cmake: linker: Use the same linker for cmake checks and final build

Currently, the linker that is used when performing various cmake checks
(check_c_compiler_flag, for example) may be different than the linker that
will be used during the actual build. This happens as we currently specify
'-fuse-ld' to force the appropriate linker a) after many such checks have
already happened and b) in a way which is not automatically propagated to
check_c_compiler_flag (and friends). As a result, the toolchain's default
linker will generally be used for such checks regardless of which linker
was selected in Zephyr.

This can lead to a number of surprises when building Zephyr, particularly
when building with clang. For example:

- If the linker is misconfigured, where the build will fail can vary
  depending on whether the linker is the toolchain's default. When the
  configured linker happens to be the toolchain's default, the build
  (helpfully) fails quickly on the checks for a basic working toochain.
  When the configured linker isn't the default, the build won't fail until
  the final link steps.
- The build can fail due to issues with a linker other than the one
  configured by the user in Zephyr. For example, LLVM toolchains without
  lld will generally fail to build Zephyr (the checks for a basic
  working toochain will fail) for targets where lld is the default in LLVM
  even if GNU ld is configured in Zephyr and would otherwise be used in the
  final build.
- Flags which are only added if check_c_compiler_flag (or similar) succeeds
  may be unexpectedly omitted during the final build if the flag is
  supported in the configured linker but is unsupported in the toolchain's
  default linker (as check_c_compiler_flag will test using the default
  one).

Note that this isn't limited to clang--even when we are building with
Zephyr's SDK and force ld.bfd, we seem to use the 'ld' variant during the
cmake checks (though this generally seems fairly harmless compared to
mixing ld/lld or other proprietary linkers).

To fix this, ensure the appropriate 'fuse-ld' is set early enough and in
such a way that the same linker will be used throughout the entire build.

Signed-off-by: Jonathon Penix <[email protected]>

Author: jonathonpenix

cmake/linker/ld/target.cmake

@@ -6,6 +6,14 @@ set(CMAKE_LINKER ${GNULD_LINKER})
 
 set_ifndef(LINKERFLAGPREFIX -Wl)
 
+if((${CMAKE_LINKER} STREQUAL "${CROSS_COMPILE}ld.bfd") OR
+   ${GNULD_LINKER_IS_BFD})
+  # ld.bfd was found so let's explicitly use that for linking, see #32237
+  list(APPEND TOOLCHAIN_LD_FLAGS -fuse-ld=bfd)
+  list(APPEND CMAKE_REQUIRED_FLAGS -fuse-ld=bfd)
+  string(REPLACE ";" " " CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
+endif()
+
 if(NOT "${ZEPHYR_TOOLCHAIN_VARIANT}" STREQUAL "host")
   if(CONFIG_CPP_EXCEPTIONS AND LIBGCC_DIR)
     # When building with C++ Exceptions, it is important that crtbegin and crtend
@@ -118,16 +126,9 @@ function(toolchain_ld_link_elf)
     ${ARGN}                                                   # input args to parse
   )
 
-  if((${CMAKE_LINKER} STREQUAL "${CROSS_COMPILE}ld.bfd") OR
-     ${GNULD_LINKER_IS_BFD})
-    # ld.bfd was found so let's explicitly use that for linking, see #32237
-    set(use_linker "-fuse-ld=bfd")
-  endif()
-
   target_link_libraries(
     ${TOOLCHAIN_LD_LINK_ELF_TARGET_ELF}
     ${TOOLCHAIN_LD_LINK_ELF_LIBRARIES_PRE_SCRIPT}
-    ${use_linker}
     ${TOPT}
     ${TOOLCHAIN_LD_LINK_ELF_LINKER_SCRIPT}
     ${TOOLCHAIN_LD_LINK_ELF_LIBRARIES_POST_SCRIPT}

cmake/linker/lld/target.cmake

@@ -6,6 +6,10 @@ set(CMAKE_LINKER ${LLVMLLD_LINKER})
 
 set_ifndef(LINKERFLAGPREFIX -Wl)
 
+list(APPEND TOOLCHAIN_LD_FLAGS -fuse-ld=lld)
+list(APPEND CMAKE_REQUIRED_FLAGS -fuse-ld=lld)
+string(REPLACE ";" " " CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
+
 # Run $LINKER_SCRIPT file through the C preprocessor, producing ${linker_script_gen}
 # NOTE: ${linker_script_gen} will be produced at build-time; not at configure-time
 macro(configure_linker_script linker_script_gen linker_pass_define)

cmake/linker/lld/target_baremetal.cmake

@@ -12,13 +12,6 @@ macro(toolchain_ld_baremetal)
     ${LINKERFLAGPREFIX},-N
   )
 
-  # Force LLVM to use built-in lld linker
-  if(NOT CONFIG_LLVM_USE_LD)
-    zephyr_ld_options(
-      -fuse-ld=lld
-  )
-  endif()
-
   # Funny thing is if this is set to =error, some architectures will
   # skip this flag even though the compiler flag check passes
   # (e.g. ARC and Xtensa). So warning should be the default for now.