[clang] [clang][cmake] Apply bolt optimizations as part of the clang target (PR #119896)

[Petr Hosek via cfe-commits]

================
@@ -134,3 +147,42 @@ if(CLANG_ORDER_FILE AND
     set_target_properties(clang PROPERTIES LINK_DEPENDS ${CLANG_ORDER_FILE})
   endif()
 endif()
+
+if (CLANG_BOLT AND NOT LLVM_BUILD_INSTRUMENTED)
+  # Add a clang-bolt target for backwards compatibility.
+  add_custom_target(clang-bolt DEPENDS clang)
+
+  set(CLANG_BOLT_INSTRUMENTED "clang-bolt.inst" CACHE STRING
+    "Name of BOLT-instrumented Clang binary")
+  set(CLANG_INSTRUMENTED ${LLVM_RUNTIME_OUTPUT_INTDIR}/${CLANG_BOLT_INSTRUMENTED})
+  set(PERF_TRAINING_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/../../utils/perf-training)
+  set(BOLT_FDATA ${PERF_TRAINING_BINARY_DIR}/prof.fdata)
+  get_llvm_lit_path(
+    lit_base_dir
+    lit_file_name
+    ALLOW_EXTERNAL
+  )
+  set(LIT_COMMAND "${lit_base_dir}/${lit_file_name}")
+
+  # This POST_BUILD command is executed unconditionally even if the clang target
+  # is already built.  We need to wrap the whole bolt optimization process in
+  # a single python wrapper, so that we can first check if the binary has
+  # already been optimized and then exit early with a 0 status if it has.
+  add_custom_command(
+    TARGET clang POST_BUILD
+    COMMAND  "${Python3_EXECUTABLE}" ${CMAKE_CURRENT_SOURCE_DIR}/../../utils/perf-training/perf-helper.py
+             bolt-optimize
+	     --method ${CLANG_BOLT}
+	     --input $<TARGET_FILE:clang>
+	     --instrumented-output ${CLANG_INSTRUMENTED}
+	     --fdata ${BOLT_FDATA}
+	     --perf-training-binary-dir ${PERF_TRAINING_BINARY_DIR}
+	     --readelf $<TARGET_FILE:llvm-readobj>
----------------
petrhosek wrote:

```suggestion
	     --readelf $<TARGET_FILE:llvm-readelf>
```

https://github.com/llvm/llvm-project/pull/119896