target/mips: Simplify MSA TCG logic

Only decode MSA opcodes if MSA is present (implemented).

Now than check_msa_access() will only be called if MSA is
present, the only way to have MIPS_HFLAG_MSA unset is if
MSA is disabled (bit CP0C5_MSAEn cleared, see previous
commit). Therefore we can remove the 'reserved instruction'
exception.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201208003702.4088927-5-f4bug@amsat.org>

target/mips/translate.c

@@ -28567,13 +28567,8 @@ static inline int check_msa_access(DisasContext *ctx)
     }
     if (unlikely(!(ctx->hflags & MIPS_HFLAG_MSA))) {
-        if (ctx->insn_flags & ASE_MSA) {
-            generate_exception_end(ctx, EXCP_MSADIS);
-            return 0;
-        } else {
-            gen_reserved_instruction(ctx);
-            return 0;
-        }
+        generate_exception_end(ctx, EXCP_MSADIS);
+        return 0;
     }
     return 1;
 }
@@ -30417,7 +30412,7 @@ static void gen_msa_vec(CPUMIPSState *env, DisasContext *ctx)
 static void gen_msa(CPUMIPSState *env, DisasContext *ctx)
 {
     uint32_t opcode = ctx->opcode;
-    check_insn(ctx, ASE_MSA);
+
     check_msa_access(ctx);
     switch (MASK_MSA_MINOR(opcode)) {
@@ -31047,9 +31042,11 @@ static bool decode_opc_legacy(CPUMIPSState *env, DisasContext *ctx)
         case OPC_BNZ_H:
         case OPC_BNZ_W:
         case OPC_BNZ_D:
-            check_insn(ctx, ASE_MSA);
-            gen_msa_branch(env, ctx, op1);
-            break;
+            if (ase_msa_available(env)) {
+                gen_msa_branch(env, ctx, op1);
+                break;
+            }
+            /* fall through */
         default:
             MIPS_INVAL("cp1");
             gen_reserved_instruction(ctx);
@@ -31238,7 +31235,9 @@ static bool decode_opc_legacy(CPUMIPSState *env, DisasContext *ctx)
 #endif
         } else {
             /* MDMX: Not implemented. */
-            gen_msa(env, ctx);
+            if (ase_msa_available(env)) {
+                gen_msa(env, ctx);
+            }
         }
         break;
     case OPC_PCREL: