[flang][driver] Add -fno-fortran-main (link time) option to remove Fortran_main from link line

clang/include/clang/Driver/Options.td

@@ -6354,6 +6354,12 @@ def J : JoinedOrSeparate<["-"], "J">,
   Group<gfortran_Group>,
   Alias<module_dir>;
 
+let Visibility = [FlangOption] in {
+def no_fortran_main : Flag<["-"], "fno-fortran-main">,
+  Visibility<[FlangOption]>, Group<f_Group>,
+  HelpText<"Do not include Fortran_main.a (provided by Flang) when linking">;
+} // let Visibility = [ FlangOption ]
+
 //===----------------------------------------------------------------------===//
 // FC1 Options
 //===----------------------------------------------------------------------===//

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1120,55 +1120,61 @@ void tools::addFortranRuntimeLibs(const ToolChain &TC, const ArgList &Args,
                                   llvm::opt::ArgStringList &CmdArgs) {
   // These are handled earlier on Windows by telling the frontend driver to add
   // the correct libraries to link against as dependents in the object file.
-  if (!TC.getTriple().isKnownWindowsMSVCEnvironment()) {
-    // The --whole-archive option needs to be part of the link line to
-    // make sure that the main() function from Fortran_main.a is pulled
-    // in by the linker.  Determine if --whole-archive is active when
-    // flang will try to link Fortran_main.a.  If it is, don't add the
-    // --whole-archive flag to the link line.  If it's not, add a proper
-    // --whole-archive/--no-whole-archive bracket to the link line.
-    bool WholeArchiveActive = false;
-    for (auto *Arg : Args.filtered(options::OPT_Wl_COMMA))
-      if (Arg)
-        for (StringRef ArgValue : Arg->getValues()) {
-          if (ArgValue == "--whole-archive")
-            WholeArchiveActive = true;
-          if (ArgValue == "--no-whole-archive")
-            WholeArchiveActive = false;
-        }
 
-    if (!WholeArchiveActive)
-      CmdArgs.push_back("--whole-archive");
-    CmdArgs.push_back("-lFortran_main");
-    if (!WholeArchiveActive)
-      CmdArgs.push_back("--no-whole-archive");
+  // if -fno-fortran-main has been passed, skip linking Fortran_main.a
+  bool LinkFortranMain = !Args.hasArg(options::OPT_no_fortran_main);
+  if (!TC.getTriple().isKnownWindowsMSVCEnvironment()) {
+    if (LinkFortranMain) {
+      // The --whole-archive option needs to be part of the link line to
+      // make sure that the main() function from Fortran_main.a is pulled
+      // in by the linker.  Determine if --whole-archive is active when
+      // flang will try to link Fortran_main.a.  If it is, don't add the
+      // --whole-archive flag to the link line.  If it's not, add a proper
+      // --whole-archive/--no-whole-archive bracket to the link line.
+      bool WholeArchiveActive = false;
+      for (auto *Arg : Args.filtered(options::OPT_Wl_COMMA))
+        if (Arg)
+          for (StringRef ArgValue : Arg->getValues()) {
+            if (ArgValue == "--whole-archive")
+              WholeArchiveActive = true;
+            if (ArgValue == "--no-whole-archive")
+              WholeArchiveActive = false;
+          }
 
+      if (!WholeArchiveActive)
+        CmdArgs.push_back("--whole-archive");
+      CmdArgs.push_back("-lFortran_main");
+      if (!WholeArchiveActive)
+        CmdArgs.push_back("--no-whole-archive");
+    }
     // Perform regular linkage of the remaining runtime libraries.
     CmdArgs.push_back("-lFortranRuntime");
     CmdArgs.push_back("-lFortranDecimal");
   } else {
-    unsigned RTOptionID = options::OPT__SLASH_MT;
-    if (auto *rtl = Args.getLastArg(options::OPT_fms_runtime_lib_EQ)) {
-      RTOptionID = llvm::StringSwitch<unsigned>(rtl->getValue())
-                       .Case("static", options::OPT__SLASH_MT)
-                       .Case("static_dbg", options::OPT__SLASH_MTd)
-                       .Case("dll", options::OPT__SLASH_MD)
-                       .Case("dll_dbg", options::OPT__SLASH_MDd)
-                       .Default(options::OPT__SLASH_MT);
-    }
-    switch (RTOptionID) {
-    case options::OPT__SLASH_MT:
-      CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.static.lib");
-      break;
-    case options::OPT__SLASH_MTd:
-      CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.static_dbg.lib");
-      break;
-    case options::OPT__SLASH_MD:
-      CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.dynamic.lib");
-      break;
-    case options::OPT__SLASH_MDd:
-      CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.dynamic_dbg.lib");
-      break;
+    if (LinkFortranMain) {
+      unsigned RTOptionID = options::OPT__SLASH_MT;
+      if (auto *rtl = Args.getLastArg(options::OPT_fms_runtime_lib_EQ)) {
+        RTOptionID = llvm::StringSwitch<unsigned>(rtl->getValue())
+                         .Case("static", options::OPT__SLASH_MT)
+                         .Case("static_dbg", options::OPT__SLASH_MTd)
+                         .Case("dll", options::OPT__SLASH_MD)
+                         .Case("dll_dbg", options::OPT__SLASH_MDd)
+                         .Default(options::OPT__SLASH_MT);
+      }
+      switch (RTOptionID) {
+      case options::OPT__SLASH_MT:
+        CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.static.lib");
+        break;
+      case options::OPT__SLASH_MTd:
+        CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.static_dbg.lib");
+        break;
+      case options::OPT__SLASH_MD:
+        CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.dynamic.lib");
+        break;
+      case options::OPT__SLASH_MDd:
+        CmdArgs.push_back("/WHOLEARCHIVE:Fortran_main.dynamic_dbg.lib");
+        break;
+      }
     }
   }
 }

