[clang][perf-training] Support excluding LLVM build from PGO training (#126876)

Using LLVM build itself for PGO training is convenient and a great
starting point but it also has several issues:

* LLVM build implicitly depends on tools other than CMake and C/C++
compiler and if those tools aren't available in PATH, the build will
fail.
* LLVM build also requires standard headers and libraries which may not
always be available in the default location requiring an explicit
sysroot.
* Building a single configuration (-DCMAKE_BUILD_TYPE=Release) only
exercises the -O3 pipeline and can pesimize other configurations.
* Building for the host target doesn't exercise all other targets.
* Since LLVMSupport is a static library, this doesn't exercise the
linker (beyond what the CMake itself does).

Rather than using LLVM build, ideally we would provide a more minimal,
purpose built corpus. While we're working on building such a corpus,
provide a CMake option that lets vendors disable the use LLVM build for
PGO training.

Author: petrhosek

clang/utils/perf-training/CMakeLists.txt

@@ -6,6 +6,12 @@ set(CLANG_PGO_TRAINING_DATA "${CMAKE_CURRENT_SOURCE_DIR}" CACHE PATH
 set(CLANG_PGO_TRAINING_DATA_SOURCE_DIR OFF CACHE STRING "Path to source directory containing cmake project with source files to use for generating pgo data")
 set(CLANG_PGO_TRAINING_DEPS "" CACHE STRING "Extra dependencies needed to build the PGO training data.")
 
+option(CLANG_PGO_TRAINING_USE_LLVM_BUILD "Use LLVM build for generating PGO data" ON)
+
+llvm_canonicalize_cmake_booleans(
+  CLANG_PGO_TRAINING_USE_LLVM
+)
+
 if(LLVM_BUILD_INSTRUMENTED)
   configure_lit_site_cfg(
     ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in

clang/utils/perf-training/lit.cfg

@@ -27,6 +27,9 @@ config.clang = lit.util.which('clang', config.clang_tools_dir).replace('\\', '/'
 config.name = 'Clang Perf Training'
 config.suffixes = ['.c', '.cc', '.cpp', '.m', '.mm', '.cu', '.ll', '.cl', '.s', '.S', '.modulemap', '.test']
 
+if not config.use_llvm_build:
+    config.excludes = ['llvm-support']
+
 cc1_wrapper = '%s %s/perf-helper.py cc1' % (config.python_exe, config.perf_helper_dir)
 
 use_lit_shell = os.environ.get("LIT_USE_INTERNAL_SHELL")

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

@@ -11,6 +11,7 @@ config.python_exe = "@Python3_EXECUTABLE@"
 config.cmake_exe = "@CMAKE_COMMAND@"
 config.llvm_src_dir ="@CMAKE_SOURCE_DIR@"
 config.cmake_generator ="@CMAKE_GENERATOR@"
+config.use_llvm_build = @CLANG_PGO_TRAINING_USE_LLVM_BUILD@
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@CLANG_SOURCE_DIR@/utils/perf-training/lit.cfg")