[flang][Driver] Let the linker fail on multiple definitions of main() (#73124)

The flang driver was silently ignoring the `main()` function in
`Fortran_main.a` for entry into the Fortran program unit if an external
`main()` as supplied (e.g., via cross-language linkage with Fortran and
C/C++). This PR fixes this by making sure that the linker always pulls
in the `main()` definition from `Fortran_main.a` and consequently fails
due to multiple definitions of the same symbol if another object file
also has a definition of `main()`.

15 files changed, 81 insertions, 14 deletions

diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 1f31c6395206..efb8b71f24a9 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -977,14 +977,60 @@ bool tools::addOpenMPRuntime(ArgStringList &CmdArgs, const ToolChain &TC,
   return true;
 }
 
-void tools::addFortranRuntimeLibs(const ToolChain &TC,
+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");
+
+    // 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;
+    }
   }
 }
 
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.h b/clang/lib/Driver/ToolChains/CommonArgs.h
index f364c9793c9b..0a0951c5386e 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.h
+++ b/clang/lib/Driver/ToolChains/CommonArgs.h
@@ -116,7 +116,7 @@ bool addOpenMPRuntime(llvm::opt::ArgStringList &CmdArgs, const ToolChain &TC,
                       bool IsOffloadingHost = false, bool GompNeedsRT = false);
 
 /// Adds Fortran runtime libraries to \p CmdArgs.
-void addFortranRuntimeLibs(const ToolChain &TC,
+void addFortranRuntimeLibs(const ToolChain &TC, const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs);
 
 /// Adds the path for the Fortran runtime libraries to \p CmdArgs.
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index 1f61bb02c6ae..7c3d4982d8f6 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -678,7 +678,7 @@ void darwin::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // to generate executables.
   if (getToolChain().getDriver().IsFlangMode()) {
     addFortranRuntimeLibraryPath(getToolChain(), Args, CmdArgs);
-    addFortranRuntimeLibs(getToolChain(), CmdArgs);
+    addFortranRuntimeLibs(getToolChain(), Args, CmdArgs);
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs))
diff --git a/clang/lib/Driver/ToolChains/DragonFly.cpp b/clang/lib/Driver/ToolChains/DragonFly.cpp
index 181d0ad0d834..9942fc632e0a 100644
--- a/clang/lib/Driver/ToolChains/DragonFly.cpp
+++ b/clang/lib/Driver/ToolChains/DragonFly.cpp
@@ -153,7 +153,7 @@ void dragonfly::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // AddRunTimeLibs).
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-      addFortranRuntimeLibs(ToolChain, CmdArgs);
+      addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
       CmdArgs.push_back("-lm");
     }
 
diff --git a/clang/lib/Driver/ToolChains/FreeBSD.cpp b/clang/lib/Driver/ToolChains/FreeBSD.cpp
index d08764aeef77..d46feb3459a6 100644
--- a/clang/lib/Driver/ToolChains/FreeBSD.cpp
+++ b/clang/lib/Driver/ToolChains/FreeBSD.cpp
@@ -310,7 +310,7 @@ void freebsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // AddRunTimeLibs).
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-      addFortranRuntimeLibs(ToolChain, CmdArgs);
+      addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
       if (Profiling)
         CmdArgs.push_back("-lm_p");
       else
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index 16cf57072272..b875991844ff 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -564,7 +564,7 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // AddRunTimeLibs).
   if (D.IsFlangMode()) {
     addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-    addFortranRuntimeLibs(ToolChain, CmdArgs);
+    addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
     CmdArgs.push_back("-lm");
   }
 
diff --git a/clang/lib/Driver/ToolChains/Haiku.cpp b/clang/lib/Driver/ToolChains/Haiku.cpp
index 3fe92054ac53..e0d94035823f 100644
--- a/clang/lib/Driver/ToolChains/Haiku.cpp
+++ b/clang/lib/Driver/ToolChains/Haiku.cpp
@@ -118,7 +118,7 @@ void haiku::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // AddRunTimeLibs).
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-      addFortranRuntimeLibs(ToolChain, CmdArgs);
+      addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
     }
 
     CmdArgs.push_back("-lgcc");
diff --git a/clang/lib/Driver/ToolChains/MSVC.cpp b/clang/lib/Driver/ToolChains/MSVC.cpp
index 4966d102c51f..8a4a174c90ea 100644
--- a/clang/lib/Driver/ToolChains/MSVC.cpp
+++ b/clang/lib/Driver/ToolChains/MSVC.cpp
@@ -131,7 +131,7 @@ void visualstudio::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (C.getDriver().IsFlangMode()) {
     addFortranRuntimeLibraryPath(TC, Args, CmdArgs);
-    addFortranRuntimeLibs(TC, CmdArgs);
+    addFortranRuntimeLibs(TC, Args, 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
diff --git a/clang/lib/Driver/ToolChains/MinGW.cpp b/clang/lib/Driver/ToolChains/MinGW.cpp
index 39d767795445..5d7f8675daf8 100644
--- a/clang/lib/Driver/ToolChains/MinGW.cpp
+++ b/clang/lib/Driver/ToolChains/MinGW.cpp
@@ -249,7 +249,7 @@ void tools::MinGW::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (C.getDriver().IsFlangMode()) {
     addFortranRuntimeLibraryPath(TC, Args, CmdArgs);
-    addFortranRuntimeLibs(TC, CmdArgs);
+    addFortranRuntimeLibs(TC, Args, CmdArgs);
   }
 
