[flang][driver] Remove Fortain_main static library from linking stages (#75816)

At present, when building static or shared libraries, Flang adds
`-lFortran_main.a` (or `/WHOLEARCHIVE:Fortran.*.lib` pon Windows) to the
link line. This leads to the problem that `_QQmain` and
`_QQEnvironmentDefaults` (as of the time of this PR) are symbols marked
as used, while `main` is being defined. This should not happen and this
PR fixes this by detecting if `-shared` or `-static` is used on the
Flang command line and removing the static `Fortran_main` library.

---------

Co-authored-by: kkwli <kkwli@users.noreply.github.com>

Author: mjklemm

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1133,6 +1133,16 @@ static bool isWholeArchivePresent(const ArgList &Args) {
   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) {
@@ -1164,6 +1174,16 @@ static void addFortranRuntimeLibsMSVC(const ArgList &Args,
 // 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);

flang/docs/FlangDriver.md

@@ -163,6 +163,63 @@ forward compiler options to the frontend driver, `flang-new -fc1`.
 You can read more on the design of `clangDriver` in Clang's [Driver Design &
 Internals](https://clang.llvm.org/docs/DriverInternals.html).
 
+## Linker Driver
+When used as a linker, Flang's frontend driver assembles the command line for an
+external linker command (e.g., LLVM's `lld`) and invokes it to create the final
+executable by linking static and shared libraries together with all the
+translation units supplied as object files.
+
+By default, the Flang linker driver adds several libraries to the linker
+invocation to make sure that all entrypoints for program start
+(Fortran's program unit) and runtime routines can be resolved by the linker.
+
+An abridged example (only showing the Fortran specific linker flags, omission
+indicated by `[...]`) for such a linker invocation on a Linux system would look
+like this:
+
+```
+$ flang -v -o example example.o
+"/usr/bin/ld" [...] example.o [...] "--whole-archive" "-lFortran_main"
+"--no-whole-archive" "-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.
+
 ## Frontend Driver
 Flang's frontend driver is the main interface between compiler developers and
 the Flang frontend. The high-level design is similar to Clang's frontend

flang/test/Driver/dynamic-linker.f90

@@ -3,17 +3,20 @@
 
 ! RUN: %flang -### --target=x86_64-linux-gnu -rpath /path/to/dir -shared \
 ! RUN:     -static %s 2>&1 | FileCheck \
-! RUN:     --check-prefixes=GNU-LINKER-OPTIONS %s
+! RUN:     --check-prefixes=GNU-LINKER-OPTIONS \
+! RUN:     --implicit-check-not=GNU-LINKER-OPTIONS-NOT %s
 ! RUN: %flang -### --target=x86_64-windows-msvc -rpath /path/to/dir -shared \
 ! RUN:     -static %s 2>&1 | FileCheck \
-! RUN:     --check-prefixes=MSVC-LINKER-OPTIONS %s
+! RUN:     --check-prefixes=MSVC-LINKER-OPTIONS \
+! RUN:     --implicit-check-not=MSVC-LINKER-OPTIONS-NOT %s
 ! RUN: %flang -### --target=aarch64-linux-none -rdynamic %s 2>&1 | FileCheck --check-prefixes=RDYNAMIC-LINKER-OPTION %s
 
 ! TODO: Could the linker have an extension or a suffix?
 ! GNU-LINKER-OPTIONS: "{{.*}}ld{{(.exe)?}}"
 ! GNU-LINKER-OPTIONS-SAME: "-shared"
 ! GNU-LINKER-OPTIONS-SAME: "-static"
 ! GNU-LINKER-OPTIONS-SAME: "-rpath" "/path/to/dir"
+! GNU-LINKER-OPTIONS-NOT: "-lFortran_main.a"
 
 ! RDYNAMIC-LINKER-OPTION: "{{.*}}ld"
 ! RDYNAMIC-LINKER-OPTION-SAME: "-export-dynamic"
@@ -22,3 +25,4 @@
 ! MSVC-LINKER-OPTIONS: "{{.*}}link{{(.exe)?}}"
 ! MSVC-LINKER-OPTIONS-SAME: "-dll"
 ! MSVC-LINKER-OPTIONS-SAME: "-rpath" "/path/to/dir"
+! MSVC-LINKER-OPTIONS-NOT: "/WHOLEARCHIVE:Fortran_main"