Update link job for flang on windows

When linking a Fortran program, we need to add the runtime libraries to
the command line. This is exactly what we do for Linux/Darwin, but the
MSVC interface is slightly different (e.g. -libpath instead of -L).

We also remove oldnames and libcmt, since they're not needed at the
moment and they bring in more dependencies.

We also pass `/subsystem:console` to the linker so it can figure out the
right entry point. This is only needed for MSVC's `link.exe`. For LLD it
is redundant but doesn't hurt.

Differential Revision: https://reviews.llvm.org/D126291

Co-authored-by: Markus Mützel <markus.muetzel@gmx.de>

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -739,23 +739,39 @@ bool tools::addOpenMPRuntime(ArgStringList &CmdArgs, const ToolChain &TC,
   return true;
 }
 
-void tools::addFortranRuntimeLibs(llvm::opt::ArgStringList &CmdArgs) {
-  CmdArgs.push_back("-lFortran_main");
-  CmdArgs.push_back("-lFortranRuntime");
-  CmdArgs.push_back("-lFortranDecimal");
+void tools::addFortranRuntimeLibs(const ToolChain &TC,
+                                  llvm::opt::ArgStringList &CmdArgs) {
+  if (TC.getTriple().isKnownWindowsMSVCEnvironment()) {
+    CmdArgs.push_back("Fortran_main.lib");
+    CmdArgs.push_back("FortranRuntime.lib");
+    CmdArgs.push_back("FortranDecimal.lib");
+  } else {
+    CmdArgs.push_back("-lFortran_main");
+    CmdArgs.push_back("-lFortranRuntime");
+    CmdArgs.push_back("-lFortranDecimal");
+  }
 }
 
 void tools::addFortranRuntimeLibraryPath(const ToolChain &TC,
                                          const llvm::opt::ArgList &Args,
                                          ArgStringList &CmdArgs) {
+  // NOTE: Generating executables by Flang is considered an "experimental"
+  // feature and hence this is guarded with a command line option.
+  // TODO: Make this work unconditionally once Flang is mature enough.
+  if (!Args.hasArg(options::OPT_flang_experimental_exec))
+    return;
+
   // Default to the <driver-path>/../lib directory. This works fine on the
   // platforms that we have tested so far. We will probably have to re-fine
   // this in the future. In particular, on some platforms, we may need to use
   // lib64 instead of lib.
   SmallString<256> DefaultLibPath =
       llvm::sys::path::parent_path(TC.getDriver().Dir);
   llvm::sys::path::append(DefaultLibPath, "lib");
-  CmdArgs.push_back(Args.MakeArgString("-L" + DefaultLibPath));
+  if (TC.getTriple().isKnownWindowsMSVCEnvironment())
+    CmdArgs.push_back(Args.MakeArgString("-libpath:" + DefaultLibPath));
+  else
+    CmdArgs.push_back(Args.MakeArgString("-L" + DefaultLibPath));
 }
 
 static void addSanitizerRuntime(const ToolChain &TC, const ArgList &Args,

clang/lib/Driver/ToolChains/CommonArgs.h

@@ -121,7 +121,8 @@ bool addOpenMPRuntime(llvm::opt::ArgStringList &CmdArgs, const ToolChain &TC,
                       bool IsOffloadingHost = false, bool GompNeedsRT = false);
 
 /// Adds Fortran runtime libraries to \p CmdArgs.
-void addFortranRuntimeLibs(llvm::opt::ArgStringList &CmdArgs);
+void addFortranRuntimeLibs(const ToolChain &TC,
+                           llvm::opt::ArgStringList &CmdArgs);
 
 /// Adds the path for the Fortran runtime libraries to \p CmdArgs.
 void addFortranRuntimeLibraryPath(const ToolChain &TC,

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -637,14 +637,9 @@ void darwin::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   // Additional linker set-up and flags for Fortran. This is required in order
   // to generate executables.
-  //
-  // NOTE: Generating executables by Flang is considered an "experimental"
-  // feature and hence this is guarded with a command line option.
-  // TODO: Make this work unconditionally once Flang is mature enough.
-  if (getToolChain().getDriver().IsFlangMode() &&
-      Args.hasArg(options::OPT_flang_experimental_exec)) {
+  if (getToolChain().getDriver().IsFlangMode()) {
     addFortranRuntimeLibraryPath(getToolChain(), Args, CmdArgs);
-    addFortranRuntimeLibs(CmdArgs);
+    addFortranRuntimeLibs(getToolChain(), CmdArgs);
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs))

