Allow clang-cl to automatically use lld instead of forcing the MSVC linker

[ruiu]

lib/Driver/Tools.cpp

7987 ↗

(On Diff #16667)

Why don't you just pass "lld-link" (not "lld") to -fuse-ld option?

[zturner]

lib/Driver/Tools.cpp

7987 ↗

(On Diff #16667)

Because there should be a consistent syntax for specifying "link with lld" regardless of which platform you're on. I don't want it to be -fuse-ld=lld on Linux, and -fuse-ld=lld-link on Windows, for example.

[thakis]

lib/Driver/Tools.cpp

7990 ↗

(On Diff #16667)

Somewhat tangential: It would be nice if clang could default to lld-link if link.exe isn't found. If I'm using clang-cl targeting Windows on non-Windows, and lld-link is in the same directory as clang-cl, it'd be great if clang-cl would then transparently use lld-link to link my PE binary instead of telling me "unable to execute command: Executable "link.exe" doesn't exist!"

(Not sure if this should be part of this change, but it's in the same code at least.)

[ruiu]

lib/Driver/Tools.cpp

7987 ↗

(On Diff #16667)

Got it.

7990 ↗

(On Diff #16667)

Besides the issue how to find libraries in the environment where link.exe doesn't exist, lld currently depends on a few external command. rc.exe (resource compiler) and lib.exe (ar command) are needed. In order to make such silent fallback work, we need to eliminate that dependency.

[rnk]

I'm kind of bummed that we have to special case lld, but I guess that's the spirit of the gcc flag anyway. This seems like the right direction.

lib/Driver/Tools.cpp

7982 ↗

(On Diff #16667)

Clang code uses StudlyCaps for variable names. (Yaaay!)

7983 ↗

(On Diff #16667)

There are some idioms that can make this code more concise. getLastArgValue() exists and takes a default value, and const char * converts implicitly to StringRef. This can probably be:

StringRef Linker = Args.getLastArgValue(options::OPT_fuse_ld_EQ, "link");
if (Linker.equals_lower("lld"))
  Linker = "lld-link";

if (Linker.equals_lower("link")) {
  ...

[zturner]

Cleaned up the special casing code a little bit. Thanks for the suggestion, this is much cleaner.

[rnk]

lgtm

[zturner]

Closed by commit rL223086 (authored by @zturner).

cfe/trunk/lib/Driver/MSVCToolChain.cpp

 using namespace clang::driver;
 using namespace clang::driver::toolchains;
 using namespace clang;
 using namespace llvm::opt;
 
 MSVCToolChain::MSVCToolChain(const Driver &D, const llvm::Triple& Triple,
                              const ArgList &Args)
   : ToolChain(D, Triple, Args) {
+  getProgramPaths().push_back(getDriver().getInstalledDir());
+  if (getDriver().getInstalledDir() != getDriver().Dir)
+    getProgramPaths().push_back(getDriver().Dir);
 }
 
 Tool *MSVCToolChain::buildLinker() const {
   return new tools::visualstudio::Link(*this);
 }
 
 Tool *MSVCToolChain::buildAssembler() const {
   if (getTriple().isOSBinFormatMachO())

cfe/trunk/lib/Driver/Tools.cpp

if (A.getOption().matches(options::OPT_l)) {
       continue;
     }
 
     // Otherwise, this is some other kind of linker input option like -Wl, -z,
     // or -L. Render it, even if MSVC doesn't understand it.
     A.renderAsInput(Args, CmdArgs);
   }
 
-  // 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.
-  llvm::SmallString<128> linkPath(FindVisualStudioExecutable(
-      getToolChain(), "link.exe", C.getDriver().getClangProgramPath()));
+  // We need to special case some linker paths.  In the case of lld, we need to
+  // 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(getToolChain(), "link.exe",
+                                          C.getDriver().getClangProgramPath());
+  } else {
+    linkPath = Linker;
+    llvm::sys::path::replace_extension(linkPath, "exe");
+    linkPath = getToolChain().GetProgramPath(linkPath.c_str());
+  }
+
   const char *Exec = Args.MakeArgString(linkPath);
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs));
 }
 
 void visualstudio::Compile::ConstructJob(Compilation &C, const JobAction &JA,
                                          const InputInfo &Output,
                                          const InputInfoList &Inputs,
                                          const ArgList &Args,