arm: Fix implicit-fallthrough warnings when building with gcc-7+

gcc 7 onwards have additional heuristics to detect implicit
fallthroughs and it fails the build with warnings for ARM as a result.
There was one gcc bug[1] that I fixed but the rest are cases that gcc
cannot detect due to the point at which it does the fallthrough check.
Most of this patch adds __builtin_unreachable() hints in places that throw
this warning to indicate to gcc that the fallthrough will never
happen.

The remaining cases are actually possible fallthroughs due to
incorrect code running on the simulator; in which case an Unknown
instruction is returned.

[1] https://gcc.gnu.org/ml/gcc-patches/2018-02/msg01105.html

Change-Id: I1baa9fa0ed15181c10c755c0bd777f88b607c158
Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
Reviewed-on: https://gem5-review.googlesource.com/8541
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

9 files changed, 155 insertions, 58 deletions

diff --git a/src/arch/arm/insts/pred_inst.hh b/src/arch/arm/insts/pred_inst.hh
index b4186c8e5..ce4d41bac 100644
--- a/src/arch/arm/insts/pred_inst.hh
+++ b/src/arch/arm/insts/pred_inst.hh
@@ -150,7 +150,7 @@ simd_modified_imm(bool op, uint8_t cmode, uint8_t data, bool &immValid,
                 break;
             }
         }
-        // Fall through, immediate encoding is invalid.
+        M5_FALLTHROUGH;
       default:
         immValid = false;
         break;
diff --git a/src/arch/arm/isa/formats/aarch64.isa b/src/arch/arm/isa/formats/aarch64.isa
index 68f600698..1b9a86cad 100644
--- a/src/arch/arm/isa/formats/aarch64.isa
+++ b/src/arch/arm/isa/formats/aarch64.isa
@@ -108,6 +108,8 @@ namespace Aarch64
                 return new SubXImm(machInst, rdsp, rnsp, imm);
               case 0x3:
                 return new SubXImmCc(machInst, rdzr, rnsp, imm);
+              default:
+                M5_UNREACHABLE;
             }
           }
           case 0x4:
@@ -150,6 +152,8 @@ namespace Aarch64
                 return new EorXImm(machInst, rdsp, rn, imm);
               case 0x3:
                 return new AndXImmCc(machInst, rdzr, rn, imm);
+              default:
+                M5_UNREACHABLE;
             }
           }
           case 0x5:
@@ -167,6 +171,8 @@ namespace Aarch64
                 return new Movz(machInst, rdzr, imm16, hw * 16);
               case 0x3:
                 return new Movk(machInst, rdzr, imm16, hw * 16);
+              default:
+                M5_UNREACHABLE;
             }
           }
           case 0x6:
@@ -181,6 +187,8 @@ namespace Aarch64
                 return new Ubfm64(machInst, rdzr, rn, immr, imms);
               case 0x3:
                 return new Unknown64(machInst);
+              default:
+                M5_UNREACHABLE;
             }
           case 0x7:
           {
@@ -405,6 +413,8 @@ namespace Aarch64
                     }
                   }
                   break;
+                  default:
+                    M5_UNREACHABLE;
                 }
             } else if (bits(machInst, 25) == 0x1) {
                 uint8_t opc = bits(machInst, 24, 21);
@@ -429,8 +439,11 @@ namespace Aarch64
                     if (rn != 0x1f)
                         return new Unknown64(machInst);
                     return new FailUnimplemented("dret", machInst);
+                  default:
+                    return new Unknown64(machInst);
                 }
             }
+          M5_FALLTHROUGH;
           default:
             return new Unknown64(machInst);
         }
@@ -470,6 +483,8 @@ namespace Aarch64
                         return new STXRW64(machInst, rt, rnsp, rs);
                       case 0x3:
                         return new STXRX64(machInst, rt, rnsp, rs);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   case 0x1:
                     switch (size) {
@@ -481,6 +496,8 @@ namespace Aarch64
                         return new STLXRW64(machInst, rt, rnsp, rs);
                       case 0x3:
                         return new STLXRX64(machInst, rt, rnsp, rs);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   case 0x2:
                     switch (size) {
@@ -491,6 +508,8 @@ namespace Aarch64
                         return new STXPW64(machInst, rs, rt, rt2, rnsp);
                       case 0x3:
                         return new STXPX64(machInst, rs, rt, rt2, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
 
