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https://issues.apache.org/jira/browse/ARROW-764?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16315435#comment-16315435
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ASF GitHub Bot commented on ARROW-764:
--------------------------------------
wesm commented on a change in pull request #1422: ARROW-764: [C++] Improves
performance of CopyBitmap and adds benchmarks
URL: https://github.com/apache/arrow/pull/1422#discussion_r160057731
##########
File path: cpp/src/arrow/util/bit-util.cc
##########
@@ -109,9 +109,37 @@ Status CopyBitmap(MemoryPool* pool, const uint8_t* data,
int64_t offset, int64_t
std::shared_ptr<Buffer> buffer;
RETURN_NOT_OK(GetEmptyBitmap(pool, length, &buffer));
uint8_t* dest = buffer->mutable_data();
- for (int64_t i = 0; i < length; ++i) {
- BitUtil::SetBitTo(dest, i, BitUtil::GetBit(data, i + offset));
+
+ int64_t byte_offset = offset / 8;
+ int64_t bit_offset = offset % 8;
+ int64_t num_bytes = BitUtil::BytesForBits(length);
+ int64_t bits_to_zero = num_bytes * 8 - length;
+
+ if (bit_offset > 0) {
+ auto carry_mask = BitUtil::kBitmask[bit_offset] - 1U;
+ auto carry_shift = 8U - bit_offset;
+
+ auto carry = 0U;
+ if (BitUtil::BytesForBits(length + bit_offset) > num_bytes) {
+ carry = (data[byte_offset + num_bytes] & carry_mask) << carry_shift;
+ }
+
+ auto i = num_bytes - 1;
+ while (i + 1 > 0) {
+ auto cur_byte = data[byte_offset + i];
Review comment:
We should write down explicit types for these autos
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> [C++] Improve performance of CopyBitmap, add benchmarks
> -------------------------------------------------------
>
> Key: ARROW-764
> URL: https://issues.apache.org/jira/browse/ARROW-764
> Project: Apache Arrow
> Issue Type: Improvement
> Components: C++
> Reporter: Wes McKinney
> Assignee: Adam Seibert
> Labels: pull-request-available
> Fix For: 0.9.0
>
>
> This is follow up work after a discussion in the patch for ARROW-657
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