diff --git a/desktop/api.cpp b/desktop/api.cpp
index bba0ba1..ac476c2 100644
--- a/desktop/api.cpp
+++ b/desktop/api.cpp
@@ -261,11 +261,10 @@ struct APIInstance {
 
 #include "renderer.cpp"
 #include "theme.cpp"
-
 #define TEXT_RENDERER
 #include "text.cpp"
 #undef TEXT_RENDERER
-
+#include "profiling.cpp"
 #include "gui.cpp"
 
 #ifndef NO_API_TABLE
@@ -1106,12 +1105,13 @@ EsMessage *EsMessageReceive() {
 			ProcessMessageTiming timing = {};
 			double start = EsTimeStampMs();
 			UIProcessWindowManagerMessage((EsWindow *) message.object, &message.message, &timing);
-			EsPrint("Processed message from WM %x in %Fms (%Fms logic, %Fms layout, %Fms paint, %Fms update screen).\n", 
+			EsPrint("Processed message from WM %x in %Fms (%Fms logic, %Fms layout, %Fms paint, %Fms update screen). Profiling buffer %F%% full.\n", 
 					type, EsTimeStampMs() - start, 
 					timing.endLogic - timing.startLogic, 
 					timing.endLayout - timing.startLayout,
 					timing.endPaint - timing.startPaint,
-					timing.endUpdate - timing.startUpdate);
+					timing.endUpdate - timing.startUpdate,
+					profilingBufferSize ? profilingBufferPosition * 100.0 / profilingBufferSize : 0);
 #else
 			UIProcessWindowManagerMessage((EsWindow *) message.object, &message.message, nullptr);
 #endif
@@ -1470,6 +1470,7 @@ uintptr_t EsSystemGetOptimalWorkQueueThreadCount() {
 	return EsSyscall(ES_SYSCALL_PROCESSOR_COUNT, 0, 0, 0, 0);
 }
 
+__attribute__((no_instrument_function))
 void ThreadInitialise(ThreadLocalStorage *local) {
 	EsMemoryZero(local, sizeof(ThreadLocalStorage));
 	EsSyscall(ES_SYSCALL_THREAD_GET_ID, ES_CURRENT_THREAD, (uintptr_t) &local->id, 0, 0);
diff --git a/desktop/gui.cpp b/desktop/gui.cpp
index 13a0c09..e42b257 100644
--- a/desktop/gui.cpp
+++ b/desktop/gui.cpp
@@ -8035,6 +8035,13 @@ void InspectorVisualizeLayoutBounds(EsInstance *instance, EsElement *, EsCommand
 	EsElementRepaint(window);
 }
 
+#ifdef PROFILE_DESKTOP_FUNCTIONS
+void InspectorBeginProfiling(EsInstance *, EsElement *, EsCommand *) {
+	size_t entryCount = 3000000;
+	ProfilingSetup((ProfilingEntry *) EsHeapAllocate(sizeof(ProfilingEntry) * entryCount, false), entryCount);
+}
+#endif
+
 void InspectorAddElement2(EsMenu *menu, EsGeneric context) {
 	InspectorWindow *inspector = (InspectorWindow *) menu->instance; 
 	if (inspector->selectedElement == -1) return;
@@ -8094,6 +8101,10 @@ void InspectorSetup(EsWindow *window) {
 		EsButtonOnCommand(inspector->visualizeLayoutBounds, InspectorVisualizeLayoutBounds);
 		inspector->visualizePaintSteps = EsButtonCreate(toolbar, ES_BUTTON_TOOLBAR, 0, "Visualize paint steps");
 		EsButtonOnCommand(inspector->visualizePaintSteps, InspectorVisualizePaintSteps);
+		EsSpacerCreate(toolbar, ES_CELL_H_FILL);
+#ifdef PROFILE_DESKTOP_FUNCTIONS
+		EsButtonOnCommand(EsButtonCreate(toolbar, ES_BUTTON_TOOLBAR, 0, "Begin profiling"), InspectorBeginProfiling);
+#endif
 	}
 
