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).

7 files changed, 94 insertions, 60 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,
diff --git a/platform/android/detect.py b/platform/android/detect.py
index ad63821162..1d9bcdd932 100644
--- a/platform/android/detect.py
+++ b/platform/android/detect.py
@@ -47,6 +47,9 @@ def get_flags():
     return [
         ("arch", "arm64"),  # Default for convenience.
         ("tools", False),
+        # Benefits of LTO for Android (size, performance) haven't been clearly established yet.
+        # So for now we override the default value which may be set when using `production=yes`.
+        ("lto", "none"),
     ]
 
 
@@ -132,6 +135,15 @@ def configure(env):
         env.Append(CPPDEFINES=["_DEBUG"])
         env.Append(CPPFLAGS=["-UNDEBUG"])
 
+    # LTO
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        else:
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
+
     # Compiler configuration
 
     env["SHLIBSUFFIX"] = ".so"
diff --git a/platform/ios/detect.py b/platform/ios/detect.py
index 1a8d24d12d..3cfb25cf61 100644
--- a/platform/ios/detect.py
+++ b/platform/ios/detect.py
@@ -39,6 +39,9 @@ def get_flags():
         ("arch", "arm64"),  # Default for convenience.
         ("tools", False),
         ("use_volk", False),
+        # Disable by default even if production is set, as it makes linking in Xcode
+        # on exports very slow and that's not what most users expect.
+        ("lto", "none"),
     ]
 
 
@@ -70,9 +73,14 @@ def configure(env):
         env.Append(CCFLAGS=["-gdwarf-2", "-O0"])
         env.Append(CPPDEFINES=["_DEBUG", ("DEBUG", 1)])
 
-    if env["use_lto"]:
-        env.Append(CCFLAGS=["-flto"])
-        env.Append(LINKFLAGS=["-flto"])
+    # LTO
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        else:
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
 
     ## Compiler configuration
 
diff --git a/platform/linuxbsd/detect.py b/platform/linuxbsd/detect.py
index f5f7e65417..36644d5f29 100644
--- a/platform/linuxbsd/detect.py
+++ b/platform/linuxbsd/detect.py
@@ -31,7 +31,6 @@ def get_opts():
     return [
         EnumVariable("linker", "Linker program", "default", ("default", "bfd", "gold", "lld", "mold")),
         BoolVariable("use_llvm", "Use the LLVM compiler", False),
-        BoolVariable("use_thinlto", "Use ThinLTO (LLVM only, requires linker=lld, implies use_lto=yes)", False),
         BoolVariable("use_static_cpp", "Link libgcc and libstdc++ statically for better portability", True),
         BoolVariable("use_coverage", "Test Godot coverage", False),
         BoolVariable("use_ubsan", "Use LLVM/GCC compiler undefined behavior sanitizer (UBSAN)", False),
@@ -129,13 +128,6 @@ def configure(env):
         else:
             env.Append(LINKFLAGS=["-fuse-ld=%s" % env["linker"]])
 
-    if env["use_thinlto"]:
-        if not env["use_llvm"] or env["linker"] != "lld":
-            print("ThinLTO is only compatible with LLVM and the LLD linker, use `use_llvm=yes linker=lld`.")
-            sys.exit(255)
-        else:
-            env["use_lto"] = True  # ThinLTO implies LTO
-
     if env["use_coverage"]:
         env.Append(CCFLAGS=["-ftest-coverage", "-fprofile-arcs"])
         env.Append(LINKFLAGS=["-ftest-coverage", "-fprofile-arcs"])
@@ -178,8 +170,12 @@ def configure(env):
             env.Append(CCFLAGS=["-fsanitize-recover=memory"])
             env.Append(LINKFLAGS=["-fsanitize=memory"])
 
-    if env["use_lto"]:
-        if env["use_thinlto"]:
+    # LTO
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            if not env["use_llvm"]:
+                print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+                sys.exit(255)
             env.Append(CCFLAGS=["-flto=thin"])
             env.Append(LINKFLAGS=["-flto=thin"])
         elif not env["use_llvm"] and env.GetOption("num_jobs") > 1:
diff --git a/platform/macos/detect.py b/platform/macos/detect.py
index bd89e8d59f..bcf4776609 100644
--- a/platform/macos/detect.py
+++ b/platform/macos/detect.py
@@ -40,6 +40,9 @@ def get_flags():
     return [
         ("arch", detect_arch()),
         ("use_volk", False),
+        # Benefits of LTO for macOS (size, performance) haven't been clearly established yet.
+        # So for now we override the default value which may be set when using `production=yes`.
+        ("lto", "none"),
     ]
 
