[flang] Generate main only when a Fortran program statement is present (#89938)

This patch changes the behaviour for flang to only create and link to a
`main` entry point when the Fortran code has a program statement in it.

This means that flang-new can be used to link even when the program is
a mixed C/Fortran code with `main` present in C and no entry point
present in Fortran.

This also removes the `-fno-fortran-main` flag as this no longer has any
functionality.

Author: DavidTruby

clang/include/clang/Driver/Options.td

@@ -6589,12 +6589,6 @@ 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

@@ -1191,118 +1191,10 @@ bool tools::addOpenMPRuntime(const Compilation &C, ArgStringList &CmdArgs,
   return true;
 }
 
-/// Determines if --whole-archive is active in the list of arguments.
-static bool isWholeArchivePresent(const ArgList &Args) {
-  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;
-      }
-    }
-  }
-
-  return WholeArchiveActive;
-}
-
-/// Determine if driver is invoked to create a shared object library (-static)
-static bool isSharedLinkage(const ArgList &Args) {
-  return Args.hasArg(options::OPT_shared);
-}
-
-/// Determine if driver is invoked to create a static object library (-shared)
-static bool isStaticLinkage(const ArgList &Args) {
-  return Args.hasArg(options::OPT_static);
-}
-
-/// Add Fortran runtime libs for MSVC
-static void addFortranRuntimeLibsMSVC(const ArgList &Args,
-                                      llvm::opt::ArgStringList &CmdArgs) {
-  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;
-  }
-}
-
-// Add FortranMain runtime lib
-static void addFortranMain(const ToolChain &TC, const ArgList &Args,
-                           llvm::opt::ArgStringList &CmdArgs) {
-  // 0. Shared-library linkage
-  // If we are attempting to link a library, we should not add
-  // -lFortran_main.a to the link line, as the `main` symbol is not
-  // required for a library and should also be provided by one of
-  // the translation units of the code that this shared library
-  // will be linked against eventually.
-  if (isSharedLinkage(Args) || isStaticLinkage(Args)) {
-    return;
-  }
-
-  // 1. MSVC
-  if (TC.getTriple().isKnownWindowsMSVCEnvironment()) {
-    addFortranRuntimeLibsMSVC(Args, CmdArgs);
-    return;
-  }
-
-  // 2. GNU and similar
-  const Driver &D = TC.getDriver();
-  const char *FortranMainLinkFlag = "-lFortran_main";
-
-  // Warn if the user added `-lFortran_main` - this library is an implementation
-  // detail of Flang and should be handled automaticaly by the driver.
-  for (const char *arg : CmdArgs) {
-    if (strncmp(arg, FortranMainLinkFlag, strlen(FortranMainLinkFlag)) == 0)
-      D.Diag(diag::warn_drv_deprecated_custom)
-          << FortranMainLinkFlag
-          << "see the Flang driver documentation for correct usage";
-  }
-
-  // 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. However, it shouldn't be used if it's already active.
-  // TODO: Find an equivalent of `--whole-archive` for Darwin and AIX.
-  if (!isWholeArchivePresent(Args) && !TC.getTriple().isMacOSX() &&
-      !TC.getTriple().isOSAIX()) {
-    CmdArgs.push_back("--whole-archive");
-    CmdArgs.push_back(FortranMainLinkFlag);
-    CmdArgs.push_back("--no-whole-archive");
-    return;
-  }
-
-  CmdArgs.push_back(FortranMainLinkFlag);
-}
-
 /// Add Fortran runtime libs
 void tools::addFortranRuntimeLibs(const ToolChain &TC, const ArgList &Args,
                                   llvm::opt::ArgStringList &CmdArgs) {
-  // 1. Link FortranMain
-  // FortranMain depends on FortranRuntime, so needs to be listed first. If
-  // -fno-fortran-main has been passed, skip linking Fortran_main.a
-  if (!Args.hasArg(options::OPT_no_fortran_main))
-    addFortranMain(TC, Args, CmdArgs);
-
-  // 2. Link FortranRuntime and FortranDecimal
+  // Link FortranRuntime and FortranDecimal
   // 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.

clang/lib/Driver/ToolChains/Flang.cpp

@@ -282,7 +282,6 @@ static void processVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
   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")));
@@ -300,26 +299,20 @@ 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");
-    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");
-    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");
-    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;
@@ -328,8 +321,6 @@ 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");
-    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/docs/FlangDriver.md

@@ -179,46 +179,20 @@ like this:
 
 ```
 $ flang -v -o example example.o
-"/usr/bin/ld" [...] example.o [...] "--whole-archive" "-lFortran_main"
-"--no-whole-archive" "-lFortranRuntime" "-lFortranDecimal" [...]
+"/usr/bin/ld" [...] example.o [...] "-lFortranRuntime" "-lFortranDecimal" [...]
 ```
 
 The automatically added libraries are:
 
-* `Fortran_main`: Provides the main entry point `main` that then invokes
-  `_QQmain` with the Fortran program unit.  This library has a dependency to
-  the `FortranRuntime` library.
 * `FortranRuntime`: Provides most of the Flang runtime library.
 * `FortranDecimal`: Provides operations for decimal numbers.
 
