[libc] Use a prebuilt libc-hdrgen binary if available.

[sivachandra]

Summary

This feature will primarily by used by the runtimes build. In the
runtimes build, we build libc-hdrgen once for the host and use it
to build the libc for other targets. So, the host libc-hdrgen is
like a prebuilt passed to the target builds.

[jhuber6]

libc/CMakeLists.txt

155–156

CMake Error at /home/jhuber/Documents/llvm/llvm-project/libc/utils/HdrGen/CMakeLists.txt:3 (add_tablegen):
  Unknown CMake command "add_tablegen".

This gives me the following error when I run with -DLLVM_ENABLE_RUNTIMES=libc. I remember adding this code to work around it.

[sivachandra]

libc/CMakeLists.txt

155–156

Ah, OK. One way to fix that problem is proposed here: https://reviews.llvm.org/D141460. Essentially, libc-hdrgen is a host tool and should be built separately and not as part of the libc build. As a workaround for now, you can try building libc-hdrgen separately and then do a full build with the LIBC_HDRGEN_EXE option specified.

Also, can you explain why you need full-build for GPU builds? For headers?

[jhuber6]

libc/CMakeLists.txt

155–156

Yes, it's primarily the headers since the user's system headers are most likely not going to be compatible. For example, on my machine if I try to include the standard headers while compiling for the GPU target it'll think it's a 32-bit system and try to include some stubs which don't exist.

The second reason is that I think in order to run the code on the GPU, we need a "loader" of some kind. Which is usually handled by a language runtime, i.e. libcuda or libomptarget, so I think that's in-line with a full build.

Is there a reason we don't want to just do what I did above? A runtimes build is somewhat different from a bootstrap since we can still reliably expect to be in-tree.

if("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
  include(TableGen)
endif()

[sivachandra]

libc/CMakeLists.txt

155–156

The proposed patch that I linked to above solves this problem for the general case. In the general case, runtimes build/bootstrap build is a crossbuild. So, you cannot build libc-hdrgen for the target and run it at libc build time on the host. It should be built as a host tool separately and then used to do the actual runtimes build.

If we go with what you are suggesting, then we will build libc-hdrgen for the target as well, which is incorrect.

[jhuber6]

libc/CMakeLists.txt

155–156

I see, that makes sense for cross builds. So how do you get the libc header generator tool then? The patch linked just seems to error if it hasn't been added as a target. So how do we get the binary in the first place without first building libc?

[sivachandra]

libc/CMakeLists.txt

155–156

The runtimes build for the libc full-build is currently broken in many ways without the proposed patch. So, I actually would say that until it is fixed, you should build clang separately and then do a standalone libc full build. That way, you will mimic the runtimes build but not have to deal with broken runtimes build for the full libc.

[jhuber6]

libc/CMakeLists.txt

155–156

The whole point of -DLLVM_ENABLE_RUNTIMES= is to remove the need for two-step builds, so it would be nice if we could get it to work. What's the problem with including the target? So, if I understand this correctly, the problem is that if the user wants to cross-build (e.g. x86_64 host but AArch64 target) then the HdrGen executable will be built for AArch64 and thus not able to be run? Could we override the triple in the compilation?

[sivachandra]

libc/CMakeLists.txt

155–156

Ah, if all you want is a host build runtimes build (or a libc GPU build), you can still do it without any changes: Just list the libc project in LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES.

libc/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 # Default to C++17
 set(CMAKE_CXX_STANDARD 17)
 
 # Use old version of target_sources command which converts the source
 # file paths to full paths.
 cmake_policy(SET CMP0076 OLD)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level sourse and binary directories.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBC_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
-# For a runtimes build we need to manually include tablgen and LLVM directories.
-if ("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
-  include(TableGen)
-  set(LLVM_LIBC_INCLUDE_DIRS ${LLVM_MAIN_INCLUDE_DIR} ${LLVM_BINARY_DIR}/include)
+if(LLVM_LIBC_FULL_BUILD)
+  if(NOT LIBC_HDRGEN_EXE)
+    # We need to set up hdrgen first since other targets depend on it.
+    add_subdirectory(utils/LibcTableGenUtil)
+    add_subdirectory(utils/HdrGen)
+  else()
+    message(STATUS "Will use ${LIBC_HDRGEN_EXE} for libc header generation.")
+  endif()
+endif()
+
+if(LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD)
+  # When libc is build as part of the runtimes/bootstrap build's CMake run, we
+  # only need to build the host tools to build the libc. So, we just do enough
+  # to build libc-hdrgen and return.
+  return()
 endif()
 
