Add lto scons option, defaulting to "auto" ("full" in production, "none" in dev builds).

tools/android.py

@@ -120,4 +120,9 @@ def generate(env):
 
     env.Append(CPPDEFINES=["ANDROID_ENABLED", "UNIX_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/android/detect.py
+    # LTO benefits for Android (size, performance) haven't been clearly established yet.
+    if env["lto"] == "auto":
+        env["lto"] = "none"
+
     common_compiler_flags.generate(env)

tools/common_compiler_flags.py

@@ -22,6 +22,10 @@ def exists(env):
 
 
 def generate(env):
+    assert env["lto"] in ["thin", "full", "none"], "Unrecognized lto: {}".format(env["lto"])
+    if env["lto"] != "none":
+        print("Using LTO: " + env["lto"])
+
     # Require C++17
     if env.get("is_msvc", False):
         env.Append(CXXFLAGS=["/std:c++17"])
@@ -64,6 +68,22 @@ def generate(env):
             env.Append(LINKFLAGS=["/OPT:REF"])
         elif env["optimize"] == "debug" or env["optimize"] == "none":
             env.Append(CCFLAGS=["/Od"])
+
+        if env["lto"] == "thin":
+            if not env["use_llvm"]:
+                print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+                env.Exit(255)
+
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        elif env["lto"] == "full":
+            if env["use_llvm"]:
+                env.Append(CCFLAGS=["-flto"])
+                env.Append(LINKFLAGS=["-flto"])
+            else:
+                env.AppendUnique(CCFLAGS=["/GL"])
+                env.AppendUnique(ARFLAGS=["/LTCG"])
+                env.AppendUnique(LINKFLAGS=["/LTCG"])
     else:
         if env["debug_symbols"]:
             # Adding dwarf-4 explicitly makes stacktraces work with clang builds,
@@ -91,3 +111,13 @@ def generate(env):
             env.Append(CCFLAGS=["-Og"])
         elif env["optimize"] == "none":
             env.Append(CCFLAGS=["-O0"])
+
+        if env["lto"] == "thin":
+            if (env["platform"] == "windows" or env["platform"] == "linux") and not env["use_llvm"]:
+                print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+                env.Exit(255)
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        elif env["lto"] == "full":
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])

tools/godotcpp.py

@@ -326,6 +326,14 @@ def options(opts, env):
             ("none", "custom", "debug", "speed", "speed_trace", "size"),
         )
     )
+    opts.Add(
+        EnumVariable(
+            "lto",
+            "Link-time optimization",
+            "none",
+            ("none", "auto", "thin", "full"),
+        )
+    )
     opts.Add(BoolVariable("debug_symbols", "Build with debugging symbols", True))
     opts.Add(BoolVariable("dev_build", "Developer build with dev-only debugging code (DEV_ENABLED)", False))
     opts.Add(BoolVariable("verbose", "Enable verbose output for the compilation", False))

tools/ios.py

@@ -97,4 +97,9 @@ def generate(env):
 
     env.Append(CPPDEFINES=["IOS_ENABLED", "UNIX_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/ios/detect.py:
+    # Disable by default as it makes linking in Xcode very slow.
+    if env["lto"] == "auto":
+        env["lto"] = "none"
+
     common_compiler_flags.generate(env)

tools/linux.py

@@ -39,4 +39,8 @@ def generate(env):
 
     env.Append(CPPDEFINES=["LINUX_ENABLED", "UNIX_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/linuxbsd/detect.py
+    if env["lto"] == "auto":
+        env["lto"] = "full"
+
     common_compiler_flags.generate(env)

tools/macos.py

@@ -73,4 +73,9 @@ def generate(env):
 
     env.Append(CPPDEFINES=["MACOS_ENABLED", "UNIX_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/macos/detect.py
+    # LTO benefits for macOS (size, performance) haven't been clearly established yet.
+    if env["lto"] == "auto":
+        env["lto"] = "none"
+
     common_compiler_flags.generate(env)

tools/web.py

@@ -48,4 +48,8 @@ def generate(env):
 
     env.Append(CPPDEFINES=["WEB_ENABLED", "UNIX_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/web/detect.py
+    if env["lto"] == "auto":
+        env["lto"] = "full"
+
     common_compiler_flags.generate(env)

tools/windows.py

@@ -199,4 +199,12 @@ def generate(env):
 
     env.Append(CPPDEFINES=["WINDOWS_ENABLED"])
 
+    # Refer to https://github.com/godotengine/godot/blob/master/platform/windows/detect.py
+    if env["lto"] == "auto":
+        if env.get("is_msvc", False):
+            # No LTO by default for MSVC, doesn't help.
+            env["lto"] = "none"
+        else:  # Release
+            env["lto"] = "full"
+
     common_compiler_flags.generate(env)