[flang-commits] [flang] 9d6837d - [flang][driver] Remove Fortain_main static library from linking stages (#75816)
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Mon Dec 25 10:15:05 PST 2023
Author: Michael Klemm
Date: 2023-12-25T19:15:00+01:00
New Revision: 9d6837d595719904720e5ff68ec1f1a2665bdc2f
URL: https://github.com/llvm/llvm-project/commit/9d6837d595719904720e5ff68ec1f1a2665bdc2f
DIFF: https://github.com/llvm/llvm-project/commit/9d6837d595719904720e5ff68ec1f1a2665bdc2f.diff
LOG: [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 at users.noreply.github.com>
Added:
Modified:
clang/lib/Driver/ToolChains/CommonArgs.cpp
flang/docs/FlangDriver.md
flang/test/Driver/dynamic-linker.f90
Removed:
################################################################################
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 6eb0ed8f3fed9a..3b29e1bc75850f 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/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);
diff --git a/flang/docs/FlangDriver.md b/flang/docs/FlangDriver.md
index 5231e78335f6ad..fa39889927e0eb 100644
--- a/flang/docs/FlangDriver.md
+++ b/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
diff --git a/flang/test/Driver/dynamic-linker.f90 b/flang/test/Driver/dynamic-linker.f90
index 1cbd407d21ce09..7c3f1b5a53fe48 100644
--- a/flang/test/Driver/dynamic-linker.f90
+++ b/flang/test/Driver/dynamic-linker.f90
@@ -3,10 +3,12 @@
! 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?
@@ -14,6 +16,7 @@
! 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"
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