-The default is that, when using Flang as the linker, one of the Fortran
-translation units provides the program unit and therefore it is assumed that
-Fortran is the main code part (calling into C/C++ routines via `BIND (C)`
-interfaces).  When composing the linker commandline, Flang uses
-`--whole-archive` and `--no-whole-archive` (Windows: `/WHOLEARCHIVE:`,
-Darwin & AIX: *not implemented yet*) to make sure that all for `Fortran_main`
-is processed by the linker.  This is done to issue a proper error message when
-multiple definitions of `main` occur.  This happens, for instance, when linking
-a code that has a Fortran program unit with a C/C++ code that also defines a
-`main` function.  A user may be required to explicitly provide the C++ runtime
-libraries at link time (e.g., via `-lstdc++` for STL)
-
 If the code is C/C++ based and invokes Fortran routines, one can either use Clang
 or Flang as the linker driver.  If Clang is used, it will automatically all
 required runtime libraries needed by C++ (e.g., for STL) to the linker invocation.
 In this case, one has to explicitly provide the Fortran runtime libraries
-`FortranRuntime` and/or `FortranDecimal`.  An alternative is to use Flang to link
-and use the `-fno-fortran-main` flag.  This flag removes
-`Fortran_main` from the linker stage and hence requires one of the C/C++
-translation units to provide a definition of the `main` function. In this case,
-it may be required to explicitly supply C++ runtime libraries as mentioned above.
-
-When creating shared or static libraries using Flang with `-shared` or `-static`
-flag, Fortran_main is automatically removed from the linker stage (i.e.,
-`-fno-fortran-main` is on by default).  It is assumed that when creating a
-static or shared library, the generated library does not need a `main`
-function, as a final link stage will occur that will provide the `Fortran_main`
-library when creating the final executable.
+`FortranRuntime` and/or `FortranDecimal`.  An alternative is to use Flang to link.
+In this case, it may be required to explicitly supply C++ runtime libraries.
 
 On Darwin, the logical root where the system libraries are located (sysroot)
 must be specified. This can be done with the CMake build flag `DEFAULT_SYSROOT`

flang/include/flang/Optimizer/Builder/Runtime/EnvironmentDefaults.h

@@ -22,6 +22,7 @@
 
 namespace fir {
 class FirOpBuilder;
+class GlobalOp;
 } // namespace fir
 
 namespace mlir {
@@ -37,7 +38,7 @@ namespace fir::runtime {
 /// Create the list of environment variable defaults for the runtime to set. The
 /// form of the generated list is defined in the runtime header file
 /// environment-default-list.h
-void genEnvironmentDefaults(
+fir::GlobalOp genEnvironmentDefaults(
     fir::FirOpBuilder &builder, mlir::Location loc,
     const std::vector<Fortran::lower::EnvironmentDefault> &envDefaults);
 

flang/include/flang/Optimizer/Builder/Runtime/Main.h

@@ -0,0 +1,28 @@
+//===-- Main.h - generate main runtime API calls ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_OPTIMIZER_BUILDER_RUNTIME_MAIN_H
+#define FORTRAN_OPTIMIZER_BUILDER_RUNTIME_MAIN_H
+
+namespace mlir {
+class Location;
+} // namespace mlir
+
+namespace fir {
+class FirOpBuilder;
+class GlobalOp;
+} // namespace fir
+
+namespace fir::runtime {
+
+void genMain(fir::FirOpBuilder &builder, mlir::Location loc,
+             fir::GlobalOp &env);
+
+}
+
+#endif // FORTRAN_OPTIMIZER_BUILDER_RUNTIME_MAIN_H

flang/lib/Lower/Bridge.cpp

@@ -36,6 +36,7 @@
 #include "flang/Optimizer/Builder/Runtime/Character.h"
 #include "flang/Optimizer/Builder/Runtime/Derived.h"
 #include "flang/Optimizer/Builder/Runtime/EnvironmentDefaults.h"
+#include "flang/Optimizer/Builder/Runtime/Main.h"
 #include "flang/Optimizer/Builder/Runtime/Ragged.h"
 #include "flang/Optimizer/Builder/Runtime/Stop.h"
 #include "flang/Optimizer/Builder/Todo.h"
@@ -359,8 +360,10 @@ class FirConverter : public Fortran::lower::AbstractConverter {
         // not need to be generated even if no defaults are specified.
         // However, generating main or changing when the runtime reads
         // environment variables is required to do so.
-        fir::runtime::genEnvironmentDefaults(*builder, toLocation(),
-                                             bridge.getEnvironmentDefaults());
+        auto env = fir::runtime::genEnvironmentDefaults(
+            *builder, toLocation(), bridge.getEnvironmentDefaults());
+
+        fir::runtime::genMain(*builder, toLocation(), env);
       });
 
     finalizeOpenACCLowering();

flang/lib/Optimizer/Builder/CMakeLists.txt

@@ -23,6 +23,7 @@ add_flang_library(FIRBuilder
   Runtime/Execute.cpp
   Runtime/Inquiry.cpp
   Runtime/Intrinsics.cpp
+  Runtime/Main.cpp
   Runtime/Numeric.cpp
   Runtime/Pointer.cpp
   Runtime/Ragged.cpp

flang/lib/Optimizer/Builder/Runtime/EnvironmentDefaults.cpp

@@ -13,7 +13,7 @@
 #include "flang/Optimizer/Support/InternalNames.h"
 #include "llvm/ADT/ArrayRef.h"
 