                   case 0x3:
@@ -502,6 +521,8 @@ namespace Aarch64
                         return new STLXPW64(machInst, rs, rt, rt2, rnsp);
                       case 0x3:
                         return new STLXPX64(machInst, rs, rt, rt2, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
 
                   case 0x4:
@@ -514,6 +535,8 @@ namespace Aarch64
                         return new LDXRW64(machInst, rt, rnsp, rs);
                       case 0x3:
                         return new LDXRX64(machInst, rt, rnsp, rs);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   case 0x5:
                     switch (size) {
@@ -525,6 +548,8 @@ namespace Aarch64
                         return new LDAXRW64(machInst, rt, rnsp, rs);
                       case 0x3:
                         return new LDAXRX64(machInst, rt, rnsp, rs);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   case 0x6:
                     switch (size) {
@@ -535,6 +560,8 @@ namespace Aarch64
                         return new LDXPW64(machInst, rt, rt2, rnsp);
                       case 0x3:
                         return new LDXPX64(machInst, rt, rt2, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
 
                   case 0x7:
@@ -546,6 +573,8 @@ namespace Aarch64
                         return new LDAXPW64(machInst, rt, rt2, rnsp);
                       case 0x3:
                         return new LDAXPX64(machInst, rt, rt2, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
 
                   case 0x9:
@@ -558,6 +587,8 @@ namespace Aarch64
                         return new STLRW64(machInst, rt, rnsp);
                       case 0x3:
                         return new STLRX64(machInst, rt, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   case 0xd:
                     switch (size) {
@@ -569,6 +600,8 @@ namespace Aarch64
                         return new LDARW64(machInst, rt, rnsp);
                       case 0x3:
                         return new LDARX64(machInst, rt, rnsp);
+                      default:
+                        M5_UNREACHABLE;
                     }
                   default:
                     return new Unknown64(machInst);
@@ -1021,9 +1054,13 @@ namespace Aarch64
                         return new Unknown64(machInst);
                     }
                   }
+                  default:
+                    M5_UNREACHABLE;
                 }
             }
           }
+          default:
+            M5_UNREACHABLE;
         }
         return new FailUnimplemented("Unhandled Case1", machInst);
     }
@@ -1070,6 +1107,8 @@ namespace Aarch64
                 return new AndXSRegCc(machInst, rdzr, rn, rm, imm6, type);
               case 0x7:
                 return new BicXSRegCc(machInst, rdzr, rn, rm, imm6, type);
+              default:
+                M5_UNREACHABLE;
             }
           }
           case 0x1:
@@ -1096,6 +1135,8 @@ namespace Aarch64
                     return new SubXSReg(machInst, rdzr, rn, rm, imm6, type);
                   case 0x3:
                     return new SubXSRegCc(machInst, rdzr, rn, rm, imm6, type);
+                  default:
+                    M5_UNREACHABLE;
                 }
             } else {
                 if (bits(machInst, 23, 22) != 0 || bits(machInst, 12, 10) > 0x4)
@@ -1119,6 +1160,8 @@ namespace Aarch64
                     return new SubXEReg(machInst, rdsp, rnsp, rm, type, imm3);
                   case 0x3:
                     return new SubXERegCc(machInst, rdzr, rnsp, rm, type, imm3);
+                  default:
+                    M5_UNREACHABLE;
                 }
             }
           }
@@ -1145,6 +1188,8 @@ namespace Aarch64
                     return new SbcXSReg(machInst, rdzr, rn, rm, 0, LSL);
                   case 0x3:
                     return new SbcXSRegCc(machInst, rdzr, rn, rm, 0, LSL);
+                  default:
+                    M5_UNREACHABLE;
                 }
               }
               case 0x1:
@@ -1198,6 +1243,8 @@ namespace Aarch64
                     return new Csinv64(machInst, rdzr, rn, rm, cond);
                   case 0x3:
                     return new Csneg64(machInst, rdzr, rn, rm, cond);
+                  default:
+                    M5_UNREACHABLE;
                 }
               }
               case 0x3:
@@ -1261,8 +1308,12 @@ namespace Aarch64
                         return new Clz64(machInst, rdzr, rn);
                       case 0x5:
                         return new Cls64(machInst, rdzr, rn);
+                      default:
+                        return new Unknown64(machInst);
                     }
                 }
+              default:
+                M5_UNREACHABLE;
             }
           }
           case 0x3:
@@ -1303,6 +1354,8 @@ namespace Aarch64
                 return new Unknown64(machInst);
             }
           }
+          default:
+            M5_UNREACHABLE;
         }
         return new FailUnimplemented("Unhandled Case2", machInst);
     }
@@ -1484,6 +1537,8 @@ namespace Aarch64
                   default:
                     return new Unknown64(machInst);
                 }
+              default:
+                return new Unknown64(machInst);
             }
         } else {
             // 30=0, 28:24=11110, 21=1
@@ -1946,6 +2001,8 @@ namespace Aarch64
                 else
                     return new Unknown64(machInst);
               }
+              default:
+                M5_UNREACHABLE;
             }
         }
         return new FailUnimplemented("Unhandled Case4", machInst);
diff --git a/src/arch/arm/isa/formats/data.isa b/src/arch/arm/isa/formats/data.isa
index eab081827..b5e29c583 100644
--- a/src/arch/arm/isa/formats/data.isa
+++ b/src/arch/arm/isa/formats/data.isa
@@ -76,17 +76,17 @@ def format ArmMiscMedia() {{
 
