python: pgo-llvm-builder: Build with ThinLTO if performing BOLT

Signed-off-by: Nathan Chancellor <[email protected]>

python/pgo-llvm-builder/build.py

@@ -267,9 +267,11 @@
     #   https://github.com/llvm/llvm-project/issues/55004)
     # - We are on aarch64 with LLVM greater than or equal to 18.x (due to
     #   https://github.com/llvm/llvm-project/issues/71822)
+    # Enable ThinLTO if BOLT is enabled, as it adds more speed gains (but it
+    # appears to regress PGO's wins without BOLT)
     maj_ver = int(VERSION.split('.', 1)[0])
     if (maj_ver >= 16 and MACHINE == 'x86_64') or (maj_ver >= 18 and MACHINE == 'aarch64'):
-        build_cmd.append('--bolt')
+        build_cmd += ['--bolt', '--lto', 'thin']
     subprocess.run(build_cmd, check=True)
 
     llvm_tarball = Path(llvm_install.parent, f"{llvm_install.name}.tar")