Re: [Qemu-devel] [PATCH v3 4/6] target/arm/translate: ensure gen_goto_tb sets exit flags

[Richard Henderson]

On 07/11/2017 09:20 AM, Alex Bennée wrote:
> Richard Henderson <address@hidden> writes:
>
> > On 07/11/2017 07:59 AM, Alex Bennée wrote:
> > >        if (use_goto_tb(s, dest)) {
> > >            tcg_gen_goto_tb(n);
> > >            gen_set_pc_im(s, dest);
> > >            tcg_gen_exit_tb((uintptr_t)s->tb + n);
> > > +        s->is_jmp = DISAS_TB_JUMP;
> > >        } else {
> > >            gen_set_pc_im(s, dest);
> > >            gen_goto_ptr();
> > > +        s->is_jmp = DISAS_JUMP;
> > >        }
> >
> > I think DISAS_TB_JUMP is appropriate for both cases.  When not using
> > goto_tb, the jump is still static and we still chain to the next TB
> > via goto_ptr.
>
> OK - I guess we need to nail down what the essential difference is
> between the two. I understood DISAS_TB_JUMP as a static known PC which
> can be patched in the generated code because we know the two addresses
> are in the same page - whereas DISAS_JUMP is a "computed" jump although
> in this case the PC is already known.
>
> Does making a distinction between computed and non-computer inter-page
> jumps make any sense anyway?

*shrug* probably not.

Honestly, the only thing that's really interesting here is that is_jmp indicate 
that we have already exited the TB and that all following code is dead.

As a response to one of Lluis' threads, I suggested that the generic term for 
this be DISAS_NORETURN.  Which also covers exiting the TB via calling a helper 
that does not return, e.g. throwing an illegal opcode exception, aka DISAS_EXC 
in the current target/arm sources.


r~