On Fri, 14 Nov 2025, Christopher Bazley wrote:
>
> On 12/11/2025 14:27, Richard Biener wrote:
> > On Tue, 11 Nov 2025, Christopher Bazley wrote:
> >
> >> On 05/11/2025 12:08, Richard Biener wrote:
> >>> On Tue, 4 Nov 2025, Christopher Bazley wrote:
> >>>
> >>>> On 04/11/2025 13:57, Christopher Bazley wrote:
> >>>>> On 28/10/2025 13:29, Richard Biener wrote:
> >>>>>> Isn't SLP_TREE_CAN_USE_PARTIAL_VECTORS_P redundant given
> >>>>>> SLP_TREE_CAN_USE_MASK_P || SLP_TREE_CAN_USE_LEN_P should be exactly
> >>>>>> this?
> >>>>>>
> >>>>>> SLP_TREE_CAN_USE_PARTIAL_VECTORS_P might be sth for an SLP instance
> >>>>>> (or a subgraph with multiple entries (instances)) if we want to have
> >>>>>> consistent len vs. mask use? (but I see no particular reason to force
> >>>>>> consistency)
> >>>>> SLP_TREE_CAN_USE_PARTIAL_VECTORS_P is initialised to true and may
> >>>>> subsequently be set to false via vect_cannot_use_partial_vectors.
> >>>>>
> >>>>> vect_analyze_stmt uses the value of SLP_TREE_CAN_USE_PARTIAL_VECTORS_P
> >>>>> to
> >>>>> decided whether to return a failure result in cases where
> >>>>> tail-predication
> >>>>> is required. If SLP_TREE_CAN_USE_MASK_P || SLP_TREE_CAN_USE_LEN_P were
> >>>>> used
> >>>>> for that purpose instead, it would follow that neither
> >>>>> SLP_TREE_CAN_USE_MASK_P nor SLP_TREE_CAN_USE_LEN_P could be set to true
> >>>>> in
> >>>>> cases where vect_cannot_use_partial_vectors might subsequently be called
> >>>>> (which seems impossible because vect_load_lanes_supported can be called
> >>>>> with
> >>>>> different values of 'count', and we cannot predict those values) , or
> >>>>> else
> >>>>> that vect_cannot_use_partial_vectors would have to set both
> >>>>> SLP_TREE_CAN_USE_MASK_P and SLP_TREE_CAN_USE_LEN_P to false.
> >>>>>
> >>>>> Setting both flags to false in vect_cannot_use_partial_vectors would be
> >>>>> trivial, but SLP_TREE_CAN_USE_PARTIAL_VECTORS_P has another purpose: it
> >>>>> gives the return value of vect_can_use_partial_vectors_p. How did you
> >>>>> envisage the return value of vect_can_use_partial_vectors_p being
> >>>>> decided
> >>>>> for BB SLP? If it always returns true then I think that the vectoriser
> >>>>> might
> >>>>> carry on trying to use partial vectors when it should have already given
> >>>>> up;
> >>>>> if it returns SLP_TREE_CAN_USE_MASK_P || SLP_TREE_CAN_USE_LEN_P then
> >>>>> that
> >>>>> would prevent check_load_store_for_partial_vectors from being called for
> >>>>> the
> >>>>> first time.
> >>>>>
> >>>>> Together, the three flags allow the following states to be represented:
> >>>>>
> >>>>> 1. Might be able to operate on partial vectors, but don't yet know
> >>>>> whether
> >>>>> we would use len or mask.
> >>>>>
> >>>>> 2. Might be able to operate on partial vectors with len.
> >>>>>
> >>>>> 3. Might be able to operate on partial vectors with mask.
> >>>>>
> >>>>> 4. (Invalid) Might be able to operate on partial vectors with both len
> >>>>> and
> >>>>> mask.
> >>>>>
> >>>>> 5. Cannot operate on partial vectors.
> >>>>>
> >>>>> 6. (Strictly redundant) Cannot operate on partial vectors although we
> >>>>> previously thought we might be able to use len.
