[CMake] Add clang-bolt target

This patch adds `CLANG_BOLT_INSTRUMENT` option that applies BOLT instrumentation
to Clang, performs a bootstrap build with the resulting Clang, merges resulting
fdata files into a single profile file, and uses it to perform BOLT optimization
on the original Clang binary.

The projects and targets used for bootstrap/profile collection are configurable via
`CLANG_BOLT_INSTRUMENT_PROJECTS` and `CLANG_BOLT_INSTRUMENT_TARGETS`.
The defaults are "llvm" and "count" respectively, which results in a profile with
~5.3B dynamically executed instructions.

The intended use of the functionality is through BOLT CMake cache file, similar
to PGO 2-stage build:
```
cmake <llvm-project>/llvm -C <llvm-project>/clang/cmake/caches/BOLT.cmake
ninja clang++-bolt # pulls clang-bolt
```

Stats with a recent checkout (clang-16), pre-built BOLT and Clang, 72vCPU/224G
| CMake configure with host Clang + BOLT.cmake | 1m6.592s
| Instrumenting Clang with BOLT | 2m50.508s
| CMake configure `llvm` with instrumented Clang | 5m46.364s (~5x slowdown)
| CMake build `not` with instrumented Clang |0m6.456s
| Merging fdata files | 0m9.439s
| Optimizing Clang with BOLT | 0m39.201s

Building Clang:
```cmake ../llvm-project/llvm -DCMAKE_C_COMPILER=... -DCMAKE_CXX_COMPILER=...
  -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=clang
  -DLLVM_TARGETS_TO_BUILD=Native -GNinja```

| | Release | BOLT-optimized
| cmake | 0m24.016s | 0m22.333s
| ninja clang | 5m55.692s | 4m35.122s

I know it's not rigorous, but shows a ballpark figure.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D132975

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/cmake/caches/BOLT.cmake b/clang/cmake/caches/BOLT.cmake
new file mode 100644
index 000000000000..65444c8044c3
--- /dev/null
+++ b/clang/cmake/caches/BOLT.cmake
@@ -0,0 +1,15 @@
+set(CMAKE_BUILD_TYPE Release CACHE STRING "")
+set(CLANG_BOLT_INSTRUMENT ON CACHE BOOL "")
+set(CLANG_BOLT_INSTRUMENT_PROJECTS "llvm" CACHE STRING "")
+set(CLANG_BOLT_INSTRUMENT_TARGETS "count" CACHE STRING "")
+set(CMAKE_EXE_LINKER_FLAGS "-Wl,--emit-relocs,-znow" CACHE STRING "")
+set(CLANG_BOLT_INSTRUMENT_EXTRA_CMAKE_FLAGS "" CACHE STRING "")
+
+set(LLVM_ENABLE_PROJECTS "bolt;clang" CACHE STRING "")
+set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
+
+# Disable function splitting enabled by default in GCC8+
+if("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fno-reorder-blocks-and-partition")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-reorder-blocks-and-partition")
+endif()