[libc] Make clang-tidy use host compiler's resource dir.

[PaulkaToast]

Summary

When building llvm-libc with linting enabled, clang-tidy would use the resource dir of the monorepo rather then the host compiler's resource dir. This presented issues when including headers from the host compiler e.g. for sanitizers. Therefore this patch explicitly tells clang-tidy to use the host compiler's resource dir.

[sivachandra]

I think I understand the problem with clang-tidy that you are trying to solve here. Also, I like the overall idea of this patch. But, see my inline comment.

libc/cmake/modules/LLVMLibCHeaderRules.cmake

131

This fallback seems more confusing than useful. For example, will it find stddef.h from /usr/include or will it find stddef.h from the compiler's resource dir? Perhaps a better alternate is to compile a stub using the -E option and grep out the path to stddef.h.

To keep it simple, may be we should just disable the header file check when we can't get the resource dir?

One minor point, this check is really not a rule. So, can we just put this whole function not as a function in libc/CMakeLists.txt?

[PaulkaToast]

libc/cmake/modules/LLVMLibCHeaderRules.cmake

131

Yeah, I'm not the biggest fan of this fallback that's why I made the TODO.

I liked the idea of disabling the check since most compiler do support the --print-resource-dir flag.

As for where the rule lives I think your suggestion is good. I'll update the revision accordingly (:

[sivachandra]

libc/CMakeLists.txt

66

Can you move this to right before/after the linting related code.

libc/cmake/modules/LLVMLibCObjectRules.cmake

228

Instead of leaving it empty, may be:

COMMAND ${CMAKE_COMMAND} -E echo "Header file check skipped"

Not sure if it is going to get too noisy. Other option is to append this message to the COMMENT of the custom command.

[PaulkaToast]

libc/cmake/modules/LLVMLibCObjectRules.cmake

228

I tried it out and it doesn't seem too noisy to me. Thanks for the suggestion!

libc/CMakeLists.txt

 # The lib and test directories are added at the very end as tests
 # and libraries potentially draw from the components present in all
 # of the other directories.
 add_subdirectory(lib)
 if(LLVM_INCLUDE_TESTS)
   add_subdirectory(test)
   add_subdirectory(fuzzing)
 endif()
+
+# Check --print-resource-dir to find the compiler resource dir if this flag
+# is supported by the compiler.
+execute_process(
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+  COMMAND ${CMAKE_CXX_COMPILER} --print-resource-dir
+  RESULT_VARIABLE COMMAND_RETURN_CODE
+  OUTPUT_VARIABLE COMPILER_RESOURCE_DIR
+)
+# Retrieve the host compiler's resource dir.
+if(COMMAND_RETURN_CODE EQUAL 0)
+  set(COMPILER_RESOURCE_DIR
+    "${COMPILER_RESOURCE_DIR}" CACHE PATH "path to compiler resource dir"
+  )
+  message(STATUS "Set COMPILER_RESOURCE_DIR to
+                  ${COMPILER_RESOURCE_DIR} using --print-resource-dir")
+else()
+  set(COMPILER_RESOURCE_DIR OFF)
+  message(STATUS "COMPILER_RESOURCE_DIR not set
+                  --print-resource-dir not supported by host compiler")
+endif()

libc/cmake/modules/LLVMLibCHeaderRules.cmake

function(add_gen_header target_name)
 
   add_custom_command(
     OUTPUT ${out_file}
     COMMAND $<TARGET_FILE:libc-hdrgen> -o ${out_file} --header ${ADD_GEN_HDR_GEN_HDR}
             --def ${in_file} ${replacement_params} -I ${LIBC_SOURCE_DIR}
             ${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/api.td
 
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    DEPENDS ${in_file} ${fq_data_files} ${td_includes} 
+    DEPENDS ${in_file} ${fq_data_files} ${td_includes}
             ${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/api.td libc-hdrgen
   )
 
   get_fq_target_name(${target_name} fq_target_name)
   if(ADD_GEN_HDR_DEPENDS)
     get_fq_deps_list(fq_deps_list ${ADD_GEN_HDR_DEPENDS})
   endif()
   add_custom_target(
     ${fq_target_name}
     DEPENDS ${out_file} ${fq_deps_list}
   )
 endfunction(add_gen_header)

libc/cmake/modules/LLVMLibCObjectRules.cmake

set_target_properties(
     PROPERTIES
       "TARGET_TYPE" ${ENTRYPOINT_OBJ_TARGET_TYPE}
       "OBJECT_FILE" "${object_file}"
       "OBJECT_FILE_RAW" "${object_file_raw}"
       "DEPS" "${fq_deps_list}"
   )
 
   if(LLVM_LIBC_ENABLE_LINTING)
-    set(lint_timestamp "${CMAKE_CURRENT_BINARY_DIR}/.${target_name}.__lint_timestamp__")
 
+    # We only want a second invocation of clang-tidy to run
+    # restrict-system-libc-headers if the compiler-resource-dir was set in
+    # order to prevent false-positives due to a mismatch between the host
+    # compiler and the compiled clang-tidy.
+    if(COMPILER_RESOURCE_DIR)
+      # We run restrict-system-libc-headers with --system-headers to prevent
+      # transitive inclusion through compler provided headers.
+      set(restrict_system_headers_check_invocation
+        COMMAND $<TARGET_FILE:clang-tidy> --system-headers
+        --checks="-*,llvmlibc-restrict-system-libc-headers"
+        # We explicitly set the resource dir here to match the
+        # resource dir of the host compiler.
+        "--extra-arg=-resource-dir=${COMPILER_RESOURCE_DIR}"
+        --quiet
+        -p ${PROJECT_BINARY_DIR}
+        ${ADD_ENTRYPOINT_OBJ_SRCS}
+      )
+    else()
+      set(restrict_system_headers_check_invocation "")
+    endif()
+
+    set(lint_timestamp "${CMAKE_CURRENT_BINARY_DIR}/.${target_name}.__lint_timestamp__")
     add_custom_command(
       OUTPUT ${lint_timestamp}
       # --quiet is used to surpress warning statistics from clang-tidy like:
       #     Suppressed X warnings (X in non-user code).
       # There seems to be a bug in clang-tidy where by even with --quiet some
       # messages from clang's own diagnostics engine leak through:
       #     X warnings generated.
       # Until this is fixed upstream, we use -fno-caret-diagnostics to surpress
       # these.
       COMMAND $<TARGET_FILE:clang-tidy>
               "--extra-arg=-fno-caret-diagnostics" --quiet
               # Path to directory containing compile_commands.json
               -p ${PROJECT_BINARY_DIR}
               ${ADD_ENTRYPOINT_OBJ_SRCS}
-      # We run restrict-system-libc-headers with --system-headers to prevent
-      # transitive inclusion through compler provided headers.
-      COMMAND $<TARGET_FILE:clang-tidy> --system-headers
-              --checks="-*,llvmlibc-restrict-system-libc-headers"
-              --quiet
-              -p ${PROJECT_BINARY_DIR}
-              ${ADD_ENTRYPOINT_OBJ_SRCS}
+      # See above: this might be a second invocation of clang-tidy depending on
+      # the conditions above.
+      ${restrict_system_headers_check_invocation}
       # We have two options for running commands, add_custom_command and
       # add_custom_target. We don't want to run the linter unless source files
       # have changed. add_custom_target explicitly runs everytime therefore we
       # use add_custom_command. This function requires an output file and since
       # linting doesn't produce a file, we create a dummy file using a
       # crossplatform touch.
       COMMAND "${CMAKE_COMMAND}" -E touch ${lint_timestamp}
       COMMENT "Linting... ${target_name}"