[GitHub] [arrow] edponce commented on a change in pull request #12080: ARROW-15118: [C++] Avoid bitmap buffer if all inputs are all valid for Scalar Kernels

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edponce commented on a change in pull request #12080:
URL: https://github.com/apache/arrow/pull/12080#discussion_r779356911



##########
File path: cpp/src/arrow/compute/exec.cc
##########
@@ -370,12 +370,15 @@ class NullPropagator {
  public:
   NullPropagator(KernelContext* ctx, const ExecBatch& batch, ArrayData* output)
       : ctx_(ctx), batch_(batch), output_(output) {
+    bool are_all_ins_valids = false;

Review comment:
       Maybe `are_all_ins_valids` --> `are_all_values_valid`

##########
File path: cpp/src/arrow/compute/exec.cc
##########
@@ -370,12 +370,15 @@ class NullPropagator {
  public:
   NullPropagator(KernelContext* ctx, const ExecBatch& batch, ArrayData* output)
       : ctx_(ctx), batch_(batch), output_(output) {
+    bool are_all_ins_valids = false;
     for (const Datum& datum : batch_.values) {
       auto null_generalization = NullGeneralization::Get(datum);
 
       if (null_generalization == NullGeneralization::ALL_NULL) {
         is_all_null_ = true;
       }
+      are_all_ins_valids =
+          are_all_ins_valids && (null_generalization == 
NullGeneralization::ALL_VALID);

Review comment:
       This logic is not correct bc `are_all_ins_valids` is initialized with 
`false` and performing and AND will always result in `false`.
   
   Also, since `null_generalization` is already checked against "not 
ALL_VALID", you can invert the logic to reduce the number of operations and 
restructure the checks.
   ```c++
   // This would also reset the bitmap if there are no buffer values, which 
AFAIK should not occur and if it occurs, then resetting the null bitmap also 
makes sense.
   bool are_all_ins_valids = true;
   for (...) {
     if (null_generalization != NullGeneralization::ALL_VALID) {
       are_all_ins_valids = false;
       if (null_generalization == NullGeneralization::ALL_NULL) {
         is_all_null_ = true;
       }
       if (datum.kind() == Datum::ARRAY) {
         arrays_with_nulls_.push_back(datum.array().get());
       }
     }
     ...
   }
   ```




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