   // TODO: Add profile stuff here
diff --git a/clang/lib/Driver/ToolChains/NetBSD.cpp b/clang/lib/Driver/ToolChains/NetBSD.cpp
index 90b195a007ca..240bf5764b9c 100644
--- a/clang/lib/Driver/ToolChains/NetBSD.cpp
+++ b/clang/lib/Driver/ToolChains/NetBSD.cpp
@@ -325,7 +325,7 @@ void netbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // AddRunTimeLibs).
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-      addFortranRuntimeLibs(ToolChain, CmdArgs);
+      addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
       CmdArgs.push_back("-lm");
     }
 
diff --git a/clang/lib/Driver/ToolChains/OpenBSD.cpp b/clang/lib/Driver/ToolChains/OpenBSD.cpp
index 133b5ff13a4c..21b9004ef2d5 100644
--- a/clang/lib/Driver/ToolChains/OpenBSD.cpp
+++ b/clang/lib/Driver/ToolChains/OpenBSD.cpp
@@ -237,7 +237,7 @@ void openbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // AddRunTimeLibs).
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
-      addFortranRuntimeLibs(ToolChain, CmdArgs);
+      addFortranRuntimeLibs(ToolChain, Args, CmdArgs);
       if (Profiling)
         CmdArgs.push_back("-lm_p");
       else
diff --git a/clang/lib/Driver/ToolChains/Solaris.cpp b/clang/lib/Driver/ToolChains/Solaris.cpp
index 32b6cbe647da..485730da7df1 100644
--- a/clang/lib/Driver/ToolChains/Solaris.cpp
+++ b/clang/lib/Driver/ToolChains/Solaris.cpp
@@ -226,7 +226,7 @@ void solaris::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     // these dependencies need to be listed before the C runtime below.
     if (D.IsFlangMode()) {
       addFortranRuntimeLibraryPath(getToolChain(), Args, CmdArgs);
-      addFortranRuntimeLibs(getToolChain(), CmdArgs);
+      addFortranRuntimeLibs(getToolChain(), Args, CmdArgs);
       CmdArgs.push_back("-lm");
     }
     if (Args.hasArg(options::OPT_fstack_protector) ||
diff --git a/flang/test/Driver/Inputs/no_duplicate_main.ll b/flang/test/Driver/Inputs/no_duplicate_main.ll
new file mode 100644
index 000000000000..ff2b19dfe9e2
--- /dev/null
+++ b/flang/test/Driver/Inputs/no_duplicate_main.ll
@@ -0,0 +1,7 @@
+; Create the symbol 'main'; does not have to be the correct
+; signature for 'main', we just need the symbol for the linker
+; to fail during the test.
+
+define i32 @main() {
+  ret i32 0
+}
diff --git a/flang/test/Driver/linker-flags.f90 b/flang/test/Driver/linker-flags.f90
index 85c4d60b3f09..ea91946316cf 100644
--- a/flang/test/Driver/linker-flags.f90
+++ b/flang/test/Driver/linker-flags.f90
@@ -28,7 +28,7 @@
 !       executable and may find the GNU linker from MinGW or Cygwin.
 ! UNIX-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! UNIX-SAME: "[[object_file]]"
-! UNIX-SAME: "-lFortran_main" "-lFortranRuntime" "-lFortranDecimal" "-lm"
+! UNIX-SAME: "--whole-archive" "-lFortran_main" "--no-whole-archive" "-lFortranRuntime" "-lFortranDecimal" "-lm"
 
 ! DARWIN-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! DARWIN-SAME: "[[object_file]]"
@@ -38,7 +38,7 @@
 
 ! HAIKU-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! HAIKU-SAME: "[[object_file]]"
-! HAIKU-SAME: "-lFortran_main" "-lFortranRuntime" "-lFortranDecimal"
+! HAIKU-SAME: "--whole-archive" "-lFortran_main" "--no-whole-archive" "-lFortranRuntime" "-lFortranDecimal"
 
 ! MINGW-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! MINGW-SAME: "[[object_file]]"
diff --git a/flang/test/Driver/no_duplicate_main.f90 b/flang/test/Driver/no_duplicate_main.f90
new file mode 100644
index 000000000000..4e33f4f2aeba
--- /dev/null
+++ b/flang/test/Driver/no_duplicate_main.f90
@@ -0,0 +1,14 @@
+! UNSUPPORTED: system-windows
+
+! RUN: %flang -x ir -o %t.c-object -c %S/Inputs/no_duplicate_main.ll
+! RUN: %flang -o %t -c %s
+! RUN: not %flang -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.
+
+program main_dupes
+    ! Irrelevant what to do in here.
+    ! Test is supposed to fail at link time.
+end program main_dupes