[flang-commits] [flang] 876df74 - [flang][driver] Allow main program to be in an archive

Shao-Ce SUN via flang-commits flang-commits at lists.llvm.org
Thu Apr 27 18:53:34 PDT 2023


Author: Shao-Ce SUN
Date: 2023-04-28T09:53:25+08:00
New Revision: 876df74dd47196a9ca3b4fff21ffb5441491a0a0

URL: https://github.com/llvm/llvm-project/commit/876df74dd47196a9ca3b4fff21ffb5441491a0a0
DIFF: https://github.com/llvm/llvm-project/commit/876df74dd47196a9ca3b4fff21ffb5441491a0a0.diff

LOG: [flang][driver] Allow main program to be in an archive

Add --undefined=_QQmain to the link line, so that a Fortran main program
will be included in the link job even if it is in an archive (unless we
are building a shared object). For now, this is only applied to the Gnu
toolchain.

We also add a section on the linker invocation to docs/FlangDriver.md.

The new tests require llvm-ar to construct an archive we can include in
the link job. This is a new dependency for flang/test (which already
depends on similar tools such as llvm-objdump).

See discussions in
https://github.com/llvm/llvm-project/issues/54787
which this patch fixes.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D134821

Added: 
    flang/test/Driver/link-c-main.c
    flang/test/Driver/link-f90-main.f90

Modified: 
    clang/lib/Driver/ToolChains/Gnu.cpp
    flang/docs/FlangDriver.md
    flang/test/CMakeLists.txt
    flang/test/Driver/linker-flags.f90

Removed: 
    


################################################################################
diff  --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
index b0716322bc141..c4a276126b653 100644
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -598,6 +598,14 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   // these dependencies need to be listed before the C runtime below (i.e.
   // AddRuntTimeLibs).
   if (D.IsFlangMode()) {
+    // A Fortran main program will be lowered to a function named _QQmain. Make
+    // _QQmain an undefined symbol, so that it's correctly resolved even when
+    // creating executable from archives. This is a workaround for how and where
+    // Flang's `main` is defined. For more context, see:
+    //   *  https://github.com/llvm/llvm-project/issues/54787
+    if (!Args.hasArg(options::OPT_shared))
+      CmdArgs.push_back("--undefined=_QQmain");
+
     addFortranRuntimeLibraryPath(ToolChain, Args, CmdArgs);
     addFortranRuntimeLibs(ToolChain, CmdArgs);
     CmdArgs.push_back("-lm");

