Re: [PATCH 08/13] target/i386: make cc_op handling more explicit for repeated string instructions.

[Richard Henderson]

On 12/15/24 03:06, Paolo Bonzini wrote:
>       fn(s, ot);
>       gen_op_add_reg_im(s, s->aflag, R_ECX, -1);
> +    gen_update_cc_op(s);
> +
> +    /* Leave if REP condition fails.  */
>       if (is_repz_nz) {
>           int nz = (s->prefix & PREFIX_REPNZ) ? 1 : 0;
> -        gen_jcc(s, (JCC_Z << 1) | (nz ^ 1), done);
> +        gen_jcc_noeob(s, (JCC_Z << 1) | (nz ^ 1), done);

The comment in gen_jcc would still seem to apply:

    CCPrepare cc = gen_prepare_cc(s, b, NULL);

    /*
     * Note that this must be _after_ gen_prepare_cc, because it
     * can change the cc_op from CC_OP_DYNAMIC to CC_OP_EFLAGS!
     */
    gen_update_cc_op(s);

via any path through gen_prepare_cc that reaches gen_compute_eflags.

However!  Because this is JCC_Z, we will never call gen_compute_eflags, we will always go 
through the gen_prepare_eflags_z, which doesn't have the same problem.

This subtlety deserves a comment and maybe an assert.  Perhaps

    gen_jcc_noeob(...);
    assert(!s->cc_op_dirty);


r~