build: use find_package(Python3) if available

This is primarily motivated by the desire to move from Python2 to
Python3.  `PYTHON_EXECUTABLE` is ambiguous.  This explicitly identifies
the python interpreter in use.  Since the LLVM build seems to be able to
completed successfully with python3, use that across the build.  The old
path aliases `PYTHON_EXECUTABLE` to be treated as Python3.

clang-tools-extra/test/lit.site.cfg.py.in

@@ -8,7 +8,7 @@ config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
 config.clang_tools_binary_dir = "@CLANG_TOOLS_BINARY_DIR@"
 config.clang_tools_dir = "@CLANG_TOOLS_DIR@"
 config.clang_libs_dir = "@SHLIBDIR@"
-config.python_executable = "@PYTHON_EXECUTABLE@"
+config.python_executable = "@Python3_EXECUTABLE@"
 config.target_triple = "@TARGET_TRIPLE@"
 config.clang_staticanalyzer = @CLANG_ENABLE_STATIC_ANALYZER@
 

clang/CMakeLists.txt

@@ -130,16 +130,38 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
 
   if(LLVM_INCLUDE_TESTS)
-    include(FindPythonInterp)
-    if(NOT PYTHONINTERP_FOUND)
-      message(FATAL_ERROR
-"Unable to find Python interpreter, required for builds and testing.
+    if(CMAKE_VERSION VERSION_LESS 3.12)
+      include(FindPythonInterp)
+      if(NOT PYTHONINTERP_FOUND)
+        message(FATAL_ERROR
+  "Unable to find Python interpreter, required for builds and testing.
 
-Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
-    endif()
+  Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
+      endif()
+
+      if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
+        message(FATAL_ERROR "Python 2.7 or newer is required")
+      endif()
 
-    if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
-      message(FATAL_ERROR "Python 2.7 or newer is required")
+      add_executable(Python3::Interpreter IMPORTED)
+      set_target_properties(Python3::Interpreter PROPERTIES
+        IMPORTED_LOCATION ${PYTHON_EXECUTABLE})
+      set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
+    else()
+      find_package(Python3 COMPONENTS Interpreter)
+      if(NOT Python3_Interpreter_FOUND)
+        message(WARNING "Python3 not found, using python2 as a fallback")
+        find_package(Python2 COMPONENTS Interpreter REQUIRED)
+        if(Python2_VERSION VERSION_LESS 2.7)
+          message(SEND_ERROR "Python 2.7 or newer is required")
+        endif()
+
+        # Treat python2 as python3
+        add_executable(Python3::Interpreter IMPORTED)
+        set_target_properties(Python3::Interpreter PROPERTIES
+          IMPORTED_LOCATION ${Python2_EXECUTABLE})
+        set(Python3_EXECUTABLE ${Python2_EXECUTABLE})
+      endif()
     endif()
 
     # Check prebuilt llvm/utils.

clang/bindings/python/tests/CMakeLists.txt

@@ -3,7 +3,7 @@
 add_custom_target(check-clang-python
     COMMAND ${CMAKE_COMMAND} -E env
             CLANG_LIBRARY_PATH=$<TARGET_FILE_DIR:libclang>
-            ${PYTHON_EXECUTABLE} -m unittest discover
+            "${Python3_EXECUTABLE}" -m unittest discover
     DEPENDS libclang
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/..)
 

clang/test/lit.site.cfg.py.in

@@ -27,7 +27,7 @@ config.enable_backtrace = @ENABLE_BACKTRACES@
 config.enable_experimental_new_pass_manager = @ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER@
 config.enable_threads = @LLVM_ENABLE_THREADS@
 config.host_arch = "@HOST_ARCH@"
-config.python_executable = "@PYTHON_EXECUTABLE@"
+config.python_executable = "@Python3_EXECUTABLE@"
 config.use_z3_solver = lit_config.params.get('USE_Z3_SOLVER', "@USE_Z3_SOLVER@")
 config.has_plugins = @LLVM_ENABLE_PLUGINS@
 

clang/utils/perf-training/CMakeLists.txt

@@ -23,7 +23,7 @@ if(LLVM_BUILD_INSTRUMENTED)
     )
 
   add_custom_target(clear-profraw
-    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} profraw
+    COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} profraw
     COMMENT "Clearing old profraw data")
 
   if(NOT LLVM_PROFDATA)
@@ -34,7 +34,7 @@ if(LLVM_BUILD_INSTRUMENTED)
     message(STATUS "To enable merging PGO data LLVM_PROFDATA has to point to llvm-profdata")
   else()
     add_custom_target(generate-profdata
-      COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py merge ${LLVM_PROFDATA} ${CMAKE_CURRENT_BINARY_DIR}/clang.profdata ${CMAKE_CURRENT_BINARY_DIR}
+      COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py merge ${LLVM_PROFDATA} ${CMAKE_CURRENT_BINARY_DIR}/clang.profdata ${CMAKE_CURRENT_BINARY_DIR}
       COMMENT "Merging profdata"
       DEPENDS generate-profraw)
   endif()
@@ -55,15 +55,15 @@ if(APPLE AND DTRACE)
     )
 
