Re: [PATCH for-9.0 v14 4/8] target/riscv/vector_helpers: do early exit when vstart >= vl

[Daniel Henrique Barboza]

On 3/14/24 00:52, Max Chou wrote:
> Hi Daniel,
>
> According the v spec section 15.2 & 15.3.
>
> "The vcpop.m instruction writes x[rd] even if vl=0 (with the value 0, since no 
> mask elements are active).
>    Traps on vcpop.m are always reported with a vstart of 0. The vcpop.m instruction 
> will raise an illegal instruction exception if vstart is non-zero."
>
> "The vfirst.m instruction writes x[rd] even if vl=0 (with the value -1, since 
> no mask elements are active).
>   Traps on vfirst are always reported with a vstart of 0. The vfirst instruction 
> will raise an illegal instruction exception if vstart is non-zero."
>
> Both the vcpop.m and vfirst.m instructions will raise illegal instruction 
> exception with non-zero vstart.
>
> And currently both the trans_vcpop_m and trans_vfirst_m translate functions 
> check the vstart_eq_zero flag.
> So I think the early exit checking in the vcpop.m and vfirstm helper functions 
> may be redundant.
>
> > @@ -4585,6 +4641,11 @@ target_ulong HELPER(vcpop_m)(void *v0, void *vs2, 
> > CPURISCVState *env,
> >       uint32_t vl = env->vl;
> >       int i;
> >   
> > +    if (env->vstart >= env->vl) {
> > +        env->vstart = 0;
> > +        return 0;
> > +    }
> > +
> >       for (i = env->vstart; i < vl; i++) {
> >           if (vm || vext_elem_mask(v0, i)) {
> >               if (vext_elem_mask(vs2, i)) {
>
> According v spec section 15.3
>
> ""The vfirst.m instruction writes x[rd] even if vl=0 (with the value -1, since no 
> mask elements are active)."
>
> If both the vstart and vl are 0 here, the early exit checking will return the 
> wrong value 0 (the return value should be -1) here.

Let's just remove these early exits from both vcpop.m and vfirst.m functions 
then.

Thanks,

Daniel

> > @@ -4604,6 +4665,11 @@ target_ulong HELPER(vfirst_m)(void *v0, void *vs2, 
> > CPURISCVState *env,
> >       uint32_t vl = env->vl;
> >       int i;
> >   
> > +    if (env->vstart >= env->vl) {
> > +        env->vstart = 0;
> > +        return 0;
> > +    }
> > +
> >       for (i = env->vstart; i < vl; i++) {
> >           if (vm || vext_elem_mask(v0, i)) {
> >               if (vext_elem_mask(vs2, i)) {