[flang-commits] [flang] [clang] [flang][driver] Remove Fortain_main static library from linking stages (PR #75816)

[Michael Klemm via flang-commits]

================
@@ -163,6 +163,40 @@ 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.
+The 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: *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.
+
+If the code is C/C++ based and invokes Fortran routines, either use Clang as the
+linker driver (supplying `FortranRuntime` and/or `FortranDecimal` to the linker
----------------
mjklemm wrote:

> I think this requirement of using Clang as the linker driver if the `main` is C changes the usability for users coming from XLF or gfortran.

I can reword this.  There are two options: either do C/C++ linkage and supply Fortran libraries or do Fortran linkage and supply C/C++ libraries.  Either way is equally difficult at this point.

> I think the fundamental question is why Fortran_main.c is inserting a "fake" main on the Fortran side when there is not user defined main in Fortrran code.

This was inherited from Classic Flang and is also done by other compilers (e.g., Intel).

> Is it possible to only creating a main when users define one in Fortran code?

That would also be possible.  I guess the compiler could generate all of `main` in IR and emit it.  That's what GFortran does.


https://github.com/llvm/llvm-project/pull/75816