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

[Chris B via cfe-commits]

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@@ -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
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llvm-beanz wrote:

IMO, the PGO use case is already an advanced-user feature. It's even documented in the "Advanced Builds" documentation.

If we want some more complex C++ sources there are some samples we could look at, like Kaleidoscope. The Kaleidoscope chapter 2 source builds without dependencies, and the other ones are also pretty simple to just compile.

If you're not concerned about the efficacy of the PGO build, I'm not sure why the existing single hello_world is a bad default. It isn't great but it is stable and something. If you want something that gives better performance then you must care about the efficacy, and I think you need to be somewhat concerned about the stability of those results, otherwise what's the point?

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