target/sh4: implement tas.b using atomic helper

We only emulate UP SH4, however as the tas.b instruction is used in the GNU
libc, this improve linux-user emulation.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target/sh4/translate.c

@@ -1634,19 +1634,14 @@ static void _decode_opc(DisasContext * ctx)
 	tcg_gen_shri_i32(REG(B11_8), REG(B11_8), 16);
 	return;
     case 0x401b:		/* tas.b @Rn */
-	{
-	    TCGv addr, val;
-	    addr = tcg_temp_local_new();
-	    tcg_gen_mov_i32(addr, REG(B11_8));
-	    val = tcg_temp_local_new();
-            tcg_gen_qemu_ld_i32(val, addr, ctx->memidx, MO_UB);
+        {
+            TCGv val = tcg_const_i32(0x80);
+            tcg_gen_atomic_fetch_or_i32(val, REG(B11_8), val,
+                                        ctx->memidx, MO_UB);
             tcg_gen_setcondi_i32(TCG_COND_EQ, cpu_sr_t, val, 0);
-	    tcg_gen_ori_i32(val, val, 0x80);
-            tcg_gen_qemu_st_i32(val, addr, ctx->memidx, MO_UB);
-	    tcg_temp_free(val);
-	    tcg_temp_free(addr);
-	}
-	return;
+            tcg_temp_free(val);
+        }
+        return;
     case 0xf00d: /* fsts FPUL,FRn - FPSCR: Nothing */
 	CHECK_FPU_ENABLED
 	tcg_gen_mov_i32(cpu_fregs[FREG(B11_8)], cpu_fpul);