Add optional LTO and harden Windows clipboard handling

- add --with-lto support to Premake 4 and 5 release builds
  - use ThinLTO with Clang, automatic LTO with GCC, and LTCG with MSVC
  - exclude Emscripten builds from LTO configuration
  - retry SDL2 clipboard writes on Windows when the clipboard is temporarily unavailable
  - make the UIMergeView copy regression test independent of the system clipboard

premake4.lua

@@ -43,6 +43,10 @@ function newclangtoolchain(toolchain)
 	}
 end
 
+function is_clang()
+	return _OPTIONS.platform == "clang" or os.is_real("macosx") or os.is_real("ios")
+end
+
 newplatform {
 	name = "clang",
 	description = "Clang",
@@ -182,6 +186,7 @@ newoption {
     description = "Set the shared data directory (default: /usr/share/ecode)",
 }
 newoption { trigger = "with-static-cpp", description = "Builds statically libstdc++" }
+newoption { trigger = "with-lto", description = "Enables Link-Time Optimization for release builds." }
 
 function explode(div,str)
 	if (div=='') then return false end
@@ -747,6 +752,23 @@ function parse_args()
 	if _OPTIONS["time-trace"] then
 		buildoptions { "-ftime-trace" }
 	end
+
+	if _OPTIONS["with-lto"] and not os.is_real("emscripten") then
+		configuration "release"
+
+		if is_vs() then
+			buildoptions { "/GL" }
+			linkoptions { "/LTCG" }
+		elseif is_clang() then
+			buildoptions { "-flto=thin" }
+			linkoptions { "-flto=thin" }
+		else
+			buildoptions { "-flto=auto" }
+			linkoptions { "-flto=auto" }
+		end
+
+		configuration {}
+	end
 end
 
 function add_static_links()

premake5.lua

@@ -51,6 +51,7 @@ newoption {
     description = "Set the shared data directory",
 }
 newoption { trigger = "with-static-cpp", description = "Builds statically libstdc++" }
+newoption { trigger = "with-lto", description = "Enables Link-Time Optimization for release builds." }
 
 function is_arm64_arch()
 	local arch = _OPTIONS["arch"]
@@ -714,6 +715,21 @@ function parse_args()
 	if _OPTIONS["time-trace"] then
 		buildoptions { "-ftime-trace" }
 	end
+
+	if _OPTIONS["with-lto"] then
+		filter { "configurations:release*", "toolset:gcc", "not system:emscripten" }
+			buildoptions { "-flto=auto" }
+			linkoptions { "-flto=auto" }
+
+		filter { "configurations:release*", "toolset:clang", "not system:emscripten" }
+			buildoptions { "-flto=thin" }
+			linkoptions { "-flto=thin" }
+
+		filter { "configurations:release*", "toolset:msc" }
+			linktimeoptimization "On"
+
+		filter {}
+	end
 end
 
 function add_static_links()

src/eepp/window/backend/SDL2/clipboardsdl2.cpp

@@ -33,6 +33,17 @@ void ClipboardSDL::setText( const std::string& text ) {
 #if EE_PLATFORM == EE_PLATFORM_EMSCRIPTEN
 	sContent = text;
 	emscripten_browser_clipboard::copy( text );
+#elif EE_PLATFORM == EE_PLATFORM_WIN
+	// SDL2's Windows backend retries reads when another process owns the clipboard, but not
+	// writes. Clipboard ownership is transient, so mirror the read-side retry policy here.
+	static constexpr int MAX_ATTEMPTS = 3;
+	for ( int attempt = 0; attempt < MAX_ATTEMPTS; ++attempt ) {
+		if ( SDL_SetClipboardText( text.c_str() ) == 0 )
+			return;
+		if ( attempt + 1 < MAX_ATTEMPTS )
+			SDL_Delay( 10 );
+	}
+	Log::warning( "Failed to set clipboard text: %s", SDL_GetError() );
 #else
 	SDL_SetClipboardText( text.c_str() );
 #endif

src/tests/unit_tests/uimergeview_tests.cpp

@@ -2,7 +2,6 @@
 #include <eepp/ui/doc/syntaxdefinitionmanager.hpp>
 #include <eepp/ui/tools/uimergeview.hpp>
 #include <eepp/ui/uiapplication.hpp>
-#include <eepp/window/clipboard.hpp>
 
 using namespace EE;
 using namespace EE::UI;
@@ -82,9 +81,12 @@ UTEST( UIMergeView, UsesSharedResultDocumentAndAcceptIsUndoable ) {
 	EXPECT_TRUE( view->isToolbarVisible() );
 	EXPECT_TRUE( view->hasCommand( "merge-accept-left" ) );
 	auto* leftEditor = view->getLeftEditor();
-	leftEditor->getDocument().setSelection( { { 0, 0 }, { 0, 4 } } );
-	leftEditor->getDocument().execute( "copy", leftEditor );
-	EXPECT_STREQ( "left", app.getWindow()->getClipboard()->getText().c_str() );
+	auto& leftDocument = leftEditor->getDocument();
+	// Keep this hermetic: the Windows system clipboard can be temporarily owned by another
+	// process. The regression is that replacement side documents must retain the copy command.
+	EXPECT_TRUE( leftDocument.hasCommand( "copy" ) );
+	leftDocument.setSelection( { { 0, 0 }, { 0, 4 } } );
+	EXPECT_TRUE( leftDocument.getAllSelectedText() == "left" );
 	view->setToolbarVisible( false );
 	EXPECT_FALSE( view->isToolbarVisible() );
 	view->setToolbarVisible( true );