target-arm: Check undefined opcodes for SWP in A32 decoder

Make sure we are not treating architecturally Undefined instructions
as a SWP, by verifying the opcodes as per section A8.8.229 of ARMv7-A
specification. Bits [21:20] must be zero for this to be a SWP or SWPB.
We also choose to UNDEF for the architecturally UNPREDICTABLE case of
bits [11:8] not being zero.

Signed-off-by: Onur Sahin <onursahin08@gmail.com>
[PMM: tweaked commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/translate.c

@@ -9237,11 +9237,14 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                             }
                         }
                         tcg_temp_free_i32(addr);
-                    } else {
+                    } else if ((insn & 0x00300f00) == 0) {
+                        /* 0bcccc_0001_0x00_xxxx_xxxx_0000_1001_xxxx
+                        *  - SWP, SWPB
+                        */
+
                         TCGv taddr;
                         TCGMemOp opc = s->be_data;
 
-                        /* SWP instruction */
                         rm = (insn) & 0xf;
 
                         if (insn & (1 << 22)) {
@@ -9259,6 +9262,8 @@ static void disas_arm_insn(DisasContext *s, unsigned int insn)
                                                 get_mem_index(s), opc);
                         tcg_temp_free(taddr);
                         store_reg(s, rd, tmp);
+                    } else {
+                        goto illegal_op;
                     }
                 }
             } else {