Setting the `armlink --list` option without other diagnostic flags is
misleading because it produces an empty file and prevents the user from
printing diagnostics to the standard output or redirecting them to
another file.
It's unclear why the flag was added when support for ARMClang was first
added by commit 7b0abaac31 (ARMClang: Add support for Clang-based ARM
compiler, 2019-05-13, v3.15.0-rc1~111^2).
Fixes: #21538
Given the compiler to use, `CMakeFindBinUtils.cmake` automatically
determines a number of tools including linker (CMAKE_LINKER) and archiver
(CMAKE_AR) and stores them in a generated file `CMakeCCompiler.cmake` as
non-CACHE entries. The compiler-specific ARMClang.cmake then tries to
override CMAKE_LINKER and CMAKE_AR as CACHE entries.
Following the introduction of CMP0126, which is set to NEW in the test
for a working compiler, setting a CACHE entry does not replace a normal
entry of the same name anymore, resulting in a failed test due to wrong
linker and archiver.
To fix this, set CMAKE_LINKER and CMAKE_AR for ARMClang directly in
`CMakeFindBinUtils.cmake` as is done for other compilers. Check
for them in `ARMClang.cmake` to safeguard cases when a project explicitly
includes `ARMClang.cmake` prior to compiler determination (which some
projects do to work around other problems in older CMake versions).
The compile options `--march=<arch>` and `--mcpu=<cpu>` and the
link option `--cpu=<cpu>` are automatically added by CMake based
on `CMAKE_SYSTEM_PROCESSOR` or `CMAKE_SYSTEM_ARCH`. But this is not
sufficient, because armclang also supports enabling or disabling
features using `+<feature>`:
-mcpu=<name>[+[no]<feature>+...]
For example:
-mcpu=cortex-a57+nocrypto+nofp+nosimd+crc
(Reference: https://developer.arm.com/documentation/dui0774/k/Compiler-Command-line-Options/-mcpu?lang=en)
The problem is, even if a project adds a flag with features it needs,
CMake still adds flags, resulting in code that is compiled with wrong
CPU features and unable to run.
Add policy `CMP0123` to not automatically add compile or link options,
and let projects set them instead.
Co-Author: Brad King <brad.king@kitware.com>
Fixes: #21173
3d1ed986e2 ARMClang: Fix link line generation after addition of armlink support
Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: Osama Moharam <osama.moharam.uw@renesas.com>
Merge-request: !5211
allows cmake to fall back to CMAKE_SYSTEM_ARCH in case CMAKE_SYSTEM_PROCESSOR is not in armclang -mcpu=list
additionally checks if CMAKE_SYSTEM_PROCESSOR belongs to armlink --cpu=list
Fixes: #19962