> >>>>>
> >>>>> 7. (Strictly redundant) Cannot operate on partial vectors although we
> >>>>> previously thought we might be able to use mask.
> >>>>>
> >>>>> 8. (Invalid) Cannot operate on partial vectors although we previously
> >>>>> thought we might be able to use both len and mask.
> >>>>>
> >>>>> It would be impossible to encode the four states that neither invalid
> >>>>> nor
> >>>>> redundant in only two bits. In any case, my goal was to keep the new
> >>>>> logic
> >>>>> for BB SLP as close as possible to the existing logic for loop
> >>>>> vectorisation.
> >>>> Sorry, it would clearly be possible to encode four values in 2 bits (00,
> >>>> 01,
> >>>> 10, 11) by encoding the "Might be able to operate on partial vectors"
> >>>> state
> >>>> as
> >>>> both bits clear and "Cannot operate on partial vectors" state as "Might
> >>>> be
> >>>> able to operate on partial vectors with both len and mask" (both bits
> >>>> set).
> >>>> A
> >>>> reversal of the current encoding would probably make more sense: set both
> >>>> 'len' and 'mask' bits at the start, clear the 'mask' bit when 'len' is
> >>>> chosen,
> >>>> clear the 'len' bit when 'mask' is chosen, and clear both bits when
> >>>> neither
> >>>> is
> >>>> valid. Maybe this encoding could also be applied to the loop vectoriser.
> >>>>
> >>>> Do you want me to make that change?
> >>> So I think I'm somewhat confused to the extent that with BB
> >>> vectorization we do not really have a choice - if partial vectors
> >>> are necessary (because there's padding), then if we cannot use
> >>> partial vectors, the stmt analysis should fail. I had the impression
> >> That's what this code in does vect_analyze_stmt:
> >>
> >> if (bb_vinfo)
> >> {
> >> unsigned int group_size = SLP_TREE_LANES (node);
> >> tree vectype = SLP_TREE_VECTYPE (node);
> >> poly_uint64 nunits = TYPE_VECTOR_SUBPARTS (vectype);
> >> bool needs_partial = known_lt (group_size, nunits);
> >> if (needs_partial && !SLP_TREE_CAN_USE_PARTIAL_VECTORS_P (node))
> >> return opt_result::failure_at (stmt_info->stmt,
> >> "not vectorized: SLP node needs but "
> >> "cannot use partial vectors: %G",
> >> stmt_info->stmt);
> >> if (maybe_gt (group_size, nunits))
> >> gcc_assert (multiple_p (group_size, nunits));
> >> }
> > Conservative enough I guess, some operations might not need masking,
> > so it can be improved.
>
> The current mechanism works regardless of whether operations such as 'add'
> need a mask or not.
>
> The SLP_TREE_CAN_USE_PARTIAL_VECTORS_P flag just records whether the target
> could support the operation if the vector were partial; it does not say
> anything about whether the vector is partial, or how the target would support
> the operation on a partial vector. Some kinds of node require mask/length for
> partial vectors; others don't. SLP_TREE_CAN_USE_PARTIAL_VECTORS_P returns true
> for nodes in the second category.
For loop masking we have LOOP_VINFO_MUST_USE_PARTIAL_VECTORS_P set
by operations that cannot be implemented correctly when not using masking.
I see most of the BB "partial" vector operations like those, and if
we fail to support masking for such an operation it's vectorizable_*
analysis should fail. Likewise for operations that don't mind there's
not really anything to record.
I get that your way to abstract BB vs. loop operation does not support
failing vectorizable_* this way since you do not teach vectorizable_*
whether needs_partial but you initialize
SLP_TREE_CAN_USE_PARTIAL_VECTORS_P to true in this case?
The vect_record_loop_{len,mask} abstraction should still record
what style we used for the particular SLP node.
Sorry for taking so long to respond, but the discussion seems to go
over too long time and disconnected from actual patches. Is the [v2]
patchset still current?
Richard.
