Integrate LLD for MSVC targets via experimental CMake option LDC_WITH_LLD

Results in a 7.5% bigger RelWithDebInfo ldc2.exe on Win64 with LLVM 3.9.1.

LLD is currently enforced when building with LDC_WITH_LLD=ON. And LLD
still doesn't support debuginfo (.pdb) generation for MSVC targets.

CMakeLists.txt

@@ -80,10 +80,12 @@ set(CONF_INST_DIR ${SYSCONF_INSTALL_DIR} CACHE PATH "Directory ldc.conf is insta
 # Note: LIB_SUFFIX should perhaps be renamed to LDC_LIBDIR_SUFFIX.
 set(LIB_SUFFIX "" CACHE STRING "Appended to the library installation directory. Set to '64' to install libraries into ${PREFIX}/lib64.")
 
-# The following flags are currently not well tested, expect the build to fail.
+# The following flag is currently not well tested, expect the build to fail.
 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.")
@@ -428,8 +430,6 @@ include_directories( SYSTEM
 )
 append("-I${PROJECT_SOURCE_DIR}" DDMD_DFLAGS)
 append("-I${PROJECT_BINARY_DIR}" DDMD_DFLAGS)
-
-
 if(MSVC)
     include_directories(${PROJECT_SOURCE_DIR}/vcbuild)
 endif()
@@ -448,6 +448,10 @@ if(GENERATE_OFFTI)
     append("-DGENERATE_OFFTI" LDC_CXXFLAGS)
 endif()
 
+if(LDC_WITH_LLD)
+    append("-DLDC_WITH_LLD" LDC_CXXFLAGS)
+endif()
+
 option(RISCV_LLVM_DEV, "full RISC-V support with riscv-llvm")
 mark_as_advanced(RISCV_LLVM_DEV)
 
@@ -548,6 +552,13 @@ add_custom_target(${LDMD_EXE} ALL DEPENDS ${LDMD_EXE_FULL})
 # Figure out how to link the main LDC executable, for which we need to take the
 # LLVM flags into account.
 set(LDC_LINKERFLAG_LIST "${SANITIZE_LDFLAGS};${LLVM_LIBRARIES};${LLVM_LDFLAGS}")
+if(LDC_WITH_LLD)
+    if(MSVC)
+        list(APPEND LDC_LINKERFLAG_LIST lldCOFF.lib lldCore.lib lldDriver.lib)
+    else()
+        set(LDC_LINKERFLAG_LIST "-llldCOFF;-llldCore;-llldDriver;${LDC_LINKERFLAG_LIST}")
+    endif()
+endif()
 
 set(LDC_LINK_MANUALLY OFF)
 if(UNIX AND (CMAKE_COMPILER_IS_GNUCXX OR (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")))

driver/archiver.cpp

@@ -362,19 +362,7 @@ int createStaticLibrary() {
 
 #if LDC_LLVM_VER >= 309
   if (useInternalArchiver) {
-    std::vector<const char *> fullArgs;
+    const auto fullArgs = getFullArgs(tool, args, global.params.verbose);
-    fullArgs.reserve(1 + args.size());
-    fullArgs.push_back(tool.c_str());
-    for (const auto &arg : args)
-      fullArgs.push_back(arg.c_str());
-
-    if (global.params.verbose) {
-      for (auto arg : fullArgs) {
-        fprintf(global.stdmsg, "%s ", arg);
-      }
-      fprintf(global.stdmsg, "\n");
-      fflush(global.stdmsg);
-    }
 
     const int exitCode =
         isTargetMSVC ? internalLib(fullArgs) : internalAr(fullArgs);

driver/linker-msvc.cpp

@@ -12,6 +12,10 @@
 #include "driver/tool.h"
 #include "gen/logger.h"
 
+#if LDC_WITH_LLD
+#include "lld/Driver/Driver.h"
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 
 static llvm::cl::opt<std::string> mscrtlib(
@@ -160,6 +164,20 @@ 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);
+
+    const bool success = lld::coff::link(fullArgs);
+    if (!success)
+      error(Loc(), "linking with LLD failed");
+
+    return success ? 0 : 1;
+  }
+#endif
+
   // try to call linker
   return executeToolAndWait(tool, args, global.params.verbose);
 }

driver/tool.cpp

@@ -89,6 +89,30 @@ void createDirectoryForFileOrFail(llvm::StringRef fileName) {
 
 ////////////////////////////////////////////////////////////////////////////////
 
+std::vector<const char *> getFullArgs(const std::string &tool,
+                                      const std::vector<std::string> &args,
+                                      bool printVerbose) {
+  std::vector<const char *> fullArgs;
+  fullArgs.reserve(args.size() +
+                   2); // executeToolAndWait() appends an additional null
+
+  fullArgs.push_back(tool.c_str());
+  for (const auto &arg : args)
+    fullArgs.push_back(arg.c_str());
+
+  // Print command line if requested
+  if (printVerbose) {
+    for (auto arg : fullArgs)
+      fprintf(global.stdmsg, "%s ", arg);
+    fprintf(global.stdmsg, "\n");
+    fflush(global.stdmsg);
+  }
+
+  return fullArgs;
+}
+
+////////////////////////////////////////////////////////////////////////////////
+
 int executeToolAndWait(const std::string &tool_,
                        std::vector<std::string> const &args, bool verbose) {
   const auto tool = findProgramByName(tool_);
@@ -99,24 +123,9 @@ int executeToolAndWait(const std::string &tool_,
 
   // Construct real argument list.
   // First entry is the tool itself, last entry must be NULL.
-  std::vector<const char *> realargs;
+  auto realargs = getFullArgs(tool, args, verbose);
-  realargs.reserve(args.size() + 2);
-  realargs.push_back(tool.c_str());
-  for (const auto &arg : args) {
-    realargs.push_back(arg.c_str());
-  }
   realargs.push_back(nullptr);
 
-  // Print command line if requested
-  if (verbose) {
-    // Print it
-    for (size_t i = 0; i < realargs.size() - 1; i++) {
-      fprintf(global.stdmsg, "%s ", realargs[i]);
-    }
-    fprintf(global.stdmsg, "\n");
-    fflush(global.stdmsg);
-  }
-
   // Execute tool.
   std::string errstr;
   if (int status = llvm::sys::ExecuteAndWait(tool, &realargs[0], nullptr,

driver/tool.h

@@ -28,6 +28,10 @@ std::string getProgram(const char *name,
 
 void createDirectoryForFileOrFail(llvm::StringRef fileName);
 
+std::vector<const char *> getFullArgs(const std::string &tool,
+                                      const std::vector<std::string> &args,
+                                      bool printVerbose);
+
 int executeToolAndWait(const std::string &tool,
                        std::vector<std::string> const &args,
                        bool verbose = false);