================ @@ -163,6 +163,62 @@ 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 +for 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, `Fortran_main` is automatically ---------------- kkwli wrote:
How about this? 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). https://github.com/llvm/llvm-project/pull/75816 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
