Add experimental -internal-linker switch to enable LLD opt-in

And infer CMake LDC_WITH_LLD automatically based on availability of LLVM
3.9+ with LLD headers & libs.

CMakeLists.txt

@@ -84,8 +84,6 @@ set(LIB_SUFFIX "" CACHE STRING "Appended to the library installation directory.
 option(GENERATE_OFFTI "generate complete ClassInfo.offTi arrays")
 mark_as_advanced(GENERATE_OFFTI)
 
-option(LDC_WITH_LLD "Integrate LLD, the LLVM cross-linker")
-
 if(D_VERSION EQUAL 1)
     message(FATAL_ERROR "D version 1 is no longer supported.
 Please consider using D version 2 or checkout the 'd1' git branch for the last version supporting D version 1.")
@@ -448,16 +446,23 @@ if(GENERATE_OFFTI)
     append("-DGENERATE_OFFTI" LDC_CXXFLAGS)
 endif()
 
-if(LDC_WITH_LLD)
-    append("-DLDC_WITH_LLD" LDC_CXXFLAGS)
+#
+# LLD integration (requires LLVM >= 3.9 with LLD headers & libs)
+#
+set(LDC_WITH_LLD OFF)
+if(LDC_LLVM_VER GREATER 308)
+    if(EXISTS "${LLVM_INCLUDE_DIRS}/lld/Driver/Driver.h")
+        message(STATUS "Building LDC with LLD support")
+        append("-DLDC_WITH_LLD" LDC_CXXFLAGS)
+        set(LDC_WITH_LLD ON)
+    endif()
 endif()
 
-option(RISCV_LLVM_DEV, "full RISC-V support with riscv-llvm")
-mark_as_advanced(RISCV_LLVM_DEV)
-
 #
 # Enable building with riscv-llvm, for full RISC-V support.
 #
+option(RISCV_LLVM_DEV, "full RISC-V support with riscv-llvm")
+mark_as_advanced(RISCV_LLVM_DEV)
 if(RISCV_LLVM_DEV)
     append("-DRISCV_LLVM_DEV" LDC_CXXFLAGS)
 endif()

driver/linker-gcc.cpp

@@ -420,7 +420,7 @@ class LdArgsBuilder : public ArgsBuilder {
 
 //////////////////////////////////////////////////////////////////////////////
 
-int linkObjToBinaryGcc(llvm::StringRef outputPath,
+int linkObjToBinaryGcc(llvm::StringRef outputPath, bool useInternalLinker,
                        llvm::cl::boolOrDefault fullyStaticFlag) {
   // find gcc for linking
   const std::string tool = getGcc();

driver/linker-msvc.cpp

@@ -51,7 +51,7 @@ void addMscrtLibs(std::vector<std::string> &args,
 
 //////////////////////////////////////////////////////////////////////////////
 
-int linkObjToBinaryMSVC(llvm::StringRef outputPath,
+int linkObjToBinaryMSVC(llvm::StringRef outputPath, bool useInternalLinker,
                         llvm::cl::boolOrDefault fullyStaticFlag) {
   if (!opts::ccSwitches.empty()) {
     error(Loc(), "-Xcc is not supported for MSVC");
@@ -165,7 +165,6 @@ int linkObjToBinaryMSVC(llvm::StringRef outputPath,
   logstr << "\n"; // FIXME where's flush ?
 
 #if LDC_WITH_LLD
-  const bool useInternalLinker = true; // TODO
   if (useInternalLinker) {
     const auto fullArgs =
         getFullArgs("lld-link.exe", args, global.params.verbose);

driver/linker.cpp

@@ -20,19 +20,27 @@
 
 //////////////////////////////////////////////////////////////////////////////
 
-static llvm::cl::opt<bool>
+static llvm::cl::opt<llvm::cl::boolOrDefault>
     staticFlag("static", llvm::cl::ZeroOrMore,
                llvm::cl::desc("Create a statically linked binary, including "
                               "all system dependencies"));
 
+#if LDC_WITH_LLD
+static llvm::cl::opt<bool>
+    useInternalLinker("internal-linker", llvm::cl::ZeroOrMore, llvm::cl::Hidden,
+                      llvm::cl::desc("Use internal LLD for linking"));
+#else
+constexpr bool useInternalLinker = false;
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 
 // linker-gcc.cpp
-int linkObjToBinaryGcc(llvm::StringRef outputPath,
+int linkObjToBinaryGcc(llvm::StringRef outputPath, bool useInternalLinker,
                        llvm::cl::boolOrDefault fullyStaticFlag);
 
 // linker-msvc.cpp
-int linkObjToBinaryMSVC(llvm::StringRef outputPath,
+int linkObjToBinaryMSVC(llvm::StringRef outputPath, bool useInternalLinker,
                         llvm::cl::boolOrDefault fullyStaticFlag);
 
 //////////////////////////////////////////////////////////////////////////////
@@ -135,16 +143,11 @@ int linkObjToBinary() {
 
   createDirectoryForFileOrFail(gExePath);
 
-  llvm::cl::boolOrDefault fullyStaticFlag = llvm::cl::BOU_UNSET;
-  if (staticFlag.getNumOccurrences() != 0) {
-    fullyStaticFlag = staticFlag ? llvm::cl::BOU_TRUE : llvm::cl::BOU_FALSE;
-  }
-
   if (global.params.targetTriple->isWindowsMSVCEnvironment()) {
-    return linkObjToBinaryMSVC(gExePath, fullyStaticFlag);
+    return linkObjToBinaryMSVC(gExePath, useInternalLinker, staticFlag);
   }
 
-  return linkObjToBinaryGcc(gExePath, fullyStaticFlag);
+  return linkObjToBinaryGcc(gExePath, useInternalLinker, staticFlag);
 }
 
 //////////////////////////////////////////////////////////////////////////////