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>

1 files changed, 10 insertions, 2 deletions

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;