clang-cl: Postpone Wmsvc-not-found emission until link.exe gets used.

[thakis]

Summary

Wmsvc-not-found was added in r297851 to help diagnose why link.exe can't be executed. However, it's emitted even when using -fuse-ld=lld, and in cross builds there's no way to get rid of the warning other than disabling it.

Instead, emit it when we look up link.exe. That way, when passing -fuse-ld=lld it will never be printed.

(We might want to eventually default to lld one day, at least when running on a non-Win host, but that's for another day.)

Fixes PR38016.

[zturner]

lib/Driver/ToolChains/MSVC.cpp

467–468

It looks like it's possible for this warning to be emitted even when FindVisualStudioExecutable succeeds (after looking in the install location it checks PATH). Would it make more sense to put this check after the call to FindVisualStudioExecutable, but only if it fails?

[thakis]

lib/Driver/ToolChains/MSVC.cpp

467–468

If we find link.exe on PATH but LIB isn't set, will link.exe do the right thing? Or is the warning still useful for that case?

I'll upload a patch that does what you suggest, but since I'm not sure what exactly the warning is supposed to catch, I find it difficult to say which version is better.

[thakis]

address comment

[zturner]

Lgtm. I also have a hard time saying which is best, we’re basically trying
to help people who have already strayed from the path, so there’s probably
no objectively correct answer

[thakis]

r337290, thanks!

lib/Driver/ToolChains/MSVC.h

computeMSVCVersion(const Driver *D,
                      const llvm::opt::ArgList &Args) const override;
 
   std::string ComputeEffectiveClangTriple(const llvm::opt::ArgList &Args,
                                           types::ID InputType) const override;
   SanitizerMask getSupportedSanitizers() const override;
 
   void printVerboseInfo(raw_ostream &OS) const override;
 
+  bool FoundMSVCInstall() const { return !VCToolChainPath.empty(); }
+
 protected:
   void AddSystemIncludeWithSubfolder(const llvm::opt::ArgList &DriverArgs,
                                      llvm::opt::ArgStringList &CC1Args,
                                      const std::string &folder,
                                      const Twine &subfolder1,
                                      const Twine &subfolder2 = "",
                                      const Twine &subfolder3 = "") const;
 

lib/Driver/ToolChains/MSVC.cpp

void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // translate 'lld' into 'lld-link', and in the case of the regular msvc
   // linker, we need to use a special search algorithm.
   llvm::SmallString<128> linkPath;
   StringRef Linker = Args.getLastArgValue(options::OPT_fuse_ld_EQ, "link");
   if (Linker.equals_lower("lld"))
     Linker = "lld-link";
 
   if (Linker.equals_lower("link")) {
     // If we're using the MSVC linker, it's not sufficient to just use link
     // from the program PATH, because other environments like GnuWin32 install
     // their own link.exe which may come first.
     linkPath = FindVisualStudioExecutable(TC, "link.exe");
 
+    if (!TC.FoundMSVCInstall() && !llvm::sys::fs::can_execute(linkPath))
+      C.getDriver().Diag(clang::diag::warn_drv_msvc_not_found);
+
 #ifdef _WIN32
     // When cross-compiling with VS2017 or newer, link.exe expects to have
     // its containing bin directory at the top of PATH, followed by the
     // native target bin directory.
     // e.g. when compiling for x86 on an x64 host, PATH should start with:
     // /bin/HostX64/x86;/bin/HostX64/x64
     // This doesn't attempt to handle ToolsetLayout::DevDivInternal.
     if (TC.getIsVS2017OrNewer() &&
MSVCToolChain::MSVCToolChain(const Driver &D, const llvm::Triple &Triple,
   // Failing that, just try to find the newest Visual Studio version we can
   // and use its default VC toolchain.
   findVCToolChainViaEnvironment(VCToolChainPath, VSLayout) ||
       findVCToolChainViaSetupConfig(VCToolChainPath, VSLayout) ||
       findVCToolChainViaRegistry(VCToolChainPath, VSLayout);
 }
 
 Tool *MSVCToolChain::buildLinker() const {
-  if (VCToolChainPath.empty())
-    getDriver().Diag(clang::diag::warn_drv_msvc_not_found);
   return new tools::visualstudio::Linker(*this);
 }
 
 Tool *MSVCToolChain::buildAssembler() const {
   if (getTriple().isOSBinFormatMachO())
     return new tools::darwin::Assembler(*this);
   getDriver().Diag(clang::diag::err_no_external_assembler);
   return nullptr;