 	inspector->elementList = EsListViewCreate(panel1, ES_CELL_FILL | ES_LIST_VIEW_COLUMNS | ES_LIST_VIEW_SINGLE_SELECT);
diff --git a/desktop/profiling.cpp b/desktop/profiling.cpp
new file mode 100644
index 0000000..02b8f18
--- /dev/null
+++ b/desktop/profiling.cpp
@@ -0,0 +1,55 @@
+// TODO Adjust time stamps for thread preemption.
+
+#include <stdint.h>
+#include <stddef.h>
+
+struct ProfilingEntry {
+	void *thisFunction;
+	uint64_t timeStamp;
+};
+
+extern ptrdiff_t tlsStorageOffset;
+extern "C" uintptr_t ProcessorTLSRead(uintptr_t offset);
+extern "C" uint64_t ProcessorReadTimeStamp();
+void EnterDebugger();
+
+extern size_t profilingBufferSize;
+extern uintptr_t profilingBufferPosition;
+void ProfilingSetup(ProfilingEntry *buffer, size_t size /* number of entries */);
+
+#ifdef PROFILING_IMPLEMENTATION
+
+ProfilingEntry *profilingBuffer;
+size_t profilingBufferSize;
+uintptr_t profilingBufferPosition;
+uintptr_t profilingThread;
+
+#define PROFILING_FUNCTION(_exiting) \
+	(void) callSite; \
+	\
+	if (profilingBufferPosition < profilingBufferSize && profilingThread == ProcessorTLSRead(tlsStorageOffset)) { \
+		ProfilingEntry *entry = (ProfilingEntry *) &profilingBuffer[profilingBufferPosition++]; \
+		entry->thisFunction = thisFunction; \
+		entry->timeStamp = ProcessorReadTimeStamp() | ((uint64_t) _exiting << 63); \
+	} else if (profilingBufferSize && profilingThread == ProcessorTLSRead(tlsStorageOffset)) { \
+		profilingBufferSize = 0; \
+		EnterDebugger(); \
+	}
+
+extern "C" void __cyg_profile_func_enter(void *thisFunction, void *callSite) {
+	PROFILING_FUNCTION(0);
+}
+
+extern "C" void __cyg_profile_func_exit(void *thisFunction, void *callSite) {
+	PROFILING_FUNCTION(1);
+}
+
+void ProfilingSetup(ProfilingEntry *buffer, size_t size) {
+	profilingThread = ProcessorTLSRead(tlsStorageOffset);
+	__sync_synchronize();
+	profilingBuffer = buffer;
+	profilingBufferSize = size;
+	profilingBufferPosition = 0;
+}
+
+#endif
diff --git a/desktop/syscall.cpp b/desktop/syscall.cpp
index 3e8f031..420d072 100644
--- a/desktop/syscall.cpp
+++ b/desktop/syscall.cpp
@@ -120,6 +120,7 @@ int EsProcessGetExitStatus(EsHandle process) {
 
 void ThreadInitialise(ThreadLocalStorage *local);
 
+__attribute__((no_instrument_function))
 void ThreadEntry(EsGeneric argument, EsThreadEntryCallback entryFunction) {
 	ThreadLocalStorage local;
 	ThreadInitialise(&local);
diff --git a/desktop/text.cpp b/desktop/text.cpp
index c414113..afd0342 100644
--- a/desktop/text.cpp
+++ b/desktop/text.cpp
@@ -822,6 +822,7 @@ void FontDatabaseFree() {
 
 // --------------------------------- Blitting rendered glyphs.
 
+__attribute__((no_instrument_function))
 inline static void DrawStringPixel(int oX, int oY, void *bitmap, size_t stride, uint32_t textColor, 
 		uint32_t selectionColor, int32_t backgroundColor, uint32_t pixel, bool selected, bool fullAlpha) {
 	uint32_t *destination = (uint32_t *) ((uint8_t *) bitmap + (oX) * 4 + (oY) * stride);
diff --git a/shared/array.cpp b/shared/array.cpp
index 63db363..25ba926 100644
--- a/shared/array.cpp
+++ b/shared/array.cpp
@@ -27,7 +27,7 @@ template <class T, EsHeap *heap = nullptr>
 struct Array {
 	T *array;
 