 def format ArmDataProcReg() {{
     pclr = '''
-        return new %(className)ssRegPclr(machInst, %(dest)s,
-                                        %(op1)s, rm, imm5,
-                                        type);
+                    if (%(dest)s == INTREG_PC) {
+                        return new %(className)ssRegPclr(machInst, %(dest)s,
+                                                         %(op1)s, rm, imm5,
+                                                         type);
+                    } else
     '''
     instDecode = '''
           case %(opcode)#x:
             if (immShift) {
                 if (setCc) {
-                    if (%(dest)s == INTREG_PC) {
-                        %(pclr)s
-                    } else {
+                    %(pclr)s {
                         return new %(className)sRegCc(machInst, %(dest)s,
                                                       %(op1)s, rm, imm5, type);
                     }
@@ -452,26 +452,26 @@ def format ArmParallelAddSubtract() {{
 
 def format ArmDataProcImm() {{
     pclr = '''
-        return new %(className)ssImmPclr(machInst, %(dest)s,
-                                        %(op1)s, imm, false);
+                if (%(dest)s == INTREG_PC) {
+                    return new %(className)ssImmPclr(machInst, %(dest)s,
+                                                     %(op1)s, imm, false);
+                } else
     '''
     adr = '''
-        return new AdrImm(machInst, %(dest)s, %(add)s,
-                                     imm, false);
+                if (%(op1)s == INTREG_PC) {
+                    return new AdrImm(machInst, %(dest)s, %(add)s,
+                                      imm, false);
+                } else
     '''
     instDecode = '''
           case %(opcode)#x:
             if (setCc) {
-                if (%(pclrInst)s && %(dest)s == INTREG_PC) {
-                    %(pclr)s
-                } else {
+                %(pclr)s {
                     return new %(className)sImmCc(machInst, %(dest)s, %(op1)s,
                                                    imm, rotC);
                 }
             } else {
-                if (%(adrInst)s && %(op1)s == INTREG_PC) {
-                    %(adr)s
-                } else {
+                %(adr)s {
                     return new %(className)sImm(machInst, %(dest)s, %(op1)s,
                                                  imm, rotC);
                 }
@@ -493,13 +493,10 @@ def format ArmDataProcImm() {{
                       "opcode": opcode,
                       "dest": dest,
                       "op1": op1,
-                      "adr": "",
-                      "adrInst": "false" }
+                      "adr": "" }
         if useDest:
-            substDict["pclrInst"] = "true"
             substDict["pclr"] = pclr % substDict
         else:
-            substDict["pclrInst"] = "false"
             substDict["pclr"] = ""
         return instDecode % substDict
 
