[libc] Allow target architecture customization

This patch provides a way to specify the default target cpu optimizations to use when compiling llvm-libc.
This ensures we don't rely on current compiler's default and allows compiling and cross compiling for a particular target.

Differential Revision: https://reviews.llvm.org/D101991

libc/CMakeLists.txt

@@ -22,6 +22,9 @@ string(TOLOWER ${LIBC_TARGET_OS} LIBC_TARGET_OS)
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)
 
+# Flags to pass down to the compiler while building the libc functions.
+set(LIBC_COMPILE_OPTIONS_DEFAULT "" CACHE STRING "Architecture to tell clang to optimize for (e.g. -march=... or -mcpu=...)")
+
 # Check --print-resource-dir to find the compiler resource dir if this flag
 # is supported by the compiler.
 execute_process(

libc/cmake/modules/LLVMLibCLibraryRules.cmake

@@ -81,11 +81,7 @@ function(add_entrypoint_library target_name)
     STATIC
     ${objects}
   )
-  set_target_properties(
-    ${target_name}
-    PROPERTIES
-      ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
-  )
+  set_target_properties(${target_name}  PROPERTIES ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 endfunction(add_entrypoint_library)
 
 # Rule to build a shared library of redirector objects.
@@ -112,18 +108,9 @@ function(add_redirector_library target_name)
     SHARED
     ${obj_files}
   )
-  set_target_properties(${target_name} PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-
-  target_link_libraries(
-    ${target_name}
-    -nostdlib -lc -lm
-  )
-
-  set_target_properties(
-    ${target_name}
-    PROPERTIES
-      LINKER_LANGUAGE "C"
-  )
+  set_target_properties(${target_name}  PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
+  target_link_libraries(${target_name}  -nostdlib -lc -lm)
+  set_target_properties(${target_name}  PROPERTIES LINKER_LANGUAGE "C")
 endfunction(add_redirector_library)
 
 set(HDR_LIBRARY_TARGET_TYPE "HDR_LIBRARY")

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -1,5 +1,9 @@
 set(OBJECT_LIBRARY_TARGET_TYPE "OBJECT_LIBRARY")
 
+function(_get_common_compile_options output_var)
+  set(${output_var} -fpie ${LLVM_CXX_STD_default} -ffreestanding ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ARGN} PARENT_SCOPE)
+endfunction()
+
 # Rule which is essentially a wrapper over add_library to compile a set of
 # sources to object files.
 # Usage:
@@ -37,12 +41,8 @@ function(add_object_library target_name)
       ${LIBC_SOURCE_DIR}
       ${LIBC_BUILD_DIR}
   )
-  if(ADD_OBJECT_COMPILE_OPTIONS)
-    target_compile_options(
-      ${fq_target_name}
-      PRIVATE ${ADD_OBJECT_COMPILE_OPTIONS}
-    )
-  endif()
+  _get_common_compile_options(compile_options ${ADD_OBJECT_COMPILE_OPTIONS})
+  target_compile_options(${fq_target_name} PRIVATE ${compile_options})
 
   get_fq_deps_list(fq_deps_list ${ADD_OBJECT_DEPENDS})
   if(fq_deps_list)
@@ -148,7 +148,7 @@ function(add_entrypoint_object target_name)
     message(FATAL_ERROR "`add_entrypoint_object` rule requires HDRS to be specified.")
   endif()
 
-  set(common_compile_options -fpie ${LLVM_CXX_STD_default} -ffreestanding ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS})
+  _get_common_compile_options(common_compile_options ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS})
   set(internal_target_name ${fq_target_name}.__internal__)
   set(include_dirs ${LIBC_BUILD_DIR}/include ${LIBC_SOURCE_DIR} ${LIBC_BUILD_DIR})
   get_fq_deps_list(fq_deps_list ${ADD_ENTRYPOINT_OBJ_DEPENDS})
@@ -176,9 +176,7 @@ function(add_entrypoint_object target_name)
     ${ADD_ENTRYPOINT_OBJ_SRCS}
     ${ADD_ENTRYPOINT_OBJ_HDRS}
   )
-  target_compile_options(
-      ${fq_target_name} BEFORE PRIVATE ${common_compile_options} -DLLVM_LIBC_PUBLIC_PACKAGING
-  )
+  target_compile_options(${fq_target_name} BEFORE PRIVATE ${common_compile_options} -DLLVM_LIBC_PUBLIC_PACKAGING)
   target_include_directories(${fq_target_name} PRIVATE ${include_dirs})
   add_dependencies(${fq_target_name} ${full_deps_list})
 
@@ -277,6 +275,6 @@ function(add_redirector_object target_name)
   )
   target_compile_options(
     ${target_name}
-    BEFORE PRIVATE -fPIC
+    BEFORE PRIVATE -fPIC ${LIBC_COMPILE_OPTIONS_DEFAULT}
   )
 endfunction(add_redirector_object)

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -127,6 +127,10 @@ function(add_libc_unittest target_name)
       ${LIBC_BUILD_DIR}
       ${LIBC_BUILD_DIR}/include
   )
+  target_compile_options(
+    ${fq_target_name}
+    PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT}
+  )
   if(LIBC_UNITTEST_COMPILE_OPTIONS)
     target_compile_options(
       ${fq_target_name}