SCons: Disable C++ exception handling

Upon investigating the extremely slow MSVC build times in godotengine#80513, I noticed
that while Godot policy is to never use exceptions, we weren't enforcing it
with compiler flags, and thus still included exception handling code and
stack unwinding.

This is wasteful on multiple aspects:

- Binary size: Around 20% binary size reduction with exceptions disabled
  for both MSVC and GCC binaries.
- Compile time:
  * More than 50% build time reduction with MSVC.
  * 10% to 25% build time reduction with GCC + LTO.
- Performance: Possibly, needs to be benchmarked.

Since users may want to re-enable exceptions in their own thirdparty code
or the libraries they compile with Godot, this behavior can be toggled with
the `disable_exceptions` SCons option, which defaults to true.

(cherry picked from commit 3907e53)

SConstruct

@@ -193,6 +193,7 @@ opts.Add(BoolVariable("vulkan", "Enable the vulkan rendering driver", True))
 opts.Add(BoolVariable("opengl3", "Enable the OpenGL/GLES3 rendering driver", True))
 opts.Add(BoolVariable("openxr", "Enable the OpenXR driver", True))
 opts.Add(BoolVariable("use_volk", "Use the volk library to load the Vulkan loader dynamically", True))
+opts.Add(BoolVariable("disable_exceptions", "Force disabling exception handling code", False))
 opts.Add("custom_modules", "A list of comma-separated directory paths containing custom modules to build.", "")
 opts.Add(BoolVariable("custom_modules_recursive", "Detect custom modules recursively for each specified path.", True))
 
@@ -700,6 +701,16 @@ if selected_platform in platform_list:
             )
             Exit(255)
 
+    # Disable exception handling. Godot doesn't use exceptions anywhere, and this
+    # saves around 20% of binary size and very significant build time (GH-80513).
+    if env["disable_exceptions"]:
+        if env.msvc:
+            env.Append(CPPDEFINES=[("_HAS_EXCEPTIONS", 0)])
+        else:
+            env.Append(CCFLAGS=["-fno-exceptions"])
+    elif env.msvc:
+        env.Append(CCFLAGS=["/EHsc"])
+
     # Configure compiler warnings
     if env.msvc:  # MSVC
         if env["warnings"] == "no":
@@ -729,9 +740,6 @@ if selected_platform in platform_list:
                 ]
             )
 
-        # Set exception handling model to avoid warnings caused by Windows system headers.
-        env.Append(CCFLAGS=["/EHsc"])
-
         if env["werror"]:
             env.Append(CCFLAGS=["/WX"])
             env.Append(LINKFLAGS=["/WX"])

modules/denoise/SCsub

@@ -109,6 +109,15 @@ env_oidn.AppendUnique(CPPDEFINES=["NDEBUG"])  # No assert() even in debug builds
 
 env_thirdparty = env_oidn.Clone()
 env_thirdparty.disable_warnings()
+
+if env["disable_exceptions"]:
+    # OIDN hard-requires exceptions, so we re-enable them here.
+    if env.msvc and ("_HAS_EXCEPTIONS", 0) in env_thirdparty["CPPDEFINES"]:
+        env_thirdparty["CPPDEFINES"].remove(("_HAS_EXCEPTIONS", 0))
+        env_thirdparty.AppendUnique(CCFLAGS=["/EHsc"])
+    elif not env.msvc and "-fno-exceptions" in env_thirdparty["CCFLAGS"]:
+        env_thirdparty["CCFLAGS"].remove("-fno-exceptions")
+
 env_thirdparty.add_source_files(thirdparty_obj, thirdparty_sources)
 env.modules_sources += thirdparty_obj
 

modules/openxr/SCsub

@@ -47,10 +47,11 @@ if env["builtin_openxr"]:
 
     env_thirdparty = env_openxr.Clone()
     env_thirdparty.disable_warnings()
+
     env_thirdparty.AppendUnique(CPPDEFINES=["DISABLE_STD_FILESYSTEM"])
+    if env["disable_exceptions"]:
+        env_thirdparty.AppendUnique(CPPDEFINES=["XRLOADER_DISABLE_EXCEPTION_HANDLING", ("JSON_USE_EXCEPTION", 0)])
 
-    if "-fno-exceptions" in env_thirdparty["CXXFLAGS"]:
-        env_thirdparty["CXXFLAGS"].remove("-fno-exceptions")
     env_thirdparty.Append(CPPPATH=[thirdparty_dir + "/src/loader"])
 
     # add in external jsoncpp dependency

platform/android/detect.py

@@ -170,10 +170,6 @@ def configure(env: "Environment"):
     env["RANLIB"] = compiler_path + "/llvm-ranlib"
     env["AS"] = compiler_path + "/clang"
 