@@ -509,9 +506,7 @@ def format ArmDataProcImm() {{
                       "opcode": opcode,
                       "dest": "rd",
                       "op1": "rn",
-                      "add": add,
-                      "pclrInst": "true",
-                      "adrInst": "true" }
+                      "add": add }
         substDict["pclr"] = pclr % substDict
         substDict["adr"] = adr % substDict
         return instDecode % substDict
@@ -611,6 +606,8 @@ def format Thumb32DataProcReg() {{
                   case 0x7:
                     return new MovRegRegCc(machInst, rd,
                             INTREG_ZERO, rn, rm, ROR);
+                  default:
+                    M5_UNREACHABLE;
                 }
             } else if (bits(op2, 3) == 0) {
                 return new Unknown(machInst);
@@ -916,6 +913,8 @@ def format Thumb16ShiftAddSubMoveCmp() {{
                 } else {
                     return new SubImmCc(machInst, rd, rn, imm3, true);
                 }
+              default:
+                M5_UNREACHABLE;
             }
           case 0x4:
             if (machInst.itstateMask) {
@@ -937,6 +936,8 @@ def format Thumb16ShiftAddSubMoveCmp() {{
             } else {
                 return new SubImmCc(machInst, rd8, rd8, imm8, true);
             }
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
@@ -1040,6 +1041,8 @@ def format Thumb16DataProcessing() {{
             } else {
                 return new MvnRegCc(machInst, rdn, INTREG_ZERO, rm, 0, LSL);
             }
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
@@ -1069,6 +1072,8 @@ def format Thumb16SpecDataAndBx() {{
                                   (IntRegIndex)(uint32_t)bits(machInst, 6, 3),
                                   COND_UC);
             }
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
@@ -1169,6 +1174,8 @@ def format Thumb16Misc() {{
                     return new Uxth(machInst, rd, 0, rm);
                   case 0x3:
                     return new Uxtb(machInst, rd, 0, rm);
+                  default:
+                    M5_UNREACHABLE;
                 }
             }
           case 0x1:
@@ -1196,6 +1203,7 @@ def format Thumb16Misc() {{
                                           ((enable ? 1 : 0) << 9);
                     return new Cps(machInst, mods);
                 }
+                return new Unknown(machInst);
             }
           case 0xa:
             {
@@ -1407,7 +1415,7 @@ def format Thumb32DataProcPlainBin() {{
                 const uint32_t satImm = bits(machInst, 4, 0);
                 return new Ssat16(machInst, rd, satImm + 1, rn);
             }
-            // Fall through on purpose...
+            M5_FALLTHROUGH;
           case 0x10:
             {
                 const uint32_t satImm = bits(machInst, 4, 0);
@@ -1440,7 +1448,7 @@ def format Thumb32DataProcPlainBin() {{
                 const uint32_t satImm = bits(machInst, 4, 0);
                 return new Usat16(machInst, rd, satImm, rn);
             }
-            // Fall through on purpose...
+            M5_FALLTHROUGH;
           case 0x18:
             {
                 const uint32_t satImm = bits(machInst, 4, 0);
diff --git a/src/arch/arm/isa/formats/fp.isa b/src/arch/arm/isa/formats/fp.isa
index 82d351e6e..009b27c8f 100644
--- a/src/arch/arm/isa/formats/fp.isa
+++ b/src/arch/arm/isa/formats/fp.isa
@@ -130,7 +130,7 @@ let {{
                     width = 1;
                     break;
                 }
-                // Fall through on purpose.
+                M5_FALLTHROUGH;
               default:
                 return new Unknown(machInst);
             }
@@ -404,6 +404,8 @@ let {{
                         } else {
                             return new VbifD<uint64_t>(machInst, vd, vn, vm);
                         }
+                      default:
+                        M5_UNREACHABLE;
                     }
                 } else {
                     switch (c) {
@@ -445,6 +447,8 @@ let {{
                             return new VornD<uint64_t>(
                                     machInst, vd, vn, vm);
                         }
+                      default:
+                        M5_UNREACHABLE;
                     }
                 }
             }
@@ -1550,6 +1554,8 @@ let {{
                     return decodeNeonSTwoMiscReg<NVnegD, NVnegQ>(