 
@@ -166,6 +169,15 @@ def configure(env):
         env["RANLIB"] = basecmd + "ranlib"
         env["AS"] = basecmd + "as"
 
+    # LTO
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        else:
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
+
     if env["use_ubsan"] or env["use_asan"] or env["use_tsan"]:
         env.extra_suffix += ".san"
         env.Append(CCFLAGS=["-DSANITIZERS_ENABLED"])
diff --git a/platform/web/detect.py b/platform/web/detect.py
index b1c1dd48a9..e055af8400 100644
--- a/platform/web/detect.py
+++ b/platform/web/detect.py
@@ -31,7 +31,6 @@ def get_opts():
     return [
         ("initial_memory", "Initial WASM memory (in MiB)", 32),
         BoolVariable("use_assertions", "Use Emscripten runtime assertions", False),
-        BoolVariable("use_thinlto", "Use ThinLTO", False),
         BoolVariable("use_ubsan", "Use Emscripten undefined behavior sanitizer (UBSAN)", False),
         BoolVariable("use_asan", "Use Emscripten address sanitizer (ASAN)", False),
         BoolVariable("use_lsan", "Use Emscripten leak sanitizer (LSAN)", False),
@@ -110,12 +109,13 @@ def configure(env):
     env["ENV"] = os.environ
 
     # LTO
-    if env["use_thinlto"]:
-        env.Append(CCFLAGS=["-flto=thin"])
-        env.Append(LINKFLAGS=["-flto=thin"])
-    elif env["use_lto"]:
-        env.Append(CCFLAGS=["-flto=full"])
-        env.Append(LINKFLAGS=["-flto=full"])
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        else:
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
 
     # Sanitizers
     if env["use_ubsan"]:
diff --git a/platform/windows/detect.py b/platform/windows/detect.py
index 5607eab342..e6e1874fc0 100644
--- a/platform/windows/detect.py
+++ b/platform/windows/detect.py
@@ -191,7 +191,6 @@ def get_opts():
         ),
         BoolVariable("use_mingw", "Use the Mingw compiler, even if MSVC is installed.", False),
         BoolVariable("use_llvm", "Use the LLVM compiler", False),
-        BoolVariable("use_thinlto", "Use ThinLTO", False),
         BoolVariable("use_static_cpp", "Link MinGW/MSVC C++ runtime libraries statically", True),
         BoolVariable("use_asan", "Use address sanitizer (ASAN)", False),
     ]
@@ -449,7 +448,10 @@ def configure_msvc(env, vcvars_msvc_config):
 
     ## LTO
 
-    if env["use_lto"]:
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+            sys.exit(255)
         env.AppendUnique(CCFLAGS=["/GL"])
         env.AppendUnique(ARFLAGS=["/LTCG"])
         if env["progress"]:
@@ -562,17 +564,19 @@ def configure_mingw(env):
         if try_cmd("gcc-ranlib --version", env["mingw_prefix"], env["arch"]):
             env["RANLIB"] = mingw_bin_prefix + "gcc-ranlib"
 
-    if env["use_lto"]:
-        if not env["use_llvm"] and env.GetOption("num_jobs") > 1:
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            if not env["use_llvm"]:
+                print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+                sys.exit(255)
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        elif not env["use_llvm"] and env.GetOption("num_jobs") > 1:
             env.Append(CCFLAGS=["-flto"])
             env.Append(LINKFLAGS=["-flto=" + str(env.GetOption("num_jobs"))])
         else:
-            if env["use_thinlto"]:
-                env.Append(CCFLAGS=["-flto=thin"])
-                env.Append(LINKFLAGS=["-flto=thin"])
-            else:
-                env.Append(CCFLAGS=["-flto"])
-                env.Append(LINKFLAGS=["-flto"])
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
 
     env.Append(LINKFLAGS=["-Wl,--stack," + str(STACK_SIZE)])