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..71dcda276646a2bed2257d4d2166b40f82b1a8d2 100644
--- a/src/mips/simulator-mips.cc
+++ b/src/mips/simulator-mips.cc
@@ -3833,12 +3833,51 @@ void Simulator::DecodeTypeRegisterSPECIAL3() {
           alu_out = static_cast<int32_t>(output);
           break;
         }
-        case SEB:
-        case SEH:
-        case WSBH:
-          alu_out = 0x12345678;
-          UNREACHABLE();
+        case SEB: {
+          uint8_t input = static_cast<uint8_t>(rt());
+          uint32_t output = input;
+          uint32_t mask = 0x00000080;
+
+          // Extending sign
+          if (mask & input) {
+            output |= 0xFFFFFF00;
+          }
+
+          alu_out = static_cast<int32_t>(output);
           break;
+        }
+        case SEH: {
+          uint16_t input = static_cast<uint16_t>(rt());
+          uint32_t output = input;
+          uint32_t mask = 0x00008000;
+
+          // Extending sign
+          if (mask & input) {
+            output |= 0xFFFF0000;
+          }
+
+          alu_out = static_cast<int32_t>(output);
+          break;
+        }
+        case WSBH: {
+          uint32_t input = static_cast<uint32_t>(rt());
+          uint32_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;