[clang] [flang][driver] Add -fno-fortran-main (link time) option to remove Fortran_main from link line (PR #74139)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Michael Klemm (mjklemm)

<details>
<summary>Changes</summary>

This is related to PR #<!-- -->74120 and (merged) PR #<!-- -->73124.

This PR adds the `-fno-fortran-main` command line option to remove `Fortran_main.a` from the link and to allow for linking Fortran code w/o program unit with C/C++ translation units that provide the `main()` entrypoint.

---
Full diff: https://github.com/llvm/llvm-project/pull/74139.diff


2 Files Affected:

- (modified) clang/include/clang/Driver/Options.td (+4) 
- (modified) clang/lib/Driver/ToolChains/CommonArgs.cpp (+25-21) 


``````````diff
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 19d04e82aed4d68..aa26344f67b3132 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -6345,6 +6345,10 @@ def J : JoinedOrSeparate<["-"], "J">,
   Group<gfortran_Group>,
   Alias<module_dir>;
 
+def no_fortran_main : Flag<["-"], "fno-fortran-main">,
+  Visibility<[FlangOption]>, Group<f_Group>,
+  HelpText<"Don't link in Fortran main">;
+
 //===----------------------------------------------------------------------===//
 // FC1 Options
 //===----------------------------------------------------------------------===//
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 0ae8e2dce32e94a..2899f07cb7490ca 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -1116,33 +1116,37 @@ bool tools::addOpenMPRuntime(ArgStringList &CmdArgs, const ToolChain &TC,
   return true;
 }
 
+// TODO: add -fno-fortran-main option and check it in this function.
 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 DontLinkFortranMain = Args.getLastArg(options::OPT_no_fortran_main) != nullptr;
+    if (!DontLinkFortranMain) {
+      // 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");

``````````

</details>


https://github.com/llvm/llvm-project/pull/74139