build: Enable LTO on-by-default for CMake and LTO off-by-default for Bazel

.bazelrc

@@ -16,11 +16,11 @@ build --cxxopt "-std=c++20"        --host_cxxopt "-std=c++20"
 build --cxxopt "-xc++"             --host_cxxopt "-xc++"
 build --cxxopt "-DBAZEL_BUILD"     --host_cxxopt "-DBAZEL_BUILD"
 
-# Default to optimized build (-c opt), O3
+# Default to optimized build (-c opt)
 build -c opt
-build:opt --copt=-O3
 
-# Enable Full-LTO (Link Time Optimization)
+# Settings for --config=opt build: Enable -O3 and Full-LTO for performance
+build:opt --copt=-O3
 build:opt --copt=-flto
 build:opt --linkopt=-flto
 

BUILD.bazel

@@ -5,6 +5,7 @@ load("@bazel_skylib//rules:common_settings.bzl", "string_flag")
 load("@openroad_rules_python//python:defs.bzl", "py_library")
 load("@rules_cc//cc:cc_binary.bzl", "cc_binary")
 load("@rules_cc//cc:cc_library.bzl", "cc_library")
+load("//bazel:notification.bzl", "notification_rule")
 load("//bazel:python_wrap_cc.bzl", "PYTHON_STABLE_API_DEFINE", "python_wrap_cc")
 load("//bazel:tcl_encode_or.bzl", "tcl_encode")
 load("//bazel:tcl_wrap_cc.bzl", "tcl_wrap_cc")
@@ -18,6 +19,22 @@ package(
     ],
 )
 
+# Notification target to notify user if --config=opt (with LTO) is not used.
+notification_rule(
+    name = "opt_notification",
+    is_opt = select(
+        {
+            ":lto_on": True,
+            "//conditions:default": False,
+        },
+    ),
+)
+
+config_setting(
+    name = "lto_on",
+    values = {"copt": "-flto"},
+)
+
 exports_files([
     "LICENSE",
     "src/Design.i",
@@ -153,6 +170,7 @@ cc_binary(
     deps = [
         ":openroad_lib",
         ":openroad_version",
+        ":opt_notification",
         ":ord",
         "//bazel:runfiles",
         "//src/cut",

CMakeLists.txt

@@ -21,13 +21,6 @@ cmake_policy(SET CMP0077 NEW)
 # Let AUTOMOC and AUTOUIC process GENERATED files.
 cmake_policy(SET CMP0071 NEW)
 
-# Interfers with Qt so off by default.
-option(LINK_TIME_OPTIMIZATION "Flag to control link time optimization: off by default" OFF)
-
-if (LINK_TIME_OPTIMIZATION)
-set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
-endif()
-
 # Allow to use external shared boost libraries
 option(USE_SYSTEM_BOOST "Use system shared Boost libraries" OFF)
 

bazel/notification.bzl

@@ -0,0 +1,24 @@
+# SPDX-License-Identifier: BSD-3-Clause
+# Copyright (c) 2025, Precision Innovations Inc.
+
+"""Rule to provide build-time notifications."""
+
+load("@rules_cc//cc/common:cc_info.bzl", "CcInfo")
+
+def _notification_impl(ctx):
+    if not ctx.attr.is_opt:
+        # buildifier: disable=print
+        print("\n" + "=" * 80 + "\n" +
+              "  NOTIFICATION: Use --config=opt to build with LTO (Link Time Optimization)\n" +
+              "                and get better performance at the expense of longer build time.\n" +
+              "=" * 80 + "\n")
+    return [CcInfo()]
+
+notification_rule = rule(
+    implementation = _notification_impl,
+    attrs = {
+        # Set to True when the build configuration enables LTO (via --config=opt).
+        # When True, the notification message is suppressed.
+        "is_opt": attr.bool(default = False),
+    },
+)