ZhangHuiGui commented on code in PR #41975:
URL: https://github.com/apache/arrow/pull/41975#discussion_r1639153037
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cpp/src/arrow/compute/exec.cc:
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@@ -1034,9 +1034,23 @@ class VectorExecutor : public
KernelExecutorImpl<VectorKernel> {
output_num_buffers_ =
static_cast<int>(output_type_.type->layout().buffers.size());
// Decide if we need to preallocate memory for this kernel
- validity_preallocated_ =
- (kernel_->null_handling != NullHandling::COMPUTED_NO_PREALLOCATE &&
- kernel_->null_handling != NullHandling::OUTPUT_NOT_NULL);
+ validity_preallocated_ = false;
+ if (output_type_.type->id() != Type::NA) {
+ if (kernel_->null_handling == NullHandling::COMPUTED_PREALLOCATE) {
+ // Override the flag if kernel asks for pre-allocation
+ validity_preallocated_ = true;
+ } else if (kernel_->null_handling == NullHandling::INTERSECTION) {
+ bool elide_validity_bitmap = true;
+ for (const auto& arg : batch.values) {
+ auto null_gen = NullGeneralization::Get(arg) ==
NullGeneralization::ALL_VALID;
+
+ // If not all valid, this becomes false
+ elide_validity_bitmap = elide_validity_bitmap && null_gen;
+ }
+ validity_preallocated_ = !elide_validity_bitmap;
+ }
+ }
Review Comment:
> follow a single code path that uses the pre-allocated buffer to put the
result of the bitmaps intersection
Are you talking about `PropagateNulls`? Currently, seems this logic is used
in `INTERSECTION`.
If not, i understand that if the kernel internal execute path must `follow a
single code path that uses the pre-allocated buffer`, `COMPUTED_PREALLOCATE`
can be set statically, which should be safer for this kernel?
Back to preallocation-validity buffer, if kernel-function sets
`NullHandling::INTERSECTION`, does it mean that kernel-function may not be sure
whether it needs a pre-allocated validity buffer and let the Executor decide
for itself? At this time, the Executor can only dynamically determine whether
to set the pre-allocate buffer based on the input data.
> And even if that is not the case (assuming all kernels are perfect at the
moment), this extra logic introduces branching in the possible states of
pre-allocated buffers based on runtime input data and not something statically
defined in the `kernel_` when it's configured.
From a performance perspective, the current logic does introduce many
branches at runtime for `INTERSECTION`, compared to `PropagateNulls`, it
actually passes `NullGeneralization::Get` introduces very few branches.
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