-	inline size_t Length() { return array ? ArrayHeader(array)->length : 0; }
+	__attribute__((no_instrument_function)) inline size_t Length() { return array ? ArrayHeader(array)->length : 0; }
 	inline T &First() { return array[0]; }
 	inline T &Last() { return array[Length() - 1]; }
 	inline void Delete(uintptr_t position) { _ArrayDelete(array, position, sizeof(T), 1); }
diff --git a/shared/common.cpp b/shared/common.cpp
index 8b6d75f..22cf359 100644
--- a/shared/common.cpp
+++ b/shared/common.cpp
@@ -95,6 +95,7 @@ EsRectangle EsRectangleBounding(EsRectangle a, EsRectangle b) {
 	return a;
 }
 
+__attribute__((no_instrument_function))
 EsRectangle EsRectangleIntersection(EsRectangle a, EsRectangle b) {
 	if (a.l < b.l) a.l = b.l;
 	if (a.t < b.t) a.t = b.t;
@@ -164,6 +165,7 @@ EsRectangle EsRectangleCut(EsRectangle a, int32_t amount, char side) {
 #if defined(SHARED_COMMON_WANT_RENDERING) || defined(SHARED_COMMON_WANT_ALL)
 
 ES_FUNCTION_OPTIMISE_O3 
+__attribute__((no_instrument_function))
 void BlendPixel(uint32_t *destinationPixel, uint32_t modified, bool fullAlpha) {
 	if ((modified & 0xFF000000) == 0xFF000000) {
 		*destinationPixel = modified;
@@ -1252,6 +1254,7 @@ double EsDoubleParse(const char *nptr, ptrdiff_t maxBytes, char **endptr) {
 
 #ifdef SHARED_COMMON_WANT_ALL
 
+__attribute__((no_instrument_function))
 void EsMemoryCopy(void *_destination, const void *_source, size_t bytes) {
 	// TODO Prevent this from being optimised out in the kernel.
 
@@ -1282,6 +1285,7 @@ void EsMemoryCopy(void *_destination, const void *_source, size_t bytes) {
 	}
 }
 
+__attribute__((no_instrument_function))
 void EsMemoryCopyReverse(void *_destination, void *_source, size_t bytes) {
 	// TODO Prevent this from being optimised out in the kernel.
 
@@ -1304,6 +1308,7 @@ void EsMemoryCopyReverse(void *_destination, void *_source, size_t bytes) {
 	}
 }
 
+__attribute__((no_instrument_function))
 void EsMemoryZero(void *destination, size_t bytes) {
 	// TODO Prevent this from being optimised out in the kernel.
 
@@ -1316,6 +1321,7 @@ void EsMemoryZero(void *destination, size_t bytes) {
 	}
 }
 
+__attribute__((no_instrument_function))
 void EsMemoryMove(void *_start, void *_end, intptr_t amount, bool zeroEmptySpace) {
 	// TODO Prevent this from being optimised out in the kernel.
 
@@ -1342,6 +1348,7 @@ void EsMemoryMove(void *_start, void *_end, intptr_t amount, bool zeroEmptySpace
 	}
 }
 
+__attribute__((no_instrument_function))
 int EsMemoryCompare(const void *a, const void *b, size_t bytes) {
 	if (!bytes) {
 		return 0;
@@ -1361,6 +1368,7 @@ int EsMemoryCompare(const void *a, const void *b, size_t bytes) {
 	return 0;
 }
 
+__attribute__((no_instrument_function))
 uint8_t EsMemorySumBytes(uint8_t *source, size_t bytes) {
 	if (!bytes) {
 		return 0;
@@ -1375,6 +1383,7 @@ uint8_t EsMemorySumBytes(uint8_t *source, size_t bytes) {
 	return total;
 }
 
+__attribute__((no_instrument_function))
 void EsMemoryFill(void *from, void *to, uint8_t byte) {
 	uint8_t *a = (uint8_t *) from;
 	uint8_t *b = (uint8_t *) to;
@@ -1679,12 +1688,14 @@ void LoadImage(const void *path, ptrdiff_t pathBytes, void *destination, int des
 
