Re: [Qemu-devel] [PATCH v3 34/34] target-alpha: Emulate LL/SC using cmpxchg helpers

[Richard Henderson]

On 09/15/2016 07:38 AM, Alex Bennée wrote:
> > +    lab_fail = gen_new_label();
> > > +    lab_done = gen_new_label();
> > > +    tcg_gen_brcond_i64(TCG_COND_NE, addr, cpu_lock_addr, lab_fail);
> > > +    tcg_temp_free_i64(addr);
> > >
> > > -        lab_fail = gen_new_label();
> > > -        lab_done = gen_new_label();
> > > -        tcg_gen_brcond_i64(TCG_COND_NE, addr, cpu_lock_addr, lab_fail);
> > > +    val = tcg_temp_new_i64();
> > > +    tcg_gen_atomic_cmpxchg_i64(val, cpu_lock_addr, cpu_lock_value,
> > > +                               load_gpr(ctx, ra), mem_idx, op);
> > > +    free_context_temps(ctx);
> I don't quite follow what free_context_temps() is doing and why it needs
> to be done twice during the building of this instructions TCGOps?

It's releasing the zero, sink, imm TCGv temporaries from DisasContext, as 
they're going out of scope within each basic block.  Since they're freed, 
they'll be re-created as necessary in the next basic block.

The two (unlikely) relevant events are: (1) address register = zero register, 
so the zero temp becomes invalid, and (2) dest register = zero register, so the 
zero/sink temps become invalid.

It's the kind of thing that really shouldn't ever happen for non-malicious 
code, but you don't want qemu to abort when invalid temps are used.  Perhaps a 
comment is warranted here...


r~