Re: [PATCH V11] VECT: Add decrement IV support in Loop Vectorizer

[Richard Biener <richard.guenther@gmail.com>]

On Fri, May 19, 2023 at 12:59 PM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> "juzhe.zhong@rivai.ai" <juzhe.zhong@rivai.ai> writes:
> >>> I don't think this is a property of decrementing IVs.  IIUC it's really
> >>> a property of rgl->factor == 1 && factor == 1, where factor would need
> >>> to be passed in by the caller.  Because of that, it should probably be
> >>> a separate patch.
> > Is it right that I just post this part code as a seperate patch then merge it?
>
> No, not in its current form.  Like I say, the test should be based on
> factors rather than TYPE_VECTOR_SUBPARTS.  But a fix for this problem
> should come before the changes to IVs.
>
> >>> That is, current LOAD_LEN targets have two properties (IIRC):
> >>> (1) all vectors used in a given piece of vector code have the same byte size
> >>> (2) lengths are measured in bytes rather than elements
> >>> For all cases, including SVE, the number of controls needed for a scalar
> >>> statement is equal to the number of vectors needed for that scalar
> >>> statement.
> >>> Because of (1), on current LOADL_LEN targets, the number of controls
> >>> needed for a scalar statement is also proportional to the total number
> >>> of bytes occupied by the vectors generated for that scalar statement.
> >>> And because of (2), the total number of bytes is the only thing that
> >>> matters, so all users of a particular control can use the same control
> >>> value.
> >>> E.g. on current LOAD_LEN targets, 2xV16QI and 2xV8HI would use the same
> >>> control (with no adjustment).  2xV16QI means 32 elements, while 2xV8HI
> >>> means 16 elements.  V16QI's nscalars_per_iter would therefore be double
> >>> V8HI's, but V8HI's factor would be double V16QI's (2 vs 1), so things
> >>> even out.
> >>> The code structurally supports targets that count in elements rather
> >>> than bytes, so that factor==1 for all element types.  See the
> >>> "rgl->factor == 1 && factor == 1" case in:
> >  >>  if (rgl->max_nscalars_per_iter < nscalars_per_iter)  >>   {  >>     /* For now, we only support cases in which all loads and stores fall back     to VnQI or none do.  */
> >    >>    gcc_assert (!rgl->max_nscalars_per_iter>>              || (rgl->factor == 1 && factor == 1)
> >                 || (rgl->max_nscalars_per_iter * rgl->factor
> >>>                   == nscalars_per_iter * factor));
> >  >>      rgl->max_nscalars_per_iter = nscalars_per_iter; >>      rgl->type = vectype; >>      rgl->factor = factor;  >>   }>> But it hasn't been tested, since no current target uses it.
> >>> I think the above part of the patch shows that the current "factor is
> >>> always 1" path is in fact broken, and the patch is a correctness fix on
> >>> targets that measure in elements rather than bytes.
> >>> So I think the above part of the patch should go in ahead of the IV changes.
> >>> But the test should be based on factor rather than TYPE_VECTOR_SUBPARTS.
> > Since the length control measured by bytes instead of bytes is not
> > appropriate for RVV.You mean I can't support RVV auto-vectorization in
> > upstream GCC middle-end and I can only support it in my downstream, is
> > that right?
>
> No.  I haven't said in this or previous reviews that something cannot be
> supported in upstream GCC.
>
> I'm saying that the code in theory supports counting in bytes *or*
> counting in elements.  But only the first one has actually been tested.
> And so, perhaps not surprisingly, the support for counting elements
> needs a fix.
>
> The fix in your patch looks like it's on the right lines, but it should be
> based on factor rather than TYPE_VECTOR_SUBPARTS.
>
> See get_len_load_store_mode for how this selection happens:
>
> (1) IFN_LOAD_LEN itself always counts in elements rather than bytes.
>
> (2) If a target has instructions that count in elements, it should
>     define load_len patterns for all vector modes that it supports.
>
> (3) If a target has instructions that count in bytes, it should define
>     load_len patterns only for byte modes.  The vectoriser will then
>     use byte loads for all vector types (even things like V8HI).

Not sure if you've covered this already in another thread but IIRC
RVV uses "with-len" not only for loads and stores but for arithmetic
instructions as well which is where (3) fails.  Fortunately RVV uses
element counts(?)

> For (2), the loop controls will always have a factor of 1.
> For (3), the loop controls will have a factor equal to the element
> size in bytes.  See:
>
>   machine_mode vmode;
>   if (get_len_load_store_mode (vecmode, is_load).exists (&vmode))
>     {
>       nvectors = group_memory_nvectors (group_size * vf, nunits);
>       vec_loop_lens *lens = &LOOP_VINFO_LENS (loop_vinfo);
>       unsigned factor = (vecmode == vmode) ? 1 : GET_MODE_UNIT_SIZE (vecmode);
>       vect_record_loop_len (loop_vinfo, lens, nvectors, vectype, factor);
>       using_partial_vectors_p = true;
>     }
>
> This part should work correctly for RVV and any future targets that
> measure in elements rather than bytes.  The problem is here:
>
>                         tree final_len
>                           = vect_get_loop_len (loop_vinfo, loop_lens,
>                                                vec_num * ncopies,
>                                                vec_num * j + i);
>                         tree ptr = build_int_cst (ref_type,
>                                                   align * BITS_PER_UNIT);
>
>                         machine_mode vmode = TYPE_MODE (vectype);
>                         opt_machine_mode new_ovmode
>                           = get_len_load_store_mode (vmode, true);
>                         machine_mode new_vmode = new_ovmode.require ();
>                         tree qi_type = unsigned_intQI_type_node;
>
> This should be rearranged so that:
>
> - new_vmode is calculated before final_len
> - a "factor" is calculated in the same way as the above code
> - this factor is passed to vect_get_loop_len
> - vect_get_loop_len then uses this information to do a division.
>
> Thanks,
> Richard