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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
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