                             q, size, machInst, vd, vm);
                 }
+              default:
+                return new Unknown64(machInst);
             }
           case 0x2:
             switch (bits(b, 4, 1)) {
@@ -1859,6 +1865,8 @@ let {{
                     // If rn == sp, then this is called vpop.
                     return new VLdmStm(machInst, rn, vd, single,
                                        true, true, true, offset);
+                  default:
+                    M5_UNREACHABLE;
                 }
             }
           case 0x2:
@@ -1870,7 +1878,7 @@ let {{
                 return new VLdmStm(machInst, rn, vd, single,
                                    false, true, true, offset);
             }
-            // Fall through on purpose
+            M5_FALLTHROUGH;
           case 0x3:
             const bool up = (bits(machInst, 23) == 1);
             const uint32_t imm = bits(machInst, 7, 0) << 2;
diff --git a/src/arch/arm/isa/formats/m5ops.isa b/src/arch/arm/isa/formats/m5ops.isa
index d3db81373..6b4112d5a 100644
--- a/src/arch/arm/isa/formats/m5ops.isa
+++ b/src/arch/arm/isa/formats/m5ops.isa
@@ -68,6 +68,7 @@ def format M5ops() {{
             case M5OP_PANIC: return new M5panic(machInst);
             case M5OP_WORK_BEGIN: return new M5workbegin(machInst);
             case M5OP_WORK_END: return new M5workend(machInst);
+            default: return new Unknown(machInst);
         }
    }
    '''
diff --git a/src/arch/arm/isa/formats/mem.isa b/src/arch/arm/isa/formats/mem.isa
index abac27021..1a30fbb9d 100644
--- a/src/arch/arm/isa/formats/mem.isa
+++ b/src/arch/arm/isa/formats/mem.isa
@@ -470,6 +470,7 @@ def format Thumb32LoadWord() {{
                     return new %(imm_puw)s(machInst, RT, RN, true, imm);
                 }
             }
+          return new Unknown(machInst);
         } else {
             return new Unknown(machInst);
         }
@@ -514,6 +515,8 @@ def format Thumb32StoreSingle() {{
                         return new %(imm_pu)s(machInst, RT, RN, true, imm);
                       case 7:
                         return new %(imm_puw)s(machInst, RT, RN, true, imm);
+                      default:
+                        M5_UNREACHABLE;
                     }
                 }
         '''
@@ -978,6 +981,8 @@ def format Thumb16MemReg() {{
             return new %(ldrb)s(machInst, rt, rn, true, 0, LSL, rm);
           case 0x7:
             return new %(ldrsh)s(machInst, rt, rn, true, 0, LSL, rm);
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
diff --git a/src/arch/arm/isa/formats/mult.isa b/src/arch/arm/isa/formats/mult.isa
index 73157dd57..142bfd67c 100644
--- a/src/arch/arm/isa/formats/mult.isa
+++ b/src/arch/arm/isa/formats/mult.isa
@@ -87,6 +87,8 @@ def format ArmMultAndMultAcc() {{
               } else {
                   return new Smlal(machInst, ra, rd, rn, rm);
               }
+            default:
+              M5_UNREACHABLE;
         }
     }
     '''
@@ -112,6 +114,8 @@ def format ArmHalfWordMultAndMultAcc() {{
                 return new SmlabtCc(machInst, rd, rn, rm, ra);
               case 0x3:
                 return new SmlattCc(machInst, rd, rn, rm, ra);
+              default:
+                M5_UNREACHABLE;
             }
           case 0x1:
             if (op) {
@@ -137,6 +141,8 @@ def format ArmHalfWordMultAndMultAcc() {{
                 return new Smlalbt(machInst, ra, rd, rn, rm);
               case 0x3:
                 return new Smlaltt(machInst, ra, rd, rn, rm);
+              default:
+                M5_UNREACHABLE;
             }
           case 0x3:
             switch (bits(machInst, 6, 5)) {
@@ -148,7 +154,11 @@ def format ArmHalfWordMultAndMultAcc() {{
                 return new Smulbt(machInst, rd, rn, rm);
               case 0x3:
                 return new Smultt(machInst, rd, rn, rm);
+              default:
+                M5_UNREACHABLE;
             }
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
@@ -201,6 +211,7 @@ def format Thumb32MulMulAccAndAbsDiff() {{
                     return new SmlattCc(machInst, rd, rn, rm, ra);
                 }
             }
+            M5_UNREACHABLE;
           case 0x2:
             if (ra == 0xf) {
                 if (bits(machInst, 4)) {
@@ -271,6 +282,8 @@ def format Thumb32MulMulAccAndAbsDiff() {{
             } else {
                 return new Usada8(machInst, rd, rn, rm, ra);
