<execution>: comment early return control flow #818

[AlexGuteniev]

Resolves #818

[AlexGuteniev]

A question about which places need put this comment.

So far I found 11 places with early return:

STL/stl/inc/execution

Lines 1289 to 1299 in ff94756

	_Cancellation_status _Process_chunk() {
	if (_Results._Complete()) {
	return _Cancellation_status::_Canceled;
	}
	const auto _Key = _Team._Get_next_key();
	if (!_Key) {
	return _Cancellation_status::_Canceled;
	}
	const auto _Range = _Basis._Get_chunk(_Key);

There are also places without early return:

STL/stl/inc/execution

Lines 1418 to 1425 in ff94756

	_Cancellation_status _Process_chunk() {
	const auto _Key = _Team._Get_next_key();
	if (!_Key) {
	return _Cancellation_status::_Canceled;
	}
	const auto _Chunk_number = _Key._Chunk_number;
	const auto _Range = _Basis._Get_chunk(_Key);

Should these be commented as well?

[StephanTLavavej]

Should these be commented as well?

I think so. I'm not an expert here, but the difference appears to be that some algorithms can return early (e.g. all_of can return early upon finding false) but other algorithms must perform all work (e.g. transform must transform all elements). Either way, once an algorithm obtains work, I don't believe it can abandon that work.

Edited to add: Hmm, after looking at all occurrences of _Get_next_key, I think perhaps we should comment just the places with early returns (which is what you're doing now), as those are the patterns I was originally interested in refactoring. I think commenting all callsites could be distracting or misleading.

[StephanTLavavej]

Thanks for making it easier to understand this code, and to write correct code in the future! 🎉