Tejas Belagod <tejas.bela...@arm.com> writes:
>>> + {
>>> + b = build3 (BIT_FIELD_REF, TREE_TYPE (f.lhs), val,
>>> + bitsize_int (step * BITS_PER_UNIT),
>>> + bitsize_int ((16 - step) * BITS_PER_UNIT));
>>> +
>>> + return gimple_build_assign (f.lhs, b);
>>> + }
>>> +
>>> + /* If VECTOR_CST_NELTS_PER_PATTERN (pred) == 2 and every multiple of
>>> + 'step_1' in
>>> + [VECTOR_CST_NPATTERNS .. VECTOR_CST_ENCODED_NELTS - 1]
>>> + is zero, then we can treat the vector as VECTOR_CST_NPATTERNS
>>> + elements followed by all inactive elements. */
>>> + if (!const_vl && VECTOR_CST_NELTS_PER_PATTERN (pred) == 2)
>>
>> Following on from the above, maybe use:
>>
>> !VECTOR_CST_NELTS (pred).is_constant ()
>>
>> instead of !const_vl here.
>>
>> I have a horrible suspicion that I'm contradicting our earlier discussion
>> here, sorry, but: I think we have to return null if NELTS_PER_PATTERN != 2.
>>
>>
>>
>> IIUC, the NPATTERNS .. ENCODED_ELTS represent the repeated part of the
> encoded
>> constant. This means the repetition occurs if NELTS_PER_PATTERN == 2, IOW the
>> base1 repeats in the encoding. This loop is checking this condition and looks
>> for a 1 in the repeated part of the NELTS_PER_PATTERN == 2 in a VL vector.
>> Please correct me if I’m misunderstanding here.
>
> NELTS_PER_PATTERN == 1 is also a repeating pattern: it means that the
> entire sequence is repeated to fill a vector. So if an NELTS_PER_PATTERN
> == 1 constant has elements {0, 1, 0, 0}, the vector is:
>
> {0, 1, 0, 0, 0, 1, 0, 0, ...}
>
>
> Wouldn’t the vect_all_same(pred, step) cover this case for a given value of
> step?
>
>
> and the optimisation can't handle that. NELTS_PER_PATTERN == 3 isn't
> likely to occur for predicates, but in principle it has the same problem.
>
>
>
> OK, I had misunderstood the encoding to always make base1 the repeating value
> by adjusting the NPATTERNS accordingly – I didn’t know you could also have the
> base2 value and beyond encoding the repeat value. In this case could I just
> remove NELTS_PER_PATTERN == 2 condition and the enclosed loop would check for
> a
> repeating ‘1’ in the repeated part of the encoded pattern?
But for NELTS_PER_PATTERN==1, the whole encoded sequence repeats.
So you would have to start the check at element 0 rather than
NPATTERNS. And then (for NELTS_PER_PATTERN==1) the loop would reject
any constant that has a nonzero element. But all valid zero-vector
cases have been handled by this point, so the effect wouldn't be useful.
It should never be the case that all elements from NPATTERNS
onwards are zero for NELTS_PER_PATTERN==3; that case should be
canonicalised to NELTS_PER_PATTERN==2 instead.
So in practice it's simpler and more obviously correct to punt
when NELTS_PER_PATTERN != 2.
Thanks,
Richard
