(
         Twine("-libpath:") +

clang/test/Driver/msvc-link.c

 // RUN: %clang -target i686-pc-windows-msvc -### %s 2>&1 | FileCheck --check-prefix=BASIC %s
 // BASIC: link.exe"
 // BASIC: "-out:a.exe"
 // BASIC: "-defaultlib:libcmt"
+// BASIC: "-defaultlib:oldnames"
 // BASIC: "-nologo"
 // BASIC-NOT: "-Brepro"
 
 // RUN: %clang -target i686-pc-windows-msvc -shared -o a.dll -### %s 2>&1 | FileCheck --check-prefix=DLL %s
 // DLL: link.exe"
 // DLL: "-out:a.dll"
 // DLL: "-defaultlib:libcmt"
+// DLL: "-defaultlib:oldnames"
 // DLL: "-nologo"
 // DLL: "-dll"
 
 // RUN: %clang -target i686-pc-windows-msvc -L/var/empty -L/usr/lib -### %s 2>&1 | FileCheck --check-prefix LIBPATH %s
 // LIBPATH: "-libpath:/var/empty"
 // LIBPATH: "-libpath:/usr/lib"
 // LIBPATH: "-nologo"