felipecrv commented on PR #41477:
URL: https://github.com/apache/arrow/pull/41477#issuecomment-2116480668
The goal is to have a definitive function that can copy bytes from any
device buffer to CPU by delegating to `MemoryManager` who should know the best
way to do it. Having to write that code that @zeroshade wrote in `bridge.cc` to
copy 4 or 8 bytes from a `Buffer` was very annoying. Without getting into the
detail of what is fast and what is slow, I think there should be a helper for
doing this:
```cpp
if (device_type_ == DeviceAllocationType::kCPU) {
auto offsets = data_->GetValues<OffsetType>(offsets_buffer_id);
// Compute visible size of buffer
int64_t buffer_size =
(c_struct_->length > 0) ? byte_width * offsets[c_struct_->length]
: 0;
return ImportBuffer(buffer_id, buffer_size);
}
// we only need the value of the last offset so let's just copy that
// one value from device to host.
auto single_value_buf =
SliceBuffer(data_->buffers[offsets_buffer_id],
c_struct_->length * sizeof(OffsetType),
sizeof(OffsetType));
ARROW_ASSIGN_OR_RAISE(
auto cpubuf, Buffer::ViewOrCopy(single_value_buf,
default_cpu_memory_manager()));
auto offsets = cpubuf->data_as<OffsetType>();
// Compute visible size of buffer
int64_t buffer_size = (c_struct_->length > 0) ? byte_width * offsets[0]
: 0;
```
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