From a53b01be00e8fc65c1abc28c8d9d3ab59f76c209 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Sun, 1 Nov 2020 02:22:09 -0800
Subject: [PATCH] ARM: Fix Addressing mode 1 shifter on rs == pc (fixes #1926)

---
 CHANGES           |  1 +
 src/arm/isa-arm.c | 24 ++++--------------------
 2 files changed, 5 insertions(+), 20 deletions(-)

diff --git a/CHANGES b/CHANGES
index 6b6966a6f..83a1829ec 100644
--- a/CHANGES
+++ b/CHANGES
@@ -15,6 +15,7 @@ Features:
 Emulation fixes:
  - ARM: Fix ALU reading PC after shifting
  - ARM: Fix STR storing PC after address calculation
+ - ARM: Fix Addressing mode 1 shifter on rs == pc (fixes mgba.io/i/1926)
  - GB: Partially fix timing for skipped BIOS
  - GB Audio: Fix serializing sweep time
  - GB MBC: Fix MBC1 mode changing behavior
diff --git a/src/arm/isa-arm.c b/src/arm/isa-arm.c
index 465e37a53..d241c58d8 100644
--- a/src/arm/isa-arm.c
+++ b/src/arm/isa-arm.c
@@ -19,15 +19,11 @@ static inline void _shiftLSL(struct ARMCore* cpu, uint32_t opcode) {
 	if (opcode & 0x00000010) {
 		int rs = (opcode >> 8) & 0x0000000F;
 		++cpu->cycles;
-		int shift = cpu->gprs[rs];
-		if (rs == ARM_PC) {
-			shift += 4;
-		}
-		shift &= 0xFF;
 		int32_t shiftVal = cpu->gprs[rm];
 		if (rm == ARM_PC) {
 			shiftVal += 4;
 		}
+		int shift = cpu->gprs[rs] & 0xFF;
 		if (!shift) {
 			cpu->shifterOperand = shiftVal;
 			cpu->shifterCarryOut = cpu->cpsr.c;
@@ -58,15 +54,11 @@ static inline void _shiftLSR(struct ARMCore* cpu, uint32_t opcode) {
 	if (opcode & 0x00000010) {
 		int rs = (opcode >> 8) & 0x0000000F;
 		++cpu->cycles;
-		int shift = cpu->gprs[rs];
-		if (rs == ARM_PC) {
-			shift += 4;
-		}
-		shift &= 0xFF;
 		uint32_t shiftVal = cpu->gprs[rm];
 		if (rm == ARM_PC) {
 			shiftVal += 4;
 		}
+		int shift = cpu->gprs[rs] & 0xFF;
 		if (!shift) {
 			cpu->shifterOperand = shiftVal;
 			cpu->shifterCarryOut = cpu->cpsr.c;
@@ -97,15 +89,11 @@ static inline void _shiftASR(struct ARMCore* cpu, uint32_t opcode) {
 	if (opcode & 0x00000010) {
 		int rs = (opcode >> 8) & 0x0000000F;
 		++cpu->cycles;
-		int shift = cpu->gprs[rs];
-		if (rs == ARM_PC) {
-			shift += 4;
-		}
-		shift &= 0xFF;
 		int shiftVal =  cpu->gprs[rm];
 		if (rm == ARM_PC) {
 			shiftVal += 4;
 		}
+		int shift = cpu->gprs[rs] & 0xFF;
 		if (!shift) {
 			cpu->shifterOperand = shiftVal;
 			cpu->shifterCarryOut = cpu->cpsr.c;
@@ -136,15 +124,11 @@ static inline void _shiftROR(struct ARMCore* cpu, uint32_t opcode) {
 	if (opcode & 0x00000010) {
 		int rs = (opcode >> 8) & 0x0000000F;
 		++cpu->cycles;
-		int shift = cpu->gprs[rs];
-		if (rs == ARM_PC) {
-			shift += 4;
-		}
-		shift &= 0xFF;
 		int shiftVal =  cpu->gprs[rm];
 		if (rm == ARM_PC) {
 			shiftVal += 4;
 		}
+		int shift = cpu->gprs[rs] & 0xFF;
 		int rotate = shift & 0x1F;
 		if (!shift) {
 			cpu->shifterOperand = shiftVal;