 #ifdef SHARED_COMMON_WANT_ALL
 
+__attribute__((no_instrument_function))
 void EsSpinlockAcquire(EsSpinlock *spinlock) {
 	__sync_synchronize();
 	while (__sync_val_compare_and_swap(&spinlock->state, 0, 1));
 	__sync_synchronize();
 }
 
+__attribute__((no_instrument_function))
 void EsSpinlockRelease(EsSpinlock *spinlock) {
 	__sync_synchronize();
 
diff --git a/shared/math.cpp b/shared/math.cpp
index dd49031..c85eb88 100644
--- a/shared/math.cpp
+++ b/shared/math.cpp
@@ -33,6 +33,7 @@ inline int DistanceSquared(int x1, int y1, int x2, int y2) {
 	return dx * dx + dy * dy;
 }
 
+__attribute__((no_instrument_function))
 inline int ClampInteger(int low, int high, int integer) {
 	if (integer < low) return low;
 	if (integer > high) return high;
@@ -51,6 +52,7 @@ inline intptr_t ClampIntptr(intptr_t low, intptr_t high, intptr_t integer) {
 	return integer;
 }
 
+__attribute__((no_instrument_function))
 inline int MaximumInteger(int a, int b) {
 	return a > b ? a : b;
 }
@@ -58,14 +60,17 @@ inline int MaximumInteger(int a, int b) {
 #define MaximumInteger3 MaximumInteger
 #define MinimumInteger3 MinimumInteger
 
+__attribute__((no_instrument_function))
 inline int MaximumInteger(int a, int b, int c) {
 	return MaximumInteger(MaximumInteger(a, b), c);
 }
 
+__attribute__((no_instrument_function))
 inline int MaximumInteger(int a, int b, int c, int d) {
 	return MaximumInteger(MaximumInteger(a, b, c), d);
 }
 
+__attribute__((no_instrument_function))
 inline int MinimumInteger(int a, int b) {
 	return a < b ? a : b;
 }
@@ -99,6 +104,7 @@ float LinearMap(float inFrom, float inTo, float outFrom, float outTo, float valu
 	return raw * (outTo - outFrom) + outFrom;
 }
 
+__attribute__((no_instrument_function))
 float LinearInterpolate(float from, float to, float progress) {
 	return from + progress * (to - from);
 }
diff --git a/util/build.c b/util/build.c
index ea1d0c5..928b919 100644
--- a/util/build.c
+++ b/util/build.c
@@ -1139,6 +1139,8 @@ void DoCommand(const char *l) {
 		BuildAndRun(OPTIMISE_OFF, true /* compile */, false /* debug */, EMULATOR_QEMU, LOG_NORMAL);
 	} else if (0 == strcmp(l, "t3") || 0 == strcmp(l, "qemu-without-compile")) {
 		BuildAndRun(OPTIMISE_OFF, false /* compile */, false /* debug */, EMULATOR_QEMU, LOG_NORMAL);
+	} else if (0 == strcmp(l, "t4")) {
+		BuildAndRun(OPTIMISE_FULL, true /* compile */, false /* debug */, EMULATOR_QEMU, LOG_NORMAL);
 	} else if (0 == strcmp(l, "e")) {
 		Run(EMULATOR_QEMU, LOG_NORMAL, DEBUG_LATER);
 	} else if (0 == strcmp(l, "k") || 0 == strcmp(l, "qemu-with-kvm")) {
diff --git a/util/build_common.h b/util/build_common.h
index 435965d..9f0c98a 100644
--- a/util/build_common.h
+++ b/util/build_common.h
@@ -286,6 +286,7 @@ Option options[] = {
 	{ "Flag.ENABLE_POSIX_SUBSYSTEM", OPTION_TYPE_BOOL, { .b = false } },
 	{ "Flag.DEBUG_BUILD", OPTION_TYPE_BOOL, { .b = true } },
 	{ "Flag.USE_SMP", OPTION_TYPE_BOOL, { .b = true } },
+	{ "Flag.PROFILE_DESKTOP_FUNCTIONS", OPTION_TYPE_BOOL, { .b = false } },
 	{ "Flag.BGA_RESOLUTION_WIDTH", OPTION_TYPE_STRING, { .s = "1600" } },
 	{ "Flag.BGA_RESOLUTION_HEIGHT", OPTION_TYPE_STRING, { .s = "900" } },
 	{ "Flag.VGA_TEXT_MODE", OPTION_TYPE_BOOL, { .b = false } },
diff --git a/util/build_core.c b/util/build_core.c
index cc513f1..1c15c38 100644
--- a/util/build_core.c
+++ b/util/build_core.c
@@ -152,6 +152,7 @@ char apiLinkFlags2[4096] = " -lgcc ";
 char apiLinkFlags3[4096] = " -Wl,--end-group -Lroot/Applications/POSIX/lib ";
 char kernelLinkFlags[4096] = " -ffreestanding -nostdlib -lgcc -g -z max-page-size=0x1000 ";
 char commonAssemblyFlags[4096] = " -Fdwarf ";
+const char *desktopProfilingFlags = "";
 
