Re: [Qemu-devel] [PATCH v5 07/17] target/m68k: add chk and chk2

[Richard Henderson <richard.henderson@linaro.org>]

On 01/01/2018 05:10 PM, Laurent Vivier wrote:
> +    SRC_EA(env, tsrc, opsize, 1, NULL);
> +    src = tcg_temp_local_new();
> +    tcg_gen_mov_i32(src, tsrc);
> +
> +    reg = tcg_temp_local_new();
> +    gen_ext(reg, DREG(insn, 9), opsize, 1);
> +    gen_flush_flags(s);
> +    update_cc_op(s);
> +
> +    l1 = gen_new_label();
> +    l2 = gen_new_label();
> +    tcg_gen_brcondi_i32(TCG_COND_GE, reg, 0, l1);
> +    tcg_gen_movi_i32(QREG_CC_N, -1);
> +    tcg_gen_movi_i32(QREG_PC, s->pc);
> +    gen_raise_exception(EXCP_CHK);
> +    tcg_gen_br(l2);

Unreachable branch after exception.

> +    gen_set_label(l1);
> +    tcg_gen_brcond_i32(TCG_COND_LE, reg, src, l2);
> +    tcg_gen_movi_i32(QREG_CC_N, 0);
> +    tcg_gen_movi_i32(QREG_PC, s->pc);
> +    gen_raise_exception(EXCP_CHK);
> +    gen_set_label(l2);
> +    tcg_temp_free(src);
> +    tcg_temp_free(reg);

Does real hardware not change flags at all if the chk passes?  The manual says
it is undefined, which would greatly simplify all this.  Especially just
assigning reg to CC_N...

Generally I put conditional traps like this into a helper, which allows
straight-line optimization of the non-trapping path to continue.  In this case,
something like

void HELPER(chk)(CPUM68kState *env, int32_t val, int32_t ub)
{
    if (val < 0 || val > ub) {
        CPUState *cs = CPU(m68k_env_get_cpu(env));

	/* Recover PC and CC_OP for the beginning of the insn.  */
        cpu_restore_state(cs, GETPC());

        /* Adjust PC and FLAGS to end of the insn.  */
        env->pc += 2;
        helper_flush_flags(env, env->cc_op);
        env->cc_n = val;

        cs->exception_index = EXCP_CHK;
        cpu_loop_exit(cs);
    }
}


r~