From 95588d3faa4bef8532e96ac20e3836a7bdedcabc Mon Sep 17 00:00:00 2001
From: MerryMage <MerryMage@users.noreply.github.com>
Date: Mon, 18 Jul 2016 22:48:23 +0100
Subject: [PATCH] Fix Thumb BLX (imm), BL (imm) for negative immediates

---
 src/frontend/translate/translate_thumb.cpp |  4 ++--
 tests/arm/test_thumb_instructions.cpp      | 16 ++++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/frontend/translate/translate_thumb.cpp b/src/frontend/translate/translate_thumb.cpp
index d16dc2a4..cdd77e7b 100644
--- a/src/frontend/translate/translate_thumb.cpp
+++ b/src/frontend/translate/translate_thumb.cpp
@@ -789,7 +789,7 @@ struct ThumbTranslatorVisitor final {
     }
 
     bool thumb32_BL_imm(Imm11 hi, Imm11 lo) {
-        s32 imm32 = Common::SignExtend<32, s32>((hi << 12) | (lo << 1));
+        s32 imm32 = Common::SignExtend<23, s32>((hi << 12) | (lo << 1));
         // BL <label>
         ir.SetRegister(Reg::LR, ir.Imm32((ir.current_location.arm_pc + 4) | 1));
         auto new_location = ir.current_location;
@@ -799,7 +799,7 @@ struct ThumbTranslatorVisitor final {
     }
 
     bool thumb32_BLX_imm(Imm11 hi, Imm11 lo) {
-        s32 imm32 = Common::SignExtend<32, s32>((hi << 12) | (lo << 1));
+        s32 imm32 = Common::SignExtend<23, s32>((hi << 12) | (lo << 1));
         if ((lo & 1) != 0) {
             return UnpredictableInstruction();
         }
diff --git a/tests/arm/test_thumb_instructions.cpp b/tests/arm/test_thumb_instructions.cpp
index cf3f42fe..23d97a96 100644
--- a/tests/arm/test_thumb_instructions.cpp
+++ b/tests/arm/test_thumb_instructions.cpp
@@ -152,3 +152,19 @@ TEST_CASE( "thumb: bl +#234584", "[thumb]" ) {
     REQUIRE( jit.Regs()[15] == 0x39458 );
     REQUIRE( jit.Cpsr() == 0x00000030 ); // Thumb, User-mode
 }
+
+TEST_CASE( "thumb: bl -#42", "[thumb]" ) {
+    Dynarmic::Jit jit{GetUserCallbacks()};
+    code_mem.fill({});
+    code_mem[0] = 0xF7FF; code_mem[1] = 0xFFE9; // bl -#42
+    code_mem[2] = 0xE7FE; // b +#0
+
+    jit.Regs()[15] = 0; // PC = 0
+    jit.Cpsr() = 0x00000030; // Thumb, User-mode
+
+    jit.Run(1);
+
+    REQUIRE( jit.Regs()[14] == (0x4 | 1) );
+    REQUIRE( jit.Regs()[15] == 0xFFFFFFD6 );
+    REQUIRE( jit.Cpsr() == 0x00000030 ); // Thumb, User-mode
+}