SCons: Refactor LTO options with `lto=<none|thin|full>`

Adds support for LTO on macOS and Android. We don't have much experience
with LTO on these platforms so for now we keep it disabled by default
even when `production=yes` is set.

Similarly for iOS where we ship object files for the user to link in
Xcode so LTO makes builds extremely slow to link.

`production=yes` defaults to full LTO.
ThinLTO is much faster for LLVM-based compilers but seems to produce
bigger binaries (at least for the Web platform).

1 files changed, 32 insertions, 30 deletions

diff --git a/SConstruct b/SConstruct
index ce586010f4..f7504f72e1 100644
--- a/SConstruct
+++ b/SConstruct
@@ -170,7 +170,7 @@ opts.Add(EnumVariable("arch", "CPU architecture", "auto", ["auto"] + architectur
 opts.Add(EnumVariable("float", "Floating-point precision", "32", ("32", "64")))
 opts.Add(EnumVariable("optimize", "Optimization type", "speed", ("speed", "size", "none")))
 opts.Add(BoolVariable("production", "Set defaults to build Godot for use in production", False))
-opts.Add(BoolVariable("use_lto", "Use link-time optimization", False))
+opts.Add(EnumVariable("lto", "Link-time optimization (for production buids)", "none", ("none", "thin", "full")))
 
 # Components
 opts.Add(BoolVariable("deprecated", "Enable compatibility code for deprecated and removed features", True))
@@ -438,35 +438,6 @@ if selected_platform in platform_list:
             )
             env.SetOption("num_jobs", safer_cpu_count)
 
-    # 'dev' and 'production' are aliases to set default options if they haven't been set
-    # manually by the user.
-    if env["dev"]:
-        env["verbose"] = methods.get_cmdline_bool("verbose", True)
-        env["warnings"] = ARGUMENTS.get("warnings", "extra")
-        env["werror"] = methods.get_cmdline_bool("werror", True)
-        if env["tools"]:
-            env["tests"] = methods.get_cmdline_bool("tests", True)
-    if env["production"]:
-        env["use_static_cpp"] = methods.get_cmdline_bool("use_static_cpp", True)
-        env["use_lto"] = methods.get_cmdline_bool("use_lto", True)
-        env["debug_symbols"] = methods.get_cmdline_bool("debug_symbols", False)
-        if not env["tools"] and env["target"] == "debug":
-            print(
-                "WARNING: Requested `production` build with `tools=no target=debug`, "
-                "this will give you a full debug template (use `target=release_debug` "
-                "for an optimized template with debug features)."
-            )
-        if env.msvc:
-            print(
-                "WARNING: For `production` Windows builds, you should use MinGW with GCC "
-                "or Clang instead of Visual Studio, as they can better optimize the "
-                "GDScript VM in a very significant way. MSVC LTO also doesn't work "
-                "reliably for our use case."
-                "If you want to use MSVC nevertheless for production builds, set "
-                "`debug_symbols=no use_lto=no` instead of the `production=yes` option."
-            )
-            Exit(255)
-
     env.extra_suffix = ""
 
     if env["extra_suffix"] != "":
@@ -517,6 +488,37 @@ if selected_platform in platform_list:
         # We apply it to CCFLAGS (both C and C++ code) in case it impacts C features.
         env.Prepend(CCFLAGS=["/std:c++17"])
 
+    # 'dev' and 'production' are aliases to set default options if they haven't been set
+    # manually by the user.
+    if env["dev"]:
+        env["verbose"] = methods.get_cmdline_bool("verbose", True)
+        env["warnings"] = ARGUMENTS.get("warnings", "extra")
+        env["werror"] = methods.get_cmdline_bool("werror", True)
+        if env["tools"]:
+            env["tests"] = methods.get_cmdline_bool("tests", True)
+    if env["production"]:
+        env["use_static_cpp"] = methods.get_cmdline_bool("use_static_cpp", True)
+        env["lto"] = ARGUMENTS.get("lto", "full")
+        env["debug_symbols"] = methods.get_cmdline_bool("debug_symbols", False)
+        if not env["tools"] and env["target"] == "debug":
+            print(
+                "WARNING: Requested `production` build with `tools=no target=debug`, "
+                "this will give you a full debug template (use `target=release_debug` "
+                "for an optimized template with debug features)."
+            )
+        if env.msvc:
+            print(
+                "WARNING: For `production` Windows builds, you should use MinGW with GCC "
+                "or Clang instead of Visual Studio, as they can better optimize the "
+                "GDScript VM in a very significant way. MSVC LTO also doesn't work "
+                "reliably for our use case."
+                "If you want to use MSVC nevertheless for production builds, set "
+                "`debug_symbols=no lto=none` instead of the `production=yes` option."
+            )
+            Exit(255)
+    if env["lto"] != "none":
+        print("Using LTO: " + env["lto"])
+
     # Enforce our minimal compiler version requirements
     cc_version = methods.get_compiler_version(env) or {
         "major": None,