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));
}
that vectorizable_* computes the per-SLP node SLP_TREE_CAN_USE_*.
That's right.
Why would it even say that it can use partial vectors when it doesn't
need to for example?
It won't through SLP_TREE_CAN_USE_MASK_P or SLP_TREE_CAN_USE_LEN_P;
however, SLP_TREE_CAN_USE_PARTIAL_VECTORS_P is initialised to true for
every SLP node and only set to false if the vectoriser wanted to use
partial vectors but cannot. I don't personally find that confusing,
because 'can' doesn't imply 'will' to me.
Consider a target that can do a mask load and
a mask store but not masked operations and
a[0] = 5 + b[0];
a[1] = 5 + b[1];
a[2] = 5 + b[2];
on unsigned int, we'd chose V4SImode and both load and store can do
SLP_TREE_CAN_USE_MASK_P. The add cannot, but it doesn't need to
mask the padding, so !SLP_TREE_CAN_USE_MASK_P and all stmt analyses
should succeed.
That's right. Admittedly, SLP_TREE_CAN_USE_MASK_P does not seem like a
good predicate name anymore.
If the target cannot do a mask load then vectorization
with V4SImode should fail vectorizable_load (), no need to set
slp-tree-would-need-partial-vectors-but-cannot.
That would require special-casing of BB-SLP in existing vectorizable_*
functions. Changing the control flow of those functions or adding
complexity is something that I explicitly wanted to avoid.
So for BB vect isn't it SLP_TREE_MUST_USE_PARTIAL_VECTORS_P
and then either or both of SLP_TREE_CAN_USE_MASK_P and
SLP_TREE_CAN_USE_LEN_P?
SLP_TREE_MUST_USE_PARTIAL_VECTORS_P does not exist. I added assertions
to check that SLP_TREE_CAN_USE_MASK_P or SLP_TREE_CAN_USE_LEN_P cannot
be set to true if the other predicate is already true. e.g., in
vect_record_mask:
gcc_checking_assert (!SLP_TREE_CAN_USE_LEN_P (slp_node));
SLP_TREE_CAN_USE_MASK_P (slp_node) = true;
It would be more accurate to say that either SLP_TREE_CAN_USE_MASK_P is
true, SLP_TREE_CAN_USE_LEN_P is true, or
SLP_TREE_CAN_USE_PARTIAL_VECTORS_P is false.
And we do not really need any upthread
decision, thus analysis would only need to decide and remember the actual
partial vector method used? If that's correct I'd rather see
a SLP_TREE_PARTIAL_VECTORS_STYLE-like enum here?
OK, I'll see if I can implement one.
With loop vect we have the global decision, so we need to record
what we can use and what not, but with BB vect we can decide
immediately given there's no "fallback".
Richard.
Understood.
Thanks,
--
Christopher Bazley
Staff Software Engineer, GNU Tools Team.
Arm Ltd, 110 Fulbourn Road, Cambridge, CB1 9NJ, UK.
http://www.arm.com/
