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

[Zachary Turner]

Hi rnk, ruiu,

http://reviews.llvm.org/D6428

Files:
  lib/Driver/MSVCToolChain.cpp
  lib/Driver/Tools.cpp

Index: lib/Driver/MSVCToolChain.cpp
===================================================================
--- lib/Driver/MSVCToolChain.cpp
+++ lib/Driver/MSVCToolChain.cpp
@@ -43,6 +43,9 @@
 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 {
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -7976,10 +7976,31 @@
     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;
+  Arg *A = Args.getLastArg(options::OPT_fuse_ld_EQ);
+  const char *ld_value = nullptr;
+  if (A) {
+    ld_value = A->getValue();
+    if (StringRef(ld_value).equals_lower("lld"))
+      ld_value = "lld-link";
+  } else {
+    ld_value = "link";
+  }
+  if (StringRef(ld_value).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 = ld_value;
+    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));
 }

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