GBA: Implement idle loop detection

CHANGES

@@ -16,6 +16,7 @@ Features:
  - Rewinding of emulation
  - Implemented BIOS routines SoftReset, RegisterRamReset, Diff8bitUnFilterWram, Diff8bitUnFilterVram, and Diff16bitUnFilter
  - Support IPv6
+ - Automatically detect and optimize out idle loops
 Bugfixes:
  - Qt: Fix issue with set frame sizes being the wrong height
  - Qt: Fix emulator crashing when full screen if a game is not running

src/gba/gba-io.c

@@ -555,6 +555,7 @@ void GBAIOWrite32(struct GBA* gba, uint32_t address, uint32_t value) {
 }
 
 uint16_t GBAIORead(struct GBA* gba, uint32_t address) {
+	gba->lastJump = -1; // IO reads need to invalidate detected idle loops
 	switch (address) {
 	case REG_TM0CNT_LO:
 		GBATimerUpdateRegister(gba, 0);

src/gba/gba-memory.c

@@ -7,6 +7,7 @@
 
 #include "macros.h"
 
+#include "decoder.h"
 #include "gba-gpio.h"
 #include "gba-io.h"
 #include "gba-serialize.h"
@@ -113,16 +114,107 @@ void GBAMemoryReset(struct GBA* gba) {
 	}
 }
 
+static void _analyzeForIdleLoop(struct GBA* gba, struct ARMCore* cpu, uint32_t address) {
+	struct ARMInstructionInfo info;
+	uint32_t nextAddress = address;
+	memset(gba->taintedRegisters, 0, sizeof(gba->taintedRegisters));
+	if (cpu->executionMode == MODE_THUMB) {
+		while (true) {
+			uint16_t opcode;
+			LOAD_16(opcode, nextAddress & cpu->memory.activeMask, cpu->memory.activeRegion);
+			ARMDecodeThumb(opcode, &info);
+			switch (info.branchType) {
+			case ARM_BRANCH_NONE:
+				if (info.operandFormat & ARM_OPERAND_MEMORY_2) {
+					if (info.mnemonic == ARM_MN_STR || gba->taintedRegisters[info.memory.baseReg]) {
+						gba->idleDetectionStep = -1;
+						return;
+					}
+					uint32_t loadAddress = gba->cachedRegisters[info.memory.baseReg];
+					uint32_t offset = 0;
+					if (info.memory.format & ARM_MEMORY_IMMEDIATE_OFFSET) {
+						offset = info.memory.offset.immediate;
+					} else if (info.memory.format & ARM_MEMORY_REGISTER_OFFSET) {
+						int reg = info.memory.offset.reg;
+						if (gba->cachedRegisters[reg]) {
+							gba->idleDetectionStep = -1;
+							return;
+						}
+						offset = gba->cachedRegisters[reg];
+					}
+					if (info.memory.format & ARM_MEMORY_OFFSET_SUBTRACT) {
+						loadAddress -= offset;
+					} else {
+						loadAddress += offset;
+					}
+					if ((loadAddress >> BASE_OFFSET) == REGION_IO) {
+						gba->idleDetectionStep = -1;
+						return;
+					}
+					if ((loadAddress >> BASE_OFFSET) < REGION_CART0 || (loadAddress >> BASE_OFFSET) > REGION_CART2_EX) {
+						gba->taintedRegisters[info.op1.reg] = true;
+					} else {
+						switch (info.memory.width) {
+						case 1:
+							gba->cachedRegisters[info.op1.reg] = GBALoad8(cpu, loadAddress, 0);
+							break;
+						case 2:
+							gba->cachedRegisters[info.op1.reg] = GBALoad16(cpu, loadAddress, 0);
+							break;
+						case 4:
+							gba->cachedRegisters[info.op1.reg] = GBALoad32(cpu, loadAddress, 0);
+							break;
+						}
+					}
+				} else if (info.operandFormat & ARM_OPERAND_AFFECTED_1) {
+					gba->taintedRegisters[info.op1.reg] = true;
+				}
+				nextAddress += WORD_SIZE_THUMB;
+				break;
+			case ARM_BRANCH:
+				if ((uint32_t) info.op1.immediate + nextAddress + WORD_SIZE_THUMB * 2 == address) {
+					gba->busyLoop = address;
+				}
+				gba->idleDetectionStep = -1;
+				return;
+			default:
+				gba->idleDetectionStep = -1;
+				return;
+			}
+		}
+	} else {
+		gba->idleDetectionStep = -1;
+	}
+}
+
 static void GBASetActiveRegion(struct ARMCore* cpu, uint32_t address) {
 	struct GBA* gba = (struct GBA*) cpu->master;
 	struct GBAMemory* memory = &gba->memory;
 
-	if (address == gba->busyLoop && memory->activeRegion != REGION_BIOS) {
+	if (address == gba->lastJump && address == gba->busyLoop && memory->activeRegion != REGION_BIOS) {
 		GBAHalt(gba);
 	}
 
 	int newRegion = address >> BASE_OFFSET;
 	if (newRegion == memory->activeRegion) {
+		if (address == gba->lastJump) {
+			switch (gba->idleDetectionStep) {
+			case 0:
+				memcpy(gba->cachedRegisters, cpu->gprs, sizeof(gba->cachedRegisters));
+				++gba->idleDetectionStep;
+				break;
+			case 1:
+				if (memcmp(gba->cachedRegisters, cpu->gprs, sizeof(gba->cachedRegisters))) {
+					gba->idleDetectionStep = -1;
+					break;
+				}
+				_analyzeForIdleLoop(gba, cpu, address);
+				break;
+			}
+		} else {
+			gba->lastJump = address;
+			gba->idleDetectionStep = 0;
+		}
 		return;
 	}
 	if (memory->activeRegion == REGION_BIOS) {

src/gba/gba.h

@@ -119,7 +119,6 @@ struct GBA {
 	int springIRQ;
 	uint32_t biosChecksum;
 	int* keySource;
-	uint32_t busyLoop;
 	struct GBARotationSource* rotationSource;
 	struct GBALuminanceSource* luminanceSource;
 	struct GBARTCSource* rtcSource;
@@ -135,6 +134,12 @@ struct GBA {
 	const char* activeFile;
 
 	int logLevel;
+
+	uint32_t busyLoop;
+	uint32_t lastJump;
+	int idleDetectionStep;
+	int32_t cachedRegisters[16];
+	bool taintedRegisters[16];
 };
 
 struct GBACartridge {