   add_custom_target(clear-dtrace-logs
-    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} dtrace
+    COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} dtrace
     COMMENT "Clearing old dtrace data")
 
   if(NOT CLANG_ORDER_FILE)
     message(FATAL_ERROR "Output clang order file is not set")
   endif()
 
   add_custom_target(generate-order-file
-    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py gen-order-file --binary $<TARGET_FILE:clang> --output ${CLANG_ORDER_FILE} ${CMAKE_CURRENT_BINARY_DIR}
+    COMMAND "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py gen-order-file --binary $<TARGET_FILE:clang> --output ${CLANG_ORDER_FILE} ${CMAKE_CURRENT_BINARY_DIR}
     COMMENT "Generating order file"
     DEPENDS generate-dtrace-logs)
 endif()

clang/utils/perf-training/lit.site.cfg.in

@@ -7,7 +7,7 @@ config.perf_helper_dir = "@CMAKE_CURRENT_SOURCE_DIR@"
 config.test_exec_root = "@CMAKE_CURRENT_BINARY_DIR@"
 config.test_source_root = "@CLANG_PGO_TRAINING_DATA@"
 config.target_triple = "@TARGET_TRIPLE@"
-config.python_exe = "@PYTHON_EXECUTABLE@"
+config.python_exe = "@Python3_EXECUTABLE@"
 
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.

clang/utils/perf-training/order-files.lit.site.cfg.in

@@ -7,7 +7,7 @@ config.perf_helper_dir = "@CMAKE_CURRENT_SOURCE_DIR@"
 config.test_exec_root = "@CMAKE_CURRENT_BINARY_DIR@"
 config.test_source_root = "@CLANG_PGO_TRAINING_DATA@"
 config.target_triple = "@TARGET_TRIPLE@"
-config.python_exe = "@PYTHON_EXECUTABLE@"
+config.python_exe = "@Python3_EXECUTABLE@"
 
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.

llvm/CMakeLists.txt

@@ -672,16 +672,38 @@ option(LLVM_ENABLE_PLUGINS "Enable plugin support" ${LLVM_ENABLE_PLUGINS_default
 
 include(HandleLLVMOptions)
 
-include(FindPythonInterp)
-if( NOT PYTHONINTERP_FOUND )
-  message(FATAL_ERROR
-"Unable to find Python interpreter, required for builds and testing.
+if(CMAKE_VERSION VERSION_LESS 3.12)
+  include(FindPythonInterp)
+  if( NOT PYTHONINTERP_FOUND )
+    message(FATAL_ERROR
+  "Unable to find Python interpreter, required for builds and testing.
 
-Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
-endif()
+  Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
+  endif()
+
+  if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
+    message(FATAL_ERROR "Python 2.7 or newer is required")
+  endif()
 
-if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
-  message(FATAL_ERROR "Python 2.7 or newer is required")
+  add_executable(Python3::Interpreter IMPORTED)
+  set_target_properties(Python3::Interpreter PROPERTIES
+    IMPORTED_LOCATION ${PYTHON_EXECUTABLE})
+  set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
+else()
+  find_package(Python3 COMPONENTS Interpreter)
+  if(NOT Python3_Interpreter_FOUND)
+    message(WARNING "Python3 not found, using python2 as a fallback")
+    find_package(Python2 COMPONENTS Interpreter REQUIRED)
+    if(Python2_VERSION VERSION_LESS 2.7)
+      message(SEND_ERROR "Python 2.7 or newer is required")
+    endif()
+
+    # Treat python2 as python3
+    add_executable(Python3::Interpreter IMPORTED)
+    set_target_properties(Python3::Interpreter PROPERTIES
+      IMPORTED_LOCATION ${Python2_EXECUTABLE})
+    set(Python3_EXECUTABLE ${Python2_EXECUTABLE})
+  endif()
 endif()
 
 ######
@@ -717,7 +739,7 @@ endif (LLVM_USE_PERF)
 
 message(STATUS "Constructing LLVMBuild project information")
 execute_process(
-  COMMAND ${PYTHON_EXECUTABLE} -B ${LLVMBUILDTOOL}
+  COMMAND "${Python3_EXECUTABLE}" -B ${LLVMBUILDTOOL}
             --native-target "${LLVM_NATIVE_ARCH}"
             --enable-targets "${LLVM_TARGETS_TO_BUILD}"
             --enable-optional-components "${LLVMOPTIONALCOMPONENTS}"

llvm/cmake/config-ix.cmake

@@ -639,7 +639,7 @@ function(find_python_module module)
     return()
   endif()
 
