GH-45820: [C++] Add optional out_offset for Buffer-returning CopyBitmap function

[raulcd]

Rationale for this change

The CopyBitmap variant that returns a std::shared_ptr<Buffer> doesn't allow passing a destination bit offset for the output.

Accepting such an argument would allow using that function to replace the null bitmap of an existing array, without changing its offset.

What changes are included in this PR?

Adding the option out_offset argument.

Are these changes tested?

Yes, the existing TestCopyBitmap has been adapted to use dest_offset or not.

Are there any user-facing changes?

No, but we allow a new optional argument (out_offset) to CopyBitmap

• GitHub Issue: [C++] Buffer-returning CopyBitmap should take an optional out offset. #45820

[pitrou]

It would be better to test with different offsets, for example:

Suggested change

	for (bool maintan_offset : {true, false}) {
	for (int64_t num_bits : lengths) {
	for (int64_t offset : offsets) {
	const int64_t copy_length = num_bits - offset;
	for (int64_t num_bits : lengths) {
	for (int64_t offset : offsets) {
	for (int64_t dest_offset : {0, offset - 1, offset + 1}) {
	const int64_t copy_length = num_bits - offset;

[pitrou]

It is actually dubious that this code is necessary, since AllocateEmptyBitmap has already zero-initialized the entire buffer. Perhaps we can try remove it?

[raulcd]

I've tried removing it and all tests are successful

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: c6e8149

Submitted crossbow builds: ursacomputing/crossbow @ actions-6ed5547d68

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-alpine-linux-cpp
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-meson
test-conda-cpp-valgrind
test-cuda-cpp-ubuntu-22.04-cuda-11.7.1
test-debian-12-cpp-amd64
test-debian-12-cpp-i386
test-fedora-39-cpp
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-bundled
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-bundled-offline
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer

[pitrou]

Valgrind tests have passed, which is a confirmation that the removed code did not serve a purpose.

[pitrou]

Thanks for this @raulcd ! CI failures are unrelated.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 5d487ca.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 19 possible false positives for unstable benchmarks that are known to sometimes produce them.