Implement byte swapping instructions on MIPS32 and MIPS64.

src/mips/simulator-mips.cc

Index: src/mips/simulator-mips.cc
diff --git a/src/mips/simulator-mips.cc b/src/mips/simulator-mips.cc
index 07cfcff5b4813e7d16276009b2ca7c3ca182b8f8..1f69ac91982fdd8d54d0f5a1ef0c12433c956493 100644
--- a/src/mips/simulator-mips.cc
+++ b/src/mips/simulator-mips.cc
@@ -3833,12 +3833,45 @@ void Simulator::DecodeTypeRegisterSPECIAL3() {
           alu_out = static_cast<int32_t>(output);
           break;
         }
-        case SEB:
-        case SEH:
-        case WSBH:
-          alu_out = 0x12345678;
-          UNREACHABLE();
+        case SEB: {
+          int8_t input = static_cast<int8_t>(rt());
+          int32_t output = 0x000000FF & input;

[balazs.kilvady, 2016/06/21 10:24:58]

In the above line if input is an int8_t then why do you mask it? Is it needed?
Question for all the similar masks below.

+          int32_t mask = 0x00000080;
+
+          if (mask & input) output += 0xFFFFFF00;
+
+          alu_out = static_cast<int32_t>(output);
+          break;
+        }
+        case SEH: {
+          int16_t input = static_cast<int16_t>(rt());
+          int32_t output = 0x0000FFFF & input;
+          int32_t mask = 0x00008000;
+
+          if (mask & input) output += 0xFFFF0000;
+
+          alu_out = static_cast<int32_t>(output);
           break;
+        }
+        case WSBH: {
+          int32_t input = static_cast<uint32_t>(rt());
+          int32_t output = 0;
+
+          uint32_t mask = 0xFF000000;
+          for (int i = 0; i < 4; i++) {
+            uint32_t tmp = mask & input;
+            if (i % 2 == 0) {
+              tmp = tmp >> 8;
+            } else {
+              tmp = tmp << 8;
+            }
+            output = output | tmp;
+            mask = mask >> 8;
+          }
+
+          alu_out = static_cast<int32_t>(output);
+          break;
+        }
         default: {
           const uint8_t bp = get_instr()->Bp2Value();
           sa >>= kBp2Bits;