Re: [PATCH v5 57/60] target/riscv: vector slide instructions

[LIU Zhiwei]

On 2020/3/17 1:42, Richard Henderson wrote:
> On 3/16/20 1:04 AM, LIU Zhiwei wrote:
> > > As a preference, I think you can do away with this helper.
> > > Simply use the slideup helper with argument 1, and then
> > > afterwards store the integer register into element 0.  You should be able to
> > > re-use code from vmv.s.x for that.
> > When I try it, I find it is some difficult, because  vmv.s.x will clean
> > the elements (0 < index < VLEN/SEW).
> Well, two things about that:
>
> (1) The 0.8 version of vmv.s.x does *not* zero the other elements, so we'll
> want to be prepared for that.
>
> (2) We have 8 insns that, in the end come down to a direct element access,
> possibly with some other processing.
>
> So we'll want basic helper functions that can locate an element by immediate
> offset and by variable offset:
>
> /* Compute the offset of vreg[idx] relative to cpu_env.
>     The index must be in range of VLMAX. */
> int vec_element_ofsi(int vreg, int idx, int sew);
>
> /* Compute a pointer to vreg[idx].
>     If need_bound is true, mask idx into VLMAX,
>     Otherwise we know a-priori that idx is already in bounds. */
> void vec_element_ofsx(DisasContext *s, TCGv_ptr base,
>                        TCGv idx, int sew, bool need_bound);
>
> /* Load idx >= VLMAX ? 0 : vreg[idx] */
> void vec_element_loadi(DisasContext *s, TCGv_i64 val,
>                         int vreg, int idx, int sew);
> void vec_element_loadx(DisasContext *s, TCGv_i64 val,
>                         int vreg, TCGv idx, int sew);
>
> /* Store vreg[imm] = val.
>     The index must be in range of VLMAX.  */
> void vec_element_storei(DisasContext *s, int vreg, int imm,
>                          TCGv_i64 val);
> void vec_element_storex(DisasContext *s, int vreg,
>                          TCGv idx, TCGv_i64 val);
>
> (3) It would be handy to have TCGv cpu_vl.
Do you mean I should define cpu_vl as a global TCG varible like cpu_pc?
So that I can check vl==0 in translation time.

Or just a temp variable?
> Then:
>
> vext.x.v:
>      If rs1 == 0,
>          Use vec_element_loadi(s, x[rd], vs2, 0, s->sew).
>      else
>          Use vec_element_loadx(s, x[rd], vs2, x[rs1], true).
>
> vmv.s.x:
>      over = gen_new_label();
>      tcg_gen_brcondi_tl(TCG_COND_EQ, cpu_vl, 0, over);
>      For 0.7.1:
>          Use tcg_gen_dup8i to zero all VLMAX elements of vd.
>          If rs1 == 0, goto done.
>      Use vec_element_storei(s, vs2, 0, x[rs1]).
>   done:
>      gen_set_label(over);
>
> vfmv.f.s:
>      Use vec_element_loadi(x, f[rd], vs2, 0).
>      NaN-box f[rd] as necessary for SEW.
>
> vfmv.s.f:
>      tcg_gen_brcondi_tl(TCG_COND_EQ, cpu_vl, 0, over);
>      For 0.7.1:
>          Use tcg_gen_dup8i to zero all VLMAX elements of vd.
>      Let tmp = f[rs1], nan-boxed as necessary for SEW.
>      Use vec_element_storei(s, vs2, 0, tmp).
>      gen_set_label(over);
>
> vslide1up.vx:
>      Ho hum, I forgot about masking.  Some options:
>      (1) Call a helper just as you did in your original patch.
>      (2) Call a helper only for !vm, for vm as below.

Sorry, I don't get it why I need a helper for !vm.
I think I can  call vslideup w/1 whether !vm or vm, then a store to vd[0].

Zhiwei
>      (3) Call vslideup w/1.
>          tcg_gen_brcondi(TCG_COND_EQ, cpu_vl, 0, over);
>          If !vm,
>              // inline test for v0[0]
>              vec_element_loadi(s, tmp, 0, 0, MO_8);
>              tcg_gen_andi_i64(tmp, tmp, 1);
>              tcg_gen_brcondi(TCG_COND_EQ, tmp, 0, over);
>          Use vec_element_store(s, vd, 0, x[rs1]).
>          gen_set_label(over);
>
> vslide1down.vx:
>      For !vm, this is complicated enough for a helper.
>      If using option 3 for vslide1up, then the store becomes:
>      tcg_gen_subi_tl(tmp, cpu_vl, 1);
>      vec_element_storex(s, base, tmp, x[rs1]);
>
> vrgather.vx:
>      If !vm or !vl_eq_vlmax, use helper.
>      vec_element_loadx(s, tmp, vs2, x[rs1]);
>      Use tcg_gen_gvec_dup_i64 to store to tmp to vd.
>
> vrgather.vi:
>      If !vm or !vl_eq_vlmax, use helper.
>      If imm >= vlmax,
>          Use tcg_gen_dup8i to zero vd;
>      else,
>          ofs = vec_element_ofsi(s, vs2, imm, s->sew);
>          tcg_gen_gvec_dup_mem(sew, vreg_ofs(vd),
>                               ofs, vlmax, vlmax);
>
>
> r~