clang/lib/Driver/ToolChains/Flang.cpp

@@ -231,6 +231,8 @@ static void processVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
                                     ArgStringList &CmdArgs) {
   assert(TC.getTriple().isKnownWindowsMSVCEnvironment() &&
          "can only add VS runtime library on Windows!");
+  // if -fno-fortran-main has been passed, skip linking Fortran_main.a
+  bool LinkFortranMain = !Args.hasArg(options::OPT_no_fortran_main);
   if (TC.getTriple().isKnownWindowsMSVCEnvironment()) {
     CmdArgs.push_back(Args.MakeArgString(
         "--dependent-lib=" + TC.getCompilerRTBasename(Args, "builtins")));
@@ -248,23 +250,26 @@ static void processVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
   case options::OPT__SLASH_MT:
     CmdArgs.push_back("-D_MT");
     CmdArgs.push_back("--dependent-lib=libcmt");
-    CmdArgs.push_back("--dependent-lib=Fortran_main.static.lib");
+    if (LinkFortranMain)
+      CmdArgs.push_back("--dependent-lib=Fortran_main.static.lib");
     CmdArgs.push_back("--dependent-lib=FortranRuntime.static.lib");
     CmdArgs.push_back("--dependent-lib=FortranDecimal.static.lib");
     break;
   case options::OPT__SLASH_MTd:
     CmdArgs.push_back("-D_MT");
     CmdArgs.push_back("-D_DEBUG");
     CmdArgs.push_back("--dependent-lib=libcmtd");
-    CmdArgs.push_back("--dependent-lib=Fortran_main.static_dbg.lib");
+    if (LinkFortranMain)
+      CmdArgs.push_back("--dependent-lib=Fortran_main.static_dbg.lib");
     CmdArgs.push_back("--dependent-lib=FortranRuntime.static_dbg.lib");
     CmdArgs.push_back("--dependent-lib=FortranDecimal.static_dbg.lib");
     break;
   case options::OPT__SLASH_MD:
     CmdArgs.push_back("-D_MT");
     CmdArgs.push_back("-D_DLL");
     CmdArgs.push_back("--dependent-lib=msvcrt");
-    CmdArgs.push_back("--dependent-lib=Fortran_main.dynamic.lib");
+    if (LinkFortranMain)
+      CmdArgs.push_back("--dependent-lib=Fortran_main.dynamic.lib");
     CmdArgs.push_back("--dependent-lib=FortranRuntime.dynamic.lib");
     CmdArgs.push_back("--dependent-lib=FortranDecimal.dynamic.lib");
     break;
@@ -273,7 +278,8 @@ static void processVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
     CmdArgs.push_back("-D_DEBUG");
     CmdArgs.push_back("-D_DLL");
     CmdArgs.push_back("--dependent-lib=msvcrtd");
-    CmdArgs.push_back("--dependent-lib=Fortran_main.dynamic_dbg.lib");
+    if (LinkFortranMain)
+      CmdArgs.push_back("--dependent-lib=Fortran_main.dynamic_dbg.lib");
     CmdArgs.push_back("--dependent-lib=FortranRuntime.dynamic_dbg.lib");
     CmdArgs.push_back("--dependent-lib=FortranDecimal.dynamic_dbg.lib");
     break;

flang/test/Driver/driver-help-hidden.f90

@@ -64,6 +64,7 @@
 ! CHECK-NEXT:                         Select Windows run-time library
 ! CHECK-NEXT: -fno-automatic          Implies the SAVE attribute for non-automatic local objects in subprograms unless RECURSIVE
 ! CHECK-NEXT: -fno-color-diagnostics  Disable colors in diagnostics
+! CHECK-NEXT: -fno-fortran-main       Do not include Fortran_main.a (provided by Flang) when linking
 ! CHECK-NEXT: -fno-integrated-as      Disable the integrated assembler
 ! CHECK-NEXT: -fno-lto                Disable LTO mode (default)
 ! CHECK-NEXT: -fno-ppc-native-vector-element-order

flang/test/Driver/driver-help.f90

@@ -54,6 +54,7 @@
 ! HELP-NEXT:                         Select Windows run-time library
 ! HELP-NEXT: -fno-automatic          Implies the SAVE attribute for non-automatic local objects in subprograms unless RECURSIVE
 ! HELP-NEXT: -fno-color-diagnostics  Disable colors in diagnostics
+! HELP-NEXT: -fno-fortran-main       Do not include Fortran_main.a (provided by Flang) when linking
 ! HELP-NEXT: -fno-integrated-as      Disable the integrated assembler
 ! HELP-NEXT: -fno-lto                Disable LTO mode (default)
 ! HELP-NEXT: -fno-ppc-native-vector-element-order

flang/test/Driver/no-duplicate-main.f90

@@ -4,6 +4,8 @@
 ! RUN: %flang -o %t -c %s
 ! RUN: not %flang -o %t.exe %t %t.c-object 2>&1
 
+! RUN: %flang -fno-fortran-main -o %t.exe %t %t.c-object 2>&1
+
 ! TODO: potentially add further checks to ensure that proper
 !       linker error messages are detected and checked via
 !       FileCheck.