-void fir::runtime::genEnvironmentDefaults(
+fir::GlobalOp fir::runtime::genEnvironmentDefaults(
     fir::FirOpBuilder &builder, mlir::Location loc,
     const std::vector<Fortran::lower::EnvironmentDefault> &envDefaults) {
   std::string envDefaultListPtrName =
@@ -34,14 +34,13 @@ void fir::runtime::genEnvironmentDefaults(
 
   // If no defaults were specified, initialize with a null pointer.
   if (envDefaults.empty()) {
-    builder.createGlobalConstant(
+    return builder.createGlobalConstant(
         loc, envDefaultListRefTy, envDefaultListPtrName,
         [&](fir::FirOpBuilder &builder) {
           mlir::Value nullVal =
               builder.createNullConstant(loc, envDefaultListRefTy);
           builder.create<fir::HasValueOp>(loc, nullVal);
         });
-    return;
   }
 
   // Create the Item list.
@@ -99,7 +98,7 @@ void fir::runtime::genEnvironmentDefaults(
       envDefaultListBuilder, linkOnce);
 
   // Define the pointer to the list used by the runtime.
-  builder.createGlobalConstant(
+  return builder.createGlobalConstant(
       loc, envDefaultListRefTy, envDefaultListPtrName,
       [&](fir::FirOpBuilder &builder) {
         mlir::Value addr = builder.create<fir::AddrOfOp>(

flang/lib/Optimizer/Builder/Runtime/Main.cpp

@@ -0,0 +1,62 @@
+//===-- Main.cpp - generate main runtime API calls --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Optimizer/Builder/Runtime/Main.h"
+#include "flang/Optimizer/Builder/BoxValue.h"
+#include "flang/Optimizer/Builder/FIRBuilder.h"
+#include "flang/Optimizer/Builder/Runtime/RTBuilder.h"
+#include "flang/Optimizer/Dialect/FIROps.h"
+#include "flang/Optimizer/Dialect/FIRType.h"
+#include "flang/Runtime/main.h"
+#include "flang/Runtime/stop.h"
+
+using namespace Fortran::runtime;
+
+/// Create a `int main(...)` that calls the Fortran entry point
+void fir::runtime::genMain(fir::FirOpBuilder &builder, mlir::Location loc,
+                           fir::GlobalOp &env) {
+  auto *context = builder.getContext();
+  auto argcTy = builder.getDefaultIntegerType();
+  auto ptrTy = mlir::LLVM::LLVMPointerType::get(context);
+
+  // void ProgramStart(int argc, char** argv, char** envp,
+  //                   _QQEnvironmentDefaults* env)
+  auto startFn = builder.createFunction(
+      loc, RTNAME_STRING(ProgramStart),
+      mlir::FunctionType::get(context, {argcTy, ptrTy, ptrTy, ptrTy}, {}));
+  // void ProgramStop()
+  auto stopFn =
+      builder.createFunction(loc, RTNAME_STRING(ProgramEndStatement),
+                             mlir::FunctionType::get(context, {}, {}));
+
+  // int main(int argc, char** argv, char** envp)
+  auto mainFn = builder.createFunction(
+      loc, "main",
+      mlir::FunctionType::get(context, {argcTy, ptrTy, ptrTy}, argcTy));
+  // void _QQmain()
+  auto qqMainFn = builder.createFunction(
+      loc, "_QQmain", mlir::FunctionType::get(context, {}, {}));
+
+  mainFn.setPublic();
+
+  auto *block = mainFn.addEntryBlock();
+  mlir::OpBuilder::InsertionGuard insertGuard(builder);
+  builder.setInsertionPointToStart(block);
+
+  llvm::SmallVector<mlir::Value, 4> args(block->getArguments());
+  auto envAddr =
+      builder.create<fir::AddrOfOp>(loc, env.getType(), env.getSymbol());
+  args.push_back(envAddr);
+
+  builder.create<fir::CallOp>(loc, startFn, args);
+  builder.create<fir::CallOp>(loc, qqMainFn);
+  builder.create<fir::CallOp>(loc, stopFn);
+
+  mlir::Value ret = builder.createIntegerConstant(loc, argcTy, 0);
+  builder.create<mlir::func::ReturnOp>(loc, ret);
+}