ARMClang: Do not automatically add cpu/arch compile or link options

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

1 files changed, 46 insertions, 26 deletions

diff --git a/Modules/Compiler/ARMClang.cmake b/Modules/Compiler/ARMClang.cmake
index 66b7c2e..354c11d 100644
--- a/Modules/Compiler/ARMClang.cmake
+++ b/Modules/Compiler/ARMClang.cmake
@@ -3,6 +3,9 @@ if(_ARMClang_CMAKE_LOADED)
 endif()
 set(_ARMClang_CMAKE_LOADED TRUE)
 
+# Save the CMP0123 setting in a variable used both below and by try_compile.
+cmake_policy(GET CMP0123 CMAKE_ARMClang_CMP0123)
+
 cmake_policy(PUSH)
 cmake_policy(SET CMP0057 NEW) # if IN_LIST
 
@@ -82,36 +85,53 @@ macro(__compiler_armclang lang)
   if(NOT CMAKE_${lang}_COMPILER_ARCH_LIST)
     __armclang_set_arch_list(${lang} CMAKE_${lang}_COMPILER_ARCH_LIST)
   endif()
-  if(NOT CMAKE_SYSTEM_PROCESSOR AND NOT CMAKE_SYSTEM_ARCH)
-    message(FATAL_ERROR "  CMAKE_SYSTEM_PROCESSOR or CMAKE_SYSTEM_ARCH must be set for ARMClang\n"
-      "  Supported processor: ${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}\n"
-      "  Supported Architecture: ${CMAKE_${lang}_COMPILER_ARCH_LIST}")
-  else()
-    __armclang_check_processor("${CMAKE_SYSTEM_ARCH}" "${CMAKE_${lang}_COMPILER_ARCH_LIST}" _CMAKE_${lang}_CHECK_ARCH_RESULT)
-    if( _CMAKE_${lang}_CHECK_ARCH_RESULT)
-      string(APPEND CMAKE_${lang}_FLAGS_INIT " -march=${CMAKE_SYSTEM_ARCH}")
-      set(__march_flag_set TRUE)
+
+  # CMAKE_SYSTEM_PROCESSOR and CMAKE_SYSTEM_ARCH are not sufficient because they provide no
+  # information of additional CPU features needed in `-mcpu=<name>[+[no]<feature>+...]`.
+  # The automatic setting of compile and link options is deprecated and projects should specify their own.
+  cmake_policy(GET CMP0123 policy_CMP0123)
+  if(NOT "x${CMAKE_ARMClang_CMP0123}x" STREQUAL "xNEWx")
+    if(NOT "x${CMAKE_ARMClang_CMP0123}x" STREQUAL "xOLDx")
+      cmake_policy(GET_WARNING CMP0123 _cmp0123_warning)
+      message(AUTHOR_WARNING
+        "${_cmp0123_warning}\n"
+        "For compatibility, CMake will automatically add cpu/arch flags based "
+        "on the CMAKE_SYSTEM_PROCESSOR and/or CMAKE_SYSTEM_ARCH variables."
+        )
     endif()
-    __armclang_check_processor("${CMAKE_SYSTEM_PROCESSOR}" "${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}" _CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
-    if(_CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
-      string(APPEND CMAKE_${lang}_FLAGS_INIT " -mcpu=${CMAKE_SYSTEM_PROCESSOR}")
-      set(__mcpu_flag_set TRUE)
+
+    if(NOT CMAKE_SYSTEM_PROCESSOR AND NOT CMAKE_SYSTEM_ARCH)
+      message(FATAL_ERROR "  CMAKE_SYSTEM_PROCESSOR or CMAKE_SYSTEM_ARCH must be set for ARMClang\n"
+        "  Supported processor: ${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}\n"
+        "  Supported Architecture: ${CMAKE_${lang}_COMPILER_ARCH_LIST}")
+    else()
+      __armclang_check_processor("${CMAKE_SYSTEM_ARCH}" "${CMAKE_${lang}_COMPILER_ARCH_LIST}" _CMAKE_${lang}_CHECK_ARCH_RESULT)
+      if( _CMAKE_${lang}_CHECK_ARCH_RESULT)
+        string(APPEND CMAKE_${lang}_FLAGS_INIT " -march=${CMAKE_SYSTEM_ARCH}")
+        set(__march_flag_set TRUE)
+      endif()
+      __armclang_check_processor("${CMAKE_SYSTEM_PROCESSOR}" "${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}" _CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
+      if(_CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
+        string(APPEND CMAKE_${lang}_FLAGS_INIT " -mcpu=${CMAKE_SYSTEM_PROCESSOR}")
+        set(__mcpu_flag_set TRUE)
+      endif()
+      if(NOT __march_flag_set AND NOT __mcpu_flag_set)
+        message(FATAL_ERROR "At least one of the variables CMAKE_SYSTEM_PROCESSOR or CMAKE_SYSTEM_ARCH must be set for ARMClang\n"
+                            "Supported processor: ${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}\n"
+                            "  Supported Architecture: ${CMAKE_${lang}_COMPILER_ARCH_LIST}")
+      endif()
+      unset(_CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
+      unset(_CMAKE_${lang}_CHECK_ARCH_RESULT)
     endif()
-    if(NOT __march_flag_set AND NOT __mcpu_flag_set)
-      message(FATAL_ERROR "At least one of the variables CMAKE_SYSTEM_PROCESSOR or CMAKE_SYSTEM_ARCH must be set for ARMClang\n"
-                          "Supported processor: ${CMAKE_${lang}_COMPILER_PROCESSOR_LIST}\n"
-                          "  Supported Architecture: ${CMAKE_${lang}_COMPILER_ARCH_LIST}")
+
+    #check if CMAKE_SYSTEM_PROCESSOR belongs to supported cpu list for armlink
+    __armlink_set_cpu_list( ${lang} CMAKE_LINKER_CPU_LIST)
+    list(TRANSFORM CMAKE_LINKER_CPU_LIST TOLOWER)
+    __armclang_check_processor("${CMAKE_SYSTEM_PROCESSOR}" "${CMAKE_LINKER_CPU_LIST}" _CMAKE_CHECK_LINK_CPU_RESULT)
+    if(_CMAKE_CHECK_LINK_CPU_RESULT)
+      string(APPEND CMAKE_${lang}_LINK_FLAGS " --cpu=${CMAKE_SYSTEM_PROCESSOR}")
     endif()
-    unset(_CMAKE_${lang}_CHECK_PROCESSOR_RESULT)
-    unset(_CMAKE_${lang}_CHECK_ARCH_RESULT)
-  endif()
 
-  #check if CMAKE_SYSTEM_PROCESSOR belongs to supported cpu list for armlink
-  __armlink_set_cpu_list( ${lang} CMAKE_LINKER_CPU_LIST)
-  list(TRANSFORM CMAKE_LINKER_CPU_LIST TOLOWER)
-  __armclang_check_processor("${CMAKE_SYSTEM_PROCESSOR}" "${CMAKE_LINKER_CPU_LIST}" _CMAKE_CHECK_LINK_CPU_RESULT)
-  if(_CMAKE_CHECK_LINK_CPU_RESULT)
-    string(APPEND CMAKE_${lang}_LINK_FLAGS " --cpu=${CMAKE_SYSTEM_PROCESSOR}")
   endif()
 
   if(__CMAKE_ARMClang_USING_armlink)