[llvm] [clang] [CMake][PGO] Build Sema.cpp to generate profdata for PGO builds (PR #77347)

[Tom Stellard via cfe-commits]

================
@@ -26,9 +30,23 @@ 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 "${Python3_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} ${CMAKE_BINARY_DIR}/profiles/
       COMMENT "Merging profdata"
       DEPENDS generate-profraw)
+    if (CLANG_PGO_TRAINING_DATA_SOURCE_DIR)
+      llvm_ExternalProject_Add(generate-profraw-external ${CLANG_PGO_TRAINING_DATA_SOURCE_DIR}
+              USE_TOOLCHAIN EXLUDE_FROM_ALL NO_INSTALL DEPENDS generate-profraw)
+      add_dependencies(generate-profdata generate-profraw-external)
+    else()
+      # Default to compiling a file from clang. This also builds all the
+      # dependencies needed to build this file, like TableGen.
+      set(generate_profraw_clang_sema tools/clang/lib/Sema/CMakeFiles/obj.clangSema.dir/Sema.cpp.o)
+      llvm_ExternalProject_Add(generate-profraw-clang ${CMAKE_CURRENT_SOURCE_DIR}/../../../llvm
+              USE_TOOLCHAIN EXLUDE_FROM_ALL NO_INSTALL DEPENDS generate-profraw
+              EXTRA_TARGETS generate_profraw_clang_sema
----------------
tstellar wrote:

> The idea I'd like explore is using libc++ (and perhaps also libc) test suite for training. It consists of self-contained test programs which should sufficiently exercise various aspects of C/C++ frontend.

I've updated the patch to use the check-cxx target to generate the profile data.  The gains (~20%) are less than with Sema.cpp (~34%) but still pretty good.  The only issue is that the profile data comes from building libcxx as well, so it's not just from the test suite.

It may be possible to get test data from just running the test suite, but that might be a lot more complicated to set up.


https://github.com/llvm/llvm-project/pull/77347