             }
+          default:
+            M5_UNREACHABLE;
         }
     }
     '''
diff --git a/src/arch/arm/isa/formats/neon64.isa b/src/arch/arm/isa/formats/neon64.isa
index e0a913a6b..b4d4fdf7b 100644
--- a/src/arch/arm/isa/formats/neon64.isa
+++ b/src/arch/arm/isa/formats/neon64.isa
@@ -181,6 +181,8 @@ namespace Aarch64
                     else
                         return new OrnDX<uint64_t>(machInst, vd, vn, vm);
                 }
+              default:
+                M5_UNREACHABLE;
             }
           case 0x04:
             if (size == 0x3)
@@ -1211,6 +1213,8 @@ namespace Aarch64
                     return new DupGprXQX<uint64_t>(machInst, vd, vn);
                 else
                     return new Unknown64(machInst);
+              default:
+                return new Unknown64(machInst);
             }
           case 0x3:
             index1 = imm5 >> (imm5_pos + 1);
@@ -2065,17 +2069,17 @@ namespace Aarch64
                 return decodeNeonUTwoMiscScFpReg<FcmltZeroScX>(
                     size & 0x1, machInst, vd, vn);
           case 0x14:
-            if (size == 0x3) {
+            switch (size) {
+              case 0x0:
+                return new SqxtnScX<int8_t>(machInst, vd, vn);
+              case 0x1:
+                return new SqxtnScX<int16_t>(machInst, vd, vn);
+              case 0x2:
+                return new SqxtnScX<int32_t>(machInst, vd, vn);
+              case 0x3:
                 return new Unknown64(machInst);
-            } else {
-                switch (size) {
-                  case 0x0:
-                    return new SqxtnScX<int8_t>(machInst, vd, vn);
-                  case 0x1:
-                    return new SqxtnScX<int16_t>(machInst, vd, vn);
-                  case 0x2:
-                    return new SqxtnScX<int32_t>(machInst, vd, vn);
-                }
+              default:
+                M5_UNREACHABLE;
             }
           case 0x1a:
             if (size < 0x2)
@@ -2145,30 +2149,30 @@ namespace Aarch64
                 return decodeNeonUTwoMiscScFpReg<FcmleZeroScX>(
                     size & 0x1, machInst, vd, vn);
           case 0x32:
-            if (size == 0x3) {
+            switch (size) {
+              case 0x0:
+                return new SqxtunScX<int8_t>(machInst, vd, vn);
+              case 0x1:
+                return new SqxtunScX<int16_t>(machInst, vd, vn);
+              case 0x2:
+                return new SqxtunScX<int32_t>(machInst, vd, vn);
+              case 0x3:
                 return new Unknown64(machInst);
-            } else {
-                switch (size) {
-                  case 0x0:
-                    return new SqxtunScX<int8_t>(machInst, vd, vn);
-                  case 0x1:
-                    return new SqxtunScX<int16_t>(machInst, vd, vn);
-                  case 0x2:
-                    return new SqxtunScX<int32_t>(machInst, vd, vn);
-                }
+              default:
+                M5_UNREACHABLE;
             }
           case 0x34:
-            if (size == 0x3) {
+            switch (size) {
+              case 0x0:
+                return new UqxtnScX<uint8_t>(machInst, vd, vn);
+              case 0x1:
+                return new UqxtnScX<uint16_t>(machInst, vd, vn);
+              case 0x2:
+                return new UqxtnScX<uint32_t>(machInst, vd, vn);
+              case 0x3:
                 return new Unknown64(machInst);
-            } else {
-                switch (size) {
-                  case 0x0:
-                    return new UqxtnScX<uint8_t>(machInst, vd, vn);
-                  case 0x1:
-                    return new UqxtnScX<uint16_t>(machInst, vd, vn);
-                  case 0x2:
-                    return new UqxtnScX<uint32_t>(machInst, vd, vn);
-                }
+              default:
+                M5_UNREACHABLE;
             }
           case 0x36:
             if (size != 0x1) {
diff --git a/src/base/compiler.hh b/src/base/compiler.hh
index 6920dad10..6b0091410 100644
--- a/src/base/compiler.hh
+++ b/src/base/compiler.hh
@@ -55,6 +55,7 @@
 #  define M5_NO_INLINE __attribute__ ((__noinline__))
 #  define M5_DEPRECATED __attribute__((deprecated))
 #  define M5_DEPRECATED_MSG(MSG) __attribute__((deprecated(MSG)))
+#  define M5_UNREACHABLE __builtin_unreachable()
 #endif
 
 #if defined(__clang__)