 // Specific configuration options:
 
@@ -568,10 +569,11 @@ void BuildDesktop(Application *application) {
 
 	snprintf(buffer, sizeof(buffer), "arch/%s/api.s", target);
 	ExecuteForApp(application, toolchainNasm, buffer, "-MD", "bin/api1.d", "-o", "bin/api1.o", ArgString(commonAssemblyFlags));
-	ExecuteForApp(application, toolchainCXX, "-MD", "-c", "desktop/api.cpp", "-o", "bin/api2.o", ArgString(commonCompileFlags));
+	ExecuteForApp(application, toolchainCXX, "-MD", "-c", "desktop/api.cpp", "-o", "bin/api2.o", ArgString(commonCompileFlags), ArgString(desktopProfilingFlags));
 	ExecuteForApp(application, toolchainCXX, "-MD", "-c", "desktop/posix.cpp", "-o", "bin/api3.o", ArgString(commonCompileFlags));
+	ExecuteForApp(application, toolchainCXX, "-MD", "-c", "desktop/profiling.cpp", "-o", "bin/api4.o", "-DPROFILING_IMPLEMENTATION", ArgString(commonCompileFlags));
 	ExecuteForApp(application, toolchainCC, "-o", "bin/Desktop", "bin/crti.o", "bin/crtbegin.o", 
-			"bin/api1.o", "bin/api2.o", "bin/api3.o", "bin/crtend.o", "bin/crtn.o", 
+			"bin/api1.o", "bin/api2.o", "bin/api3.o", "bin/api4.o", "bin/crtend.o", "bin/crtn.o", 
 			ArgString(apiLinkFlags1), ArgString(apiLinkFlags2), ArgString(apiLinkFlags3));
 	ExecuteForApp(application, toolchainStrip, "-o", "bin/Desktop.no_symbols", "--strip-all", "bin/Desktop");
 
@@ -1350,6 +1352,8 @@ int main(int argc, char **argv) {
 				} else if (0 == strcmp(s.key, "Flag.COM_OUTPUT") && atoi(s.value)) {
 					strcat(commonAssemblyFlags, " -DCOM_OUTPUT ");
 					strcat(commonCompileFlags, " -DCOM_OUTPUT ");
+				} else if (0 == strcmp(s.key, "Flag.PROFILE_DESKTOP_FUNCTIONS") && atoi(s.value)) {
+					desktopProfilingFlags = "-finstrument-functions";
 				} else if (0 == strcmp(s.key, "BuildCore.NoImportPOSIX")) {
 					noImportPOSIX = !!atoi(s.value);
 				} else if (0 == memcmp(s.key, "General.", 8)) {
@@ -1575,6 +1579,7 @@ int main(int argc, char **argv) {
 			ADD_DEPENDENCY_FILE(application, "bin/api1.d", "API1");
 			ADD_DEPENDENCY_FILE(application, "bin/api2.d", "API2");
 			ADD_DEPENDENCY_FILE(application, "bin/api3.d", "API3");
+			ADD_DEPENDENCY_FILE(application, "bin/api4.d", "API4");
 			arrput(applications, application);
 		}