diff  --git a/flang/docs/FlangDriver.md b/flang/docs/FlangDriver.md
index 6c2a473820634..9522e223b17b5 100644
--- a/flang/docs/FlangDriver.md
+++ b/flang/docs/FlangDriver.md
@@ -149,13 +149,6 @@ flang-new -ccc-print-phases -c file.f
 +- 3: backend, {2}, assembler
 4: assembler, {3}, object
 ```
-Note that currently Flang does not support code-generation and `flang-new` will
-fail during the second step above with the following error:
-```bash
-error: code-generation is not available yet
-```
-The other phases are printed nonetheless when using `-ccc-print-phases`, as
-that reflects what `clangDriver`, the library, will try to create and run.
 
 For actions specific to the frontend (e.g. preprocessing or code generation), a
 command to call the frontend driver is generated (more specifically, an
@@ -205,6 +198,41 @@ is `ParseSyntaxOnlyAction`, which corresponds to `-fsyntax-only`. In other
 words, `flang-new -fc1 <input-file>` is equivalent to `flang-new -fc1 -fsyntax-only
 <input-file>`.
 
+## Linker invocation
+> **_NOTE:_**  Linker invocation through the `flang-new` driver is so far
+> experimental. This section describes the currently intended design, and not
+> necessarily what is implemented.
+
+Calling
+```bash
+flang-new -flang-experimental-exec prog.f90
+```
+will, on a high level, do two things:
+* call the frontend driver, `flang-new -fc1`, to build the object file `prog.o`
+* call the system linker to build the executable `a.out`.
+
+In both invocations, `flang-new` will add default options to the frontend driver
+and the linker invocations. To see the exact invocations on your system, you can
+call
+```bash
+flang-new -### prog.f90
+```
+The link line will contain a fair number of options, which will depend on your
+system. Compared to when linking a C program with `clang`, the main additions
+are (on GNU/linux),
+* `--undefined=_QQmain`: A Fortran main program will appear in the corresponding
+  object file as a function called `_QQmain`. This flag makes sure that an
+  object file containing a Fortran main program (i.e., a symbol `_QQmain`) be
+  included in the linking also when it is bundled in an archive.
+* `-lFortran_main`: The Fortran_main archive is part of Flang's runtime. It
+  exports the symbol `main`, i.e., a c main function, which will make some
+  initial configuration before calling `_QQmain`, and clean up before returning.
+* `-lFortranRuntime`: Flang's Fortran runtime, containing, for example,
+  implementations of intrinsic functions.
+* `-lFortranDecimal`: Part of Flang's runtime, containing routines for parsing
+  and formatting decimal numbers.
+* `-lm`: Link with the math library, on which Flang's runtime depends.
+
 ## The `flang-to-external-fc` script
 The `flang-to-external-fc` wrapper script for `flang-new` was introduced as a
 development tool and to facilitate testing. The `flang-to-external-fc` wrapper

diff  --git a/flang/test/CMakeLists.txt b/flang/test/CMakeLists.txt
index 7d96a72e5f36d..dbb02574ec183 100644
--- a/flang/test/CMakeLists.txt
+++ b/flang/test/CMakeLists.txt
@@ -57,6 +57,7 @@ set(FLANG_TEST_DEPENDS
   fir-opt
   tco
   bbc
+  llvm-ar
   llvm-dis
   llvm-objdump
   llvm-readobj

diff  --git a/flang/test/Driver/link-c-main.c b/flang/test/Driver/link-c-main.c
new file mode 100644
index 0000000000000..e6814259a33d2
--- /dev/null
+++ b/flang/test/Driver/link-c-main.c
@@ -0,0 +1,28 @@
+/*
+Test that an object file with a C main function can be linked to an executable
+by Flang.
+
+For now, this test only covers the Gnu toolchain on Linux.
+
+REQUIRES: x86-registered-target || aarch64-registered-target || riscv64-registered-target
+REQUIRES: system-linux, c-compiler
+
+RUN: %cc -c %s -o %t.o
+RUN: %flang -target x86_64-unknown-linux-gnu %t.o -o %t.out -flang-experimental-exec
+RUN: llvm-objdump --syms %t.out | FileCheck %s --implicit-check-not Fortran
+
+Test that it also works if the c-main is bundled in an archive.
+
+RUN: llvm-ar -r %t.a %t.o
+RUN: %flang -target x86_64-unknown-linux-gnu %t.a -o %ta.out -flang-experimental-exec
+RUN: llvm-objdump --syms %ta.out | FileCheck %s --implicit-check-not Fortran
+*/
+
+int main(void) {
+    return 0;
+}
+
+/*
+CHECK-DAG: F .text {{[a-f0-9]+}} main
+CHECK-DAG: *UND* {{[a-f0-9]+}} _QQmain
+*/

diff  --git a/flang/test/Driver/link-f90-main.f90 b/flang/test/Driver/link-f90-main.f90
new file mode 100644
index 0000000000000..01f77f40d0300
--- /dev/null
+++ b/flang/test/Driver/link-f90-main.f90
@@ -0,0 +1,23 @@
+! Test that a fortran main program can be linked to an executable
+! by flang.
+!
+! For now, this test only covers the Gnu toolchain on linux.
+
+!REQUIRES: x86-registered-target || aarch64-registered-target || riscv64-registered-target
+!REQUIRES: system-linux
+
+! RUN: %flang_fc1 -emit-obj %s -o %t.o
+! RUN: %flang -target x86_64-unknown-linux-gnu %t.o -o %t.out -flang-experimental-exec
+! RUN: llvm-objdump --syms %t.out | FileCheck %s
+
+! Test that it also works if the program is bundled in an archive.
+
+! RUN: llvm-ar -r %t.a %t.o
+! RUN: %flang -target x86_64-unknown-linux-gnu %t.a -o %ta.out -flang-experimental-exec
+! RUN: llvm-objdump --syms %ta.out | FileCheck %s
+
+end program
+
+! CHECK-DAG: F .text {{[a-f0-9]+}} main
+! CHECK-DAG: F .text {{[a-f0-9]+}} _QQmain
+! CHECK-DAG: _FortranAProgramStart

diff  --git a/flang/test/Driver/linker-flags.f90 b/flang/test/Driver/linker-flags.f90
index e1350b74ae30a..f776d76129698 100644
--- a/flang/test/Driver/linker-flags.f90
+++ b/flang/test/Driver/linker-flags.f90
@@ -12,6 +12,14 @@
 !       Make sure they're not added.
 ! RUN: %flang -### -flang-experimental-exec -target aarch64-windows-msvc %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,MSVC --implicit-check-not libcmt --implicit-check-not oldnames
 
+! Check linker invocation to generate shared object (only GNU toolchain for now)
+! Output should not contain any undefined reference to _QQmain since it is not
+! considered a valid entry point for shared objects, which are usually specified
+! using the bind attribute.
+! RUN: %flang -### -flang-experimental-exec -shared -target x86_64-linux-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,GNU-SHARED --implicit-check-not _QQmain
+! RUN: %flang -### -flang-experimental-exec -shared -target aarch64-linux-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,GNU-SHARED --implicit-check-not _QQmain
+! RUN: %flang -### -flang-experimental-exec -shared -target riscv64-linux-gnu %S/Inputs/hello.f90 2>&1 | FileCheck %s --check-prefixes=CHECK,GNU-SHARED --implicit-check-not _QQmain
+
 ! Compiler invocation to generate the object file
 ! CHECK-LABEL: {{.*}} "-emit-obj"
 ! CHECK-SAME:  "-o" "[[object_file:.*\.o]]" {{.*}}Inputs/hello.f90
@@ -23,6 +31,7 @@
 !       executable and may find the GNU linker from MinGW or Cygwin.
 ! GNU-LABEL:  "{{.*}}ld{{(\.exe)?}}"
 ! GNU-SAME: "[[object_file]]"
+! GNU-SAME: --undefined=_QQmain
 ! GNU-SAME: -lFortran_main
 ! GNU-SAME: -lFortranRuntime
 ! GNU-SAME: -lFortranDecimal
@@ -50,3 +59,9 @@
 ! MSVC-SAME: FortranDecimal.lib
 ! MSVC-SAME: /subsystem:console
 ! MSVC-SAME: "[[object_file]]"
+
+! Linker invocation to generate a shared object
+! GNU-SHARED-LABEL:  "{{.*}}ld"
+! GNU-SHARED-SAME: "[[object_file]]"
+! GNU-SHARED-SAME: -lFortranRuntime
+! GNU-SHARED-SAME: -lFortranDecimal


        


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