Re: [Qemu-devel] [PATCH v11 24/29] target/arm: [tcg, a64] Port to translate_insn

[Richard Henderson <rth@twiddle.net>]

On 07/07/2017 01:18 AM, Lluís Vilanova wrote:
>>> diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
>>> index 9c870f6d07..586a01a2de 100644
>>> --- a/target/arm/translate-a64.c
>>> +++ b/target/arm/translate-a64.c
>>> @@ -11244,6 +11244,9 @@ static void aarch64_trblock_init_disas_context(DisasContextBase *dcbase,
> dc-> is_ldex = false;
> dc-> ss_same_el = (arm_debug_target_el(env) == dc->current_el);
>>> +    dc->next_page_start =
>>> +        (dc->base.pc_first & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
> 
>> I think a better solution for a fixed-length ISA is to adjust max_insns. Perhaps
>> the init_disas_context hook should be able to modify it?
> 
> ARM has the thumb instructions, so it really isn't a fixed-length ISA.

 From the filename above we're talking about AArch64 here.
Whcih is most definitely a fixed-width ISA.


That said, even for AArch32 we know by the TB flags whether or not we're going 
to be generating arm or thumb code.  I think that these hooks will allow arm 
and thumb mode to finally be split apart cleanly, instead of the tangle that 
they are now.

I see arm's gen_intermediate_code eventually looking like

   const TranslatorOps *ops = &arm_translator_ops;
   if (ARM_TBFLAG_THUMB(tb->flags)) {
     ops = &thumb_translator_ops;
   }
#ifdef TARGET_AARCH64
   if (ARM_TBFLAG_AARCH64_STATE(tb->flags)) {
     ops = &aarch64_translator_ops;
   }
#endif
   translator_loop(ops, &dc.base, cpu, tb);

There would certainly be some amount of shared code, but it would also allow 
quite a few of the existing dc->thumb checks to be eliminated.


>> And, while I'm thinking of it -- why is the init_globals hook separate? There's
>> nothing in between the two hook calls, and the more modern target front ends
>> won't need it.
> 
> You mean merging init_disas_context and init_globals? I wanted to keep
> semantically different code on separate hooks, but I can pull the init_globals
> code into init_disas_context (hoping that as targets get modernized, they won't
> initialize any global there).

I suppose you can leave the two hooks separate for now.  It doesn't hurt, and 
it's kind of a reminder of things that need cleaning up.

I do wonder if we should provide a generic empty hook, so that a target that 
does not need a particular hook need not define an empty function.  It could 
just put e.g. "translator_noop" into the structure.  Ok, maybe a better name 
than that...


r~