[flang][Driver] Enable FLANG_DEFAULT_LINKER (#149786)

The default linker can be changed by a CMake variable
CLANG_DEFAULT_LINKER. However, it also changes the default linker
invoked by clang. In fact, there already exists FLANG_DEFAULT_LINKER,
but it does not work. This patch fixes it.

Note that FLANG_DEFAULT_LINKER will have the same value as
CLANG_DEFAULT_LINKER unless it is defined explicitly. That means this
patch does not affect the current behavior.

Fixes #73153

---------

Co-authored-by: Michael Kruse <[email protected]>

Author: parabola94

flang/CMakeLists.txt

@@ -317,7 +317,7 @@ if (NOT ENABLE_LINKER_BUILD_ID)
   set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 endif()
 
-set(FLANG_DEFAULT_LINKER "" CACHE STRING
+set(FLANG_DEFAULT_LINKER "${CLANG_DEFAULT_LINKER}" CACHE STRING
   "Default linker to use (linker name or absolute path, empty for platform default)")
 
 set(FLANG_DEFAULT_RTLIB "" CACHE STRING

flang/include/flang/Config/config.h.cmake

@@ -1,10 +1,10 @@
-#===-- include/flang/Config/config.h.cmake ---------------------------------===#
-#
-# Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-# See https://llvm.org/LICENSE.txt for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-#
-#===------------------------------------------------------------------------===#
+//===-- include/flang/Config/config.h.cmake -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
 
 /* This generated file is for internal use. Do not include it from headers. */
 
@@ -16,6 +16,8 @@
 
 #define FLANG_VERSION            "${FLANG_VERSION}"
 
+#define FLANG_DEFAULT_LINKER     "${FLANG_DEFAULT_LINKER}"
+
 #endif
 
 

flang/test/Driver/linker-flags.f90

@@ -77,7 +77,7 @@
 ! MINGW-SAME: -lflang_rt.runtime
 ! MINGW-STATIC-FLANGRT: "{{.*}}{{\\|/}}libflang_rt.runtime.a"
 
-! NOTE: This also matches lld-link (when CLANG_DEFAULT_LINKER=lld) and
+! NOTE: This also matches lld-link (when FLANG_DEFAULT_LINKER=lld) and
 !       any .exe suffix that is added when resolving to the full path of
 !       (lld-)link.exe on Windows platforms. The suffix may not be added
 !       when the executable is not found or on non-Windows platforms.

flang/tools/flang-driver/driver.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/Driver/Driver.h"
+#include "flang/Config/config.h"
 #include "flang/Frontend/CompilerInvocation.h"
 #include "flang/Frontend/TextDiagnosticPrinter.h"
 #include "clang/Basic/Diagnostic.h"
@@ -137,6 +138,7 @@ int main(int argc, const char **argv) {
                                   llvm::sys::getDefaultTargetTriple(), diags,
                                   "flang LLVM compiler");
   theDriver.setTargetAndMode(targetandMode);
+  theDriver.setPreferredLinker(FLANG_DEFAULT_LINKER);
 #ifdef FLANG_RUNTIME_F128_MATH_LIB
   theDriver.setFlangF128MathLibrary(FLANG_RUNTIME_F128_MATH_LIB);
 #endif