 # Path libc/scripts directory.
 set(LIBC_BUILD_SCRIPTS_DIR "${LIBC_SOURCE_DIR}/utils/build_scripts")
 
 # 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=...)")
 
if(${last_dot_loc} EQUAL -1)
     message(FATAL "Invalid entrypoint target name ${entrypoint}; Expected a '.' "
                   "(dot) in the name.")
   endif()
   math(EXPR name_loc "${last_dot_loc} + 1")
   string(SUBSTRING ${entrypoint} ${name_loc} -1 entrypoint_name)
   list(APPEND TARGET_ENTRYPOINT_NAME_LIST ${entrypoint_name})
 endforeach()
 
-if(LLVM_LIBC_FULL_BUILD)
-  # We need to set up hdrgen first since other targets depend on it.
-  add_subdirectory(utils/LibcTableGenUtil)
-  add_subdirectory(utils/HdrGen)
-endif()
-
 set(LIBC_TARGET)
 set(LIBC_COMPONENT)
 set(LIBC_INSTALL_DEPENDS)
 set(LIBC_INSTALL_TARGET)
 if(LLVM_LIBC_FULL_BUILD)
   set(LIBC_TARGET libc)
   set(LIBC_COMPONENT libc)
   set(LIBC_INSTALL_DEPENDS "install-libc-static-archives;install-libc-headers")

libc/cmake/modules/LLVMLibCHeaderRules.cmake

function(add_gen_header target_name)
 
   set(gen_hdr_script "${LIBC_BUILD_SCRIPTS_DIR}/gen_hdr.py")
 
   file(GLOB td_includes ${LIBC_SOURCE_DIR}/spec/*.td)
 
   set(ENTRYPOINT_NAME_LIST_ARG ${TARGET_ENTRYPOINT_NAME_LIST})
   list(TRANSFORM ENTRYPOINT_NAME_LIST_ARG PREPEND "--e=")
 
+  if(LIBC_HDRGEN_EXE)
+    set(hdrgen_exe ${LIBC_HDRGEN_EXE})
+  else()
+    set(hdrgen_exe ${LIBC_TABLEGEN_EXE})
+    set(hdrgen_deps "${LIBC_TABLEGEN_EXE};${LIBC_TABLEGEN_TARGET}")
+  endif()
   add_custom_command(
     OUTPUT ${out_file}
-    COMMAND ${LIBC_TABLEGEN_EXE} -o ${out_file} --header ${ADD_GEN_HDR_GEN_HDR}
+    COMMAND ${hdrgen_exe} -o ${out_file} --header ${ADD_GEN_HDR_GEN_HDR}
             --def ${in_file} ${replacement_params} -I ${LIBC_SOURCE_DIR}
            ${ENTRYPOINT_NAME_LIST_ARG}
            ${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/api.td
 
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
     DEPENDS ${in_file} ${fq_data_files} ${td_includes}
             ${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/api.td
-            ${LIBC_TABLEGEN_EXE} ${LIBC_TABLEGEN_TARGET}
+            ${hdrgen_deps}
   )
 
   if(ADD_GEN_HDR_DEPENDS)
     get_fq_deps_list(fq_deps_list ${ADD_GEN_HDR_DEPENDS})
     # Dependencies of a add_header target can only be another add_gen_header target
     # or an add_header target.
     foreach(dep IN LISTS fq_deps_list)
       get_target_property(header_file ${dep} HEADER_FILE_PATH)

libc/test/src/CMakeLists.txt

 add_subdirectory(signal)
 add_subdirectory(spawn)
 add_subdirectory(time)
 
 if(${LIBC_TARGET_OS} STREQUAL "linux")
   add_subdirectory(pthread)
 endif()
 
+if(LLVM_RUNTIMES_BUILD OR LIBC_HDRGEN_EXE)
+  # The public API test below uses tablegen to generate the test
+  # source file. Since tablegen is not available during a runtimes
+  # build, we will skip the test.
+  # If a different libc-hdrgen binary is being used, then also we
+  # skip the api-test as we cannot generate the test source file.
+  return()
+endif()
+
 set(public_test ${CMAKE_CURRENT_BINARY_DIR}/public_api_test.cpp)
 
 set(entrypoints_name_list "")
 foreach(entry IN LISTS TARGET_LLVMLIBC_ENTRYPOINTS)
   get_target_property(entry_name ${entry} "ENTRYPOINT_NAME")
   list(APPEND entrypoints_name_list ${entry_name})
 endforeach()