-    # Disable exceptions on template builds
-    if not env.editor_build:
-        env.Append(CXXFLAGS=["-fno-exceptions"])
-
     env.Append(
         CCFLAGS=(
             "-fpic -ffunction-sections -funwind-tables -fstack-protector-strong -fvisibility=hidden -fno-strict-aliasing".split()

platform/ios/detect.py

@@ -30,7 +30,6 @@ def get_opts():
         ),
         ("IOS_SDK_PATH", "Path to the iOS SDK", ""),
         BoolVariable("ios_simulator", "Build for iOS Simulator", False),
-        BoolVariable("ios_exceptions", "Enable exceptions", False),
         ("ios_triple", "Triple for ios toolchain", ""),
     ]
 
@@ -138,11 +137,6 @@ def configure(env: "Environment"):
         env.Append(ASFLAGS=["-arch", "arm64"])
         env.Append(CPPDEFINES=["NEED_LONG_INT"])
 
-    if env["ios_exceptions"]:
-        env.Append(CCFLAGS=["-fexceptions"])
-    else:
-        env.Append(CCFLAGS=["-fno-exceptions"])
-
     # Temp fix for ABS/MAX/MIN macros in iOS SDK blocking compilation
     env.Append(CCFLAGS=["-Wno-ambiguous-macro"])
 

platform/web/detect.py

@@ -97,12 +97,9 @@ def configure(env: "Environment"):
     if env["use_assertions"]:
         env.Append(LINKFLAGS=["-s", "ASSERTIONS=1"])
 
-    if env.editor_build:
-        if env["initial_memory"] < 64:
-            print('Note: Forcing "initial_memory=64" as it is required for the web editor.')
-            env["initial_memory"] = 64
-    else:
-        env.Append(CPPFLAGS=["-fno-exceptions"])
+    if env.editor_build and env["initial_memory"] < 64:
+        print('Note: Forcing "initial_memory=64" as it is required for the web editor.')
+        env["initial_memory"] = 64
 
     env.Append(LINKFLAGS=["-s", "INITIAL_MEMORY=%sMB" % env["initial_memory"]])
 

tests/SCsub

@@ -13,10 +13,8 @@ env_tests = env.Clone()
 if env_tests["platform"] == "windows":
     env_tests.Append(CPPDEFINES=[("DOCTEST_THREAD_LOCAL", "")])
 
-# Increase number of addressable sections in object files
-# due to doctest's heavy use of templates and macros.
-if env_tests.msvc:
-    env_tests.Append(CCFLAGS=["/bigobj"])
+if env["disable_exceptions"]:
+    env_tests.Append(CPPDEFINES=["DOCTEST_CONFIG_NO_EXCEPTIONS_BUT_WITH_ALL_ASSERTS"])
 
 env_tests.add_source_files(env.tests_sources, "*.cpp")
 

tests/core/io/test_image.h

@@ -252,9 +252,7 @@ TEST_CASE("[Image] Modifying pixels of an image") {
 
 		for (const Rect2i &rect : rects) {
 			Ref<Image> img = memnew(Image(img_width, img_height, false, Image::FORMAT_RGBA8));
-			CHECK_NOTHROW_MESSAGE(
-					img->fill_rect(rect, Color(1, 1, 1, 1)),
-					"fill_rect() shouldn't throw for any rect.");
+			img->fill_rect(rect, Color(1, 1, 1, 1));
 			for (int y = 0; y < img->get_height(); y++) {
 				for (int x = 0; x < img->get_width(); x++) {
 					if (rect.abs().has_point(Point2(x, y))) {
@@ -307,7 +305,7 @@ TEST_CASE("[Image] Modifying pixels of an image") {
 	// Pre-multiply Alpha then Convert from RGBA to L8, checking alpha
 	{
 		Ref<Image> gray_image = memnew(Image(3, 3, false, Image::FORMAT_RGBA8));
-		CHECK_NOTHROW_MESSAGE(gray_image->fill_rect(Rect2i(0, 0, 3, 3), Color(1, 1, 1, 0)), "fill_rect() shouldn't throw for any rect.");
+		gray_image->fill_rect(Rect2i(0, 0, 3, 3), Color(1, 1, 1, 0));
 		gray_image->set_pixel(1, 1, Color(1, 1, 1, 1));
 		gray_image->set_pixel(1, 2, Color(0.5, 0.5, 0.5, 0.5));
 		gray_image->set_pixel(2, 1, Color(0.25, 0.05, 0.5, 1.0));

tests/scene/test_viewport.h

@@ -222,7 +222,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 
 	SUBCASE("[Viewport][GuiInputEvent] nullptr as argument doesn't lead to a crash.") {
 		ERR_PRINT_OFF;
-		CHECK_NOTHROW(root->push_input(nullptr));
+		root->push_input(nullptr);
 		ERR_PRINT_ON;
 	}