[GitHub] [arrow] lidavidm commented on a change in pull request #11041: ARROW-13812: [C++] Fix Valgrind error in Grouper.BooleanKey test

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lidavidm commented on a change in pull request #11041:
URL: https://github.com/apache/arrow/pull/11041#discussion_r700184494



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File path: cpp/src/arrow/compute/exec/key_encode.cc
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@@ -427,11 +427,19 @@ void KeyEncoder::EncoderInteger::Decode(uint32_t start_row, uint32_t num_rows,
     row_base += offset_within_row;
     uint8_t* col_base = col_prep.mutable_data(1);
     switch (col_prep.metadata().fixed_length) {
-      case 1:
+      case 1: {
         for (uint32_t i = 0; i < num_rows; ++i) {
           col_base[i] = row_base[i * row_size];
         }
+        // For booleans, we pack 8 bytes at a time, and the buffer we're
+        // writing to here may not be fully initialized - so make sure a
+        // multiple of 8 bytes are initialized to avoid Valgrind errors. The
+        // temp buffer is sized to num_rows uint16_t values, so there's more
+        // than enough space here.
+        uint32_t remainder = 8 - (num_rows % 8);

Review comment:
       It should be OK (there's a typo here, it's uint32_t values so the temporary buffer is actually overallocated by a factor of 4x). But I've adjusted it anyways.




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