-  execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c" "import ${module}"
+  execute_process(COMMAND "${Python3_EXECUTABLE}" "-c" "import ${module}"
     RESULT_VARIABLE status
     ERROR_QUIET)
 

llvm/cmake/modules/AddLLVM.cmake

@@ -117,7 +117,7 @@ function(add_llvm_symbol_exports target_name export_file)
     set(native_export_file "${target_name}.def")
 
     add_custom_command(OUTPUT ${native_export_file}
-      COMMAND ${PYTHON_EXECUTABLE} -c "import sys;print(''.join(['EXPORTS\\n']+sys.stdin.readlines(),))"
+      COMMAND "${Python3_EXECUTABLE}" -c "import sys;print(''.join(['EXPORTS\\n']+sys.stdin.readlines(),))"
         < ${export_file} > ${native_export_file}
       DEPENDS ${export_file}
       VERBATIM
@@ -1046,7 +1046,7 @@ function(export_executable_symbols target)
       set(mangling itanium)
     endif()
     add_custom_command(OUTPUT ${exported_symbol_file}
-                       COMMAND ${PYTHON_EXECUTABLE} ${LLVM_MAIN_SRC_DIR}/utils/extract_symbols.py --mangling=${mangling} ${static_libs} -o ${exported_symbol_file}
+                       COMMAND "${Python3_EXECUTABLE}" ${LLVM_MAIN_SRC_DIR}/utils/extract_symbols.py --mangling=${mangling} ${static_libs} -o ${exported_symbol_file}
                        WORKING_DIRECTORY ${LLVM_LIBRARY_OUTPUT_INTDIR}
                        DEPENDS ${LLVM_MAIN_SRC_DIR}/utils/extract_symbols.py ${static_libs}
                        VERBATIM
@@ -1447,7 +1447,7 @@ function(make_paths_relative out_pathlist basedir pathlist)
   # empty list entries. So escape the ;s in the list and do the splitting
   # ourselves. cmake has no relpath function, so use Python for that.
   string(REPLACE ";" "\\;" pathlist_escaped "${pathlist}")
-  execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c" "\n
+  execute_process(COMMAND "${Python3_EXECUTABLE}" "-c" "\n
 import os, sys\n
 base = sys.argv[1]
 def haslink(p):\n
@@ -1522,7 +1522,6 @@ function(configure_lit_site_cfg site_in site_out)
   # SHLIBDIR points the build tree.
   string(REPLACE "${CMAKE_CFG_INTDIR}" "${LLVM_BUILD_MODE}" SHLIBDIR "${LLVM_SHLIB_OUTPUT_INTDIR}")
 
-  set(PYTHON_EXECUTABLE ${PYTHON_EXECUTABLE})
   # FIXME: "ENABLE_SHARED" doesn't make sense, since it is used just for
   # plugins. We may rename it.
   if(LLVM_ENABLE_PLUGINS)
@@ -1680,7 +1679,7 @@ function(add_lit_target target comment)
     ALLOW_EXTERNAL
     )
 
-  set(LIT_COMMAND "${PYTHON_EXECUTABLE};${lit_base_dir}/${lit_file_name}")
+  set(LIT_COMMAND "${Python3_EXECUTABLE};${lit_base_dir}/${lit_file_name}")
   list(APPEND LIT_COMMAND ${LIT_ARGS})
   foreach(param ${ARG_PARAMS})
     list(APPEND LIT_COMMAND --param ${param})

llvm/test/lit.site.cfg.py.in

@@ -12,7 +12,7 @@ config.llvm_shlib_dir = path(r"@SHLIBDIR@")
 config.llvm_shlib_ext = "@SHLIBEXT@"
 config.llvm_exe_ext = "@EXEEXT@"
 config.lit_tools_dir = path(r"@LLVM_LIT_TOOLS_DIR@")
-config.python_executable = "@PYTHON_EXECUTABLE@"
+config.python_executable = "@Python3_EXECUTABLE@"
 config.gold_executable = "@GOLD_EXECUTABLE@"
 config.ld64_executable = "@LD64_EXECUTABLE@"
 config.ocamlfind_executable = "@OCAMLFIND@"

llvm/tools/llvm-shlib/CMakeLists.txt

@@ -164,7 +164,7 @@ if(LLVM_BUILD_LLVM_C_DYLIB AND MSVC)
   endif()
 
   add_custom_command(OUTPUT ${LLVM_EXPORTED_SYMBOL_FILE}
-    COMMAND ${PYTHON_EXECUTABLE} ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm "${llvm_nm}" -o ${LLVM_EXPORTED_SYMBOL_FILE}
+    COMMAND "${Python3_EXECUTABLE}" ${GEN_SCRIPT} --libsfile ${LIBSFILE} ${GEN_UNDERSCORE} --nm "${llvm_nm}" -o ${LLVM_EXPORTED_SYMBOL_FILE}
     DEPENDS ${LIB_NAMES} ${llvm_nm_target}
     COMMENT "Generating export list for LLVM-C"
     VERBATIM )