From fec4c064475615dcf5593fa8fa8d342913997936 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Mon, 12 Jun 2017 21:06:06 -0700
Subject: [PATCH] GBA Memory: Simplify prefetch logic (possibly more accurate,
 but more failing tests)

---
 src/gba/memory.c | 25 ++++++++++++-------------
 src/gba/timer.c  |  8 ++++----
 2 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/src/gba/memory.c b/src/gba/memory.c
index db4a30122..755a538a0 100644
--- a/src/gba/memory.c
+++ b/src/gba/memory.c
@@ -1505,33 +1505,32 @@ int32_t GBAMemoryStall(struct ARMCore* cpu, int32_t wait) {
 		previousLoads = dist;
 	}
 
-	int32_t s = cpu->memory.activeSeqCycles16 + 1;
-	int32_t n2s = cpu->memory.activeNonseqCycles16 - cpu->memory.activeSeqCycles16 + 1;
+	int32_t s = cpu->memory.activeSeqCycles16;
+	int32_t n2s = cpu->memory.activeNonseqCycles16 - cpu->memory.activeSeqCycles16;
 
 	// Figure out how many sequential loads we can jam in
 	int32_t stall = s;
 	int32_t loads = 1;
 
-	if (stall > wait && !previousLoads) {
-		// We might need to stall a bit extra if we haven't finished the first S cycle
-		wait = stall;
-	} else {
-		while (stall < wait) {
+	if (stall < wait) {
+		int32_t maxLoads = 8 - previousLoads;
+		while (stall < wait && loads < maxLoads) {
 			stall += s;
 			++loads;
 		}
-		if (loads + previousLoads > 8) {
-			loads = 8 - previousLoads;
-		}
 	}
+	if (stall > wait) {
+		// The wait cannot take less time than the prefetch stalls
+		wait = stall;
+	}
+
 	// This instruction used to have an N, convert it to an S.
 	wait -= n2s;
 
-	// TODO: Invalidate prefetch on branch
-	memory->lastPrefetchedPc = cpu->gprs[ARM_PC] + WORD_SIZE_THUMB * loads;
+	memory->lastPrefetchedPc = cpu->gprs[ARM_PC] + WORD_SIZE_THUMB * (loads + previousLoads - 1);
 
 	// The next |loads|S waitstates disappear entirely, so long as they're all in a row
-	cpu->cycles -= (s - 1) * loads;
+	cpu->cycles -= stall;
 	return wait;
 }
 
diff --git a/src/gba/timer.c b/src/gba/timer.c
index c62f64506..03af4fe34 100644
--- a/src/gba/timer.c
+++ b/src/gba/timer.c
@@ -83,13 +83,13 @@ void GBATimerInit(struct GBA* gba) {
 void GBATimerUpdateRegister(struct GBA* gba, int timer) {
 	struct GBATimer* currentTimer = &gba->timers[timer];
 	if (GBATimerFlagsIsEnable(currentTimer->flags) && !GBATimerFlagsIsCountUp(currentTimer->flags)) {
-		int32_t prefetchSkew = 0;
-		if (gba->memory.lastPrefetchedPc >= (uint32_t) gba->cpu->gprs[ARM_PC]) {
-			prefetchSkew = (gba->memory.lastPrefetchedPc - gba->cpu->gprs[ARM_PC]) * (gba->cpu->memory.activeSeqCycles16 + 1) / WORD_SIZE_THUMB;
+		int32_t prefetchSkew = -2;
+		if (gba->memory.lastPrefetchedPc > (uint32_t) gba->cpu->gprs[ARM_PC]) {
+			prefetchSkew += ((gba->memory.lastPrefetchedPc - gba->cpu->gprs[ARM_PC]) * gba->cpu->memory.activeSeqCycles16) / WORD_SIZE_THUMB;
 		}
 		// Reading this takes two cycles (1N+1I), so let's remove them preemptively
 		int32_t diff = gba->cpu->cycles - (currentTimer->lastEvent - gba->timing.masterCycles);
-		gba->memory.io[(REG_TM0CNT_LO + (timer << 2)) >> 1] = currentTimer->oldReload + ((diff - 2 + prefetchSkew) >> GBATimerFlagsGetPrescaleBits(currentTimer->flags));
+		gba->memory.io[(REG_TM0CNT_LO + (timer << 2)) >> 1] = currentTimer->oldReload + ((diff + prefetchSkew) >> GBATimerFlagsGetPrescaleBits(currentTimer->flags));
 	}
 }