[libc] Allow target architecture customization

[gchatelet]

Summary

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.

[gchatelet]

• Fix formatting

[gchatelet]

• include both commits

[sivachandra]

libc/cmake/modules/LLVMLibCLibraryRules.cmake

86

We should not need to set compile options here. If you notice, the add_library rule on line 79 does not compile anything. It just packs a bunch of object files into a static archive.

115

Same here as well. We should not need to set compile options.

libc/cmake/modules/LLVMLibCObjectRules.cmake

155 ↗

(On Diff #343383)

While you are at it, can you fix this: we should use the same default compile options for add_entrypoint_object and add_object_library above. So, essentially, I am saying that we should be sharing common_compiler_options between these two rules.

[gchatelet]

• Remove unnecessary target_compile_options
• Use same common compile options between add_entrypoint_object and add_object_library

[sivachandra]

libc/cmake/modules/LLVMLibCObjectRules.cmake

44 ↗

(On Diff #343601)

Instead of _set_*, I think a function with name _get_* will be more appropriate here. The function should also take an additional arg for the result:

_get_all_compile_options(all_opts ...)
target_compile_options(${fq_target_name} PRIVATE ${all_opts})

This probably a nitty comment but motivated by the fact that common is not suitable in add_object_library.

[gchatelet]

libc/cmake/modules/LLVMLibCObjectRules.cmake

44 ↗

(On Diff #343601)

I was pondering whether get or set was more suitable and I've been lazy : )
Thx for the comment it makes sense to go with get indeed.

libc/cmake/modules/LLVMLibCLibraryRules.cmake

foreach(dep IN LISTS all_deps)
     list(APPEND objects $<TARGET_OBJECTS:${dep}>)
   endforeach(dep)
   add_library(
     ${target_name}
     STATIC
     ${objects}
   )
   set_target_properties(${target_name}  PROPERTIES ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
-  target_compile_options(${target_name} PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT}
-  )
+  target_compile_options(${target_name} PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT})
 endfunction(add_entrypoint_library)
 
 # Rule to build a shared library of redirector objects.
 function(add_redirector_library target_name)
   cmake_parse_arguments(
     "REDIRECTOR_LIBRARY"
     ""
     ""
     "DEPENDS"
add_library(
     ${target_name}
     EXCLUDE_FROM_ALL
     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(${target_name}  PROPERTIES LINKER_LANGUAGE "C")
   target_compile_options(${target_name} PRIVATE ${LIBC_COMPILE_OPTIONS_DEFAULT})
 endfunction(add_redirector_library)
 
 set(HDR_LIBRARY_TARGET_TYPE "HDR_LIBRARY")
 
 # Rule to add header only libraries.
 # Usage
 #    add_header_library(
 #      <target name>