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -596,13 +596,9 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // to generate executables. As Fortran runtime depends on the C runtime,
   // these dependencies need to be listed before the C runtime below (i.e.
   // AddRuntTimeLibs).
-  //
-  // NOTE: Generating executables by Flang is considered an "experimental"
-  // feature and hence this is guarded with a command line option.
-  // TODO: Make this work unconditionally once Flang is mature enough.
-  if (D.IsFlangMode() && Args.hasArg(options::OPT_flang_experimental_exec)) {
+  if (D.IsFlangMode()) {
     addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-    addFortranRuntimeLibs(CmdArgs);
+    addFortranRuntimeLibs(ToolChain, CmdArgs);
     CmdArgs.push_back("-lm");
   }
 

clang/lib/Driver/ToolChains/MSVC.cpp

@@ -81,7 +81,7 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
         Args.MakeArgString(std::string("-out:") + Output.getFilename()));
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles) &&
-      !C.getDriver().IsCLMode()) {
+      !C.getDriver().IsCLMode() && !C.getDriver().IsFlangMode()) {
     CmdArgs.push_back("-defaultlib:libcmt");
     CmdArgs.push_back("-defaultlib:oldnames");
   }
@@ -130,6 +130,16 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
           Args.MakeArgString(std::string("-libpath:") + WindowsSdkLibPath));
   }
 
+  if (C.getDriver().IsFlangMode()) {
+    addFortranRuntimeLibraryPath(TC, Args, CmdArgs);
+    addFortranRuntimeLibs(TC, CmdArgs);
+
+    // Inform the MSVC linker that we're generating a console application, i.e.
+    // one with `main` as the "user-defined" entry point. The `main` function is
+    // defined in flang's runtime libraries.
+    CmdArgs.push_back("/subsystem:console");
+  }
+
   // Add the compiler-rt library directories to libpath if they exist to help
   // the linker find the various sanitizer, builtin, and profiling runtimes.
   for (const auto &LibPath : TC.getLibraryPaths()) {

clang/lib/Driver/ToolChains/MinGW.cpp

@@ -218,6 +218,11 @@ void tools::MinGW::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
 
+  if (C.getDriver().IsFlangMode()) {
+    addFortranRuntimeLibraryPath(TC, Args, CmdArgs);
+    addFortranRuntimeLibs(TC, CmdArgs);
+  }
+
   // TODO: Add profile stuff here
 
   if (TC.ShouldLinkCXXStdlib(Args)) {

flang/test/Driver/linker-flags.f90

@@ -2,18 +2,18 @@
 ! invocation. These libraries are added on top of other standard runtime
 ! libraries that the Clang driver will include.
 
-! NOTE: The additional linker flags tested here are currently only specified for
-! GNU and Darwin. The following line will make sure that this test is skipped on
-! Windows. If you are running this test on a yet another platform and it is
-! failing for you, please either update the following or (preferably) update the
-! linker invocation accordingly.
-! UNSUPPORTED: system-windows
+!-------------
+! RUN COMMANDS
+!-------------
+! RUN: %flang -### -flang-experimental-exec -target ppc64le-linux-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,GNU
+! RUN: %flang -### -flang-experimental-exec -target aarch64-apple-darwin %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,DARWIN
+! RUN: %flang -### -flang-experimental-exec -target x86_64-windows-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MINGW
 
-!------------
-! RUN COMMAND
-!------------
-! Use `--ld-path` so that the linker location (used in the LABEL below) is deterministic.
-! RUN: %flang -### -flang-experimental-exec --ld-path=/usr/bin/ld %S/Inputs/hello.f90 2>&1 | FileCheck %s
+! NOTE: Clang's driver library, clangDriver, usually adds 'libcmt' and
+!       'oldnames' on Windows, but they are not needed when compiling
+!       Fortran code and they might bring in additional dependencies.
+!       Make sure they're not added.
+! RUN: %flang -### -flang-experimental-exec -target aarch64-windows-msvc %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MSVC --implicit-check-not libcmt --implicit-check-not oldnames
 
 !----------------
 ! EXPECTED OUTPUT
@@ -23,9 +23,29 @@
 ! CHECK-SAME:  "-o" "[[object_file:.*\.o]]" {{.*}}Inputs/hello.f90
 
 ! Linker invocation to generate the executable
-! CHECK-LABEL:  "/usr/bin/ld"
-! CHECK-SAME: "[[object_file]]"
-! CHECK-SAME: -lFortran_main
-! CHECK-SAME: -lFortranRuntime
-! CHECK-SAME: -lFortranDecimal
-! CHECK-SAME: -lm
+! GNU-LABEL:  "{{.*}}ld" 
+! GNU-SAME: "[[object_file]]"
+! GNU-SAME: -lFortran_main
+! GNU-SAME: -lFortranRuntime
+! GNU-SAME: -lFortranDecimal
+! GNU-SAME: -lm
+
+! DARWIN-LABEL:  "{{.*}}ld" 
+! DARWIN-SAME: "[[object_file]]"
+! DARWIN-SAME: -lFortran_main
+! DARWIN-SAME: -lFortranRuntime
+! DARWIN-SAME: -lFortranDecimal
+
+! MINGW-LABEL:  "{{.*}}ld" 
+! MINGW-SAME: "[[object_file]]"
+! MINGW-SAME: -lFortran_main
+! MINGW-SAME: -lFortranRuntime
+! MINGW-SAME: -lFortranDecimal
+
+! NOTE: This check should also match if the default linker is lld-link.exe
+! MSVC-LABEL: link.exe
+! MSVC-SAME: Fortran_main.lib
+! MSVC-SAME: FortranRuntime.lib
+! MSVC-SAME: FortranDecimal.lib
+! MSVC-SAME: /subsystem:console
+! MSVC-SAME: "[[object_file]]"