[CMake][libc] Don't do CPU feature detection when cross-compiling

[phosek]

Summary

We won't be able to run the compiled program since it will be compiled
for different system. We instead allow passing the CPU features via
CMake option in that case.

[gchatelet]

libc/cmake/modules/LLVMLibCCheckCpuFeatures.cmake

17

Thx for fixing this ugly variable name : )

libc/cmake/modules/LLVMLibCCheckCpuFeatures.cmake

 # ------------------------------------------------------------------------------
 # Cpu features definition and flags
 # ------------------------------------------------------------------------------
 
 if(${LIBC_TARGET_MACHINE} MATCHES "x86|x86_64")
   set(ALL_CPU_FEATURES SSE SSE2 AVX AVX2 AVX512F)
   list(SORT ALL_CPU_FEATURES)
 endif()
 
-# Function to check whether the host supports the provided set of features.
+# Function to check whether the target CPU supports the provided set of features.
 # Usage:
-# host_supports(
+# cpu_supports(
 #   <output variable>
 #   <list of cpu features>
 # )
-function(host_supports output_var features)
-  _intersection(a "${HOST_CPU_FEATURES}" "${features}")
-  if("${a}" STREQUAL "${features}")
+function(cpu_supports output_var features)
+  _intersection(var "${LIBC_CPU_FEATURES}" "${features}")
+  if("${var}" STREQUAL "${features}")
     set(${output_var} TRUE PARENT_SCOPE)
   else()
     unset(${output_var} PARENT_SCOPE)
   endif()
 endfunction()
 
 # Function to compute the flags to pass down to the compiler.
 # Usage:
set(${output_var}
         PARENT_SCOPE)
   elseif(NOT ${compile_result})
     message(FATAL_ERROR "Failed to compile: ${compile_output}")
   else()
     message(FATAL_ERROR "Failed to run: ${run_output}")
   endif()
 endfunction()
 
-# Populates the HOST_CPU_FEATURES list.
+set(LIBC_CPU_FEATURES "" CACHE PATH "supported CPU features")
+
+if(CMAKE_CROSSCOMPILING)
+  _intersection(cpu_features "${ALL_CPU_FEATURES}" "${LIBC_CPU_FEATURES}")
+  if(NOT "${cpu_features}" STREQUAL "${LIBC_CPU_FEATURES}")
+    message(FATAL_ERROR "Unsupported CPU features: ${cpu_features}")
+  endif()
+  set(LIBC_CPU_FEATURES "${cpu_features}")
+else()
+  # Populates the LIBC_CPU_FEATURES list.
   # Use -march=native only when the compiler supports it.
   include(CheckCXXCompilerFlag)
   CHECK_CXX_COMPILER_FLAG("-march=native" COMPILER_SUPPORTS_MARCH_NATIVE)
   if(COMPILER_SUPPORTS_MARCH_NATIVE)
-  _check_defined_cpu_feature(HOST_CPU_FEATURES MARCH native)
+    _check_defined_cpu_feature(LIBC_CPU_FEATURES MARCH native)
   else()
-  _check_defined_cpu_feature(HOST_CPU_FEATURES)
+    _check_defined_cpu_feature(LIBC_CPU_FEATURES)
+  endif()
 endif()

libc/test/src/string/CMakeLists.txt

add_libc_unittest(
   SUITE
     libc_string_unittests
   SRCS
     strtok_r_test.cpp
   DEPENDS
     libc.src.string.strtok_r
 )
 
-# Tests all implementations that can run on the host.
+# Tests all implementations that can run on the target CPU.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
   foreach(fq_config_name IN LISTS fq_implementations)
     get_target_property(required_cpu_features ${fq_config_name} REQUIRE_CPU_FEATURES)
-    host_supports(can_run "${required_cpu_features}")
+    cpu_supports(can_run "${required_cpu_features}")
     if(can_run)
       add_libc_unittest(
         ${fq_config_name}_test
         SUITE
           libc_string_unittests
         DEPENDS
           ${fq_config_name}
         ${ARGN}