Re: [Qemu-devel] [PATCH 01/18] tcg: add support for 128bit vector type

[Richard Henderson]

On 01/19/2017 05:04 AM, Kirill Batuzov wrote:
> On Wed, 18 Jan 2017, Richard Henderson wrote:
>
> > On 01/17/2017 01:07 AM, Kirill Batuzov wrote:
> > > +static inline TCGv_v128 tcg_global_mem_new_v128(TCGv_ptr reg, intptr_t
> > > offset,
> > > +                                                const char *name)
> > > +{
> > > +    int idx = tcg_global_mem_new_internal(TCG_TYPE_V128, reg, offset,
> > > name);
> > > +    return MAKE_TCGV_V128(idx);
> > > +}
> >
> > You shouldn't allow a v128 type to be created if the host doesn't support it.
>
> The idea here was to create it either way, but make sure no operation
> will ever be issued if host does not support it (tcg_gen_* wrappers take
> care of this).

Huh?  If you issue *no* operation, then how is the operation being emulated?

> I'm not sure about this last part. The host may not have i64, so there
> should be another case - 4 x i32. So we'll get 4 cases for v128:

Recursively you'd get 4 x i32, but at least they'll be tagged TCG_TYPE_I64, and 
be handled by the rest of the tcg code generator like it should be.

> v128
> 2 x v64
> 2 x i64
> 4 x i32
>
> 3 cases will need to be added to tcg_temp_new_internal and
> tcg_global_new_mem_internal, two of which are rather useless (2 x i64, 4 x i32).
> Introduction of v256 will add 4 more cases two of which will be useless
> again. This sounds like too much code that serves no purpose to me.

Useless?  Surely you mean "used by hosts that don't implement v128".

I think one of us is very confused about how you intend to generate fallback 
code.  Perhaps any future patchset revision should update tcg/README first.


r~