reduce: unalias arrays for hcat/vcat (map)reduce #58490
Open
+8
−0
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
I was using `reduce(vcat, list_of_arrays)` and then modifying the resulting array. This worked great, except that in the case when `length(list_of_arrays) == 1`, the resulting array would alias the input, resulting in the input being modified and causing havoc down the road. Of course the proper fix for this is better built-in support for immutable arrays (and possibly memory ownership) to encode those assumptions explicitly. However, while this hack is unsatisfying, I do think it is somewhat defensible. One argument `hcat` and `vcat` perform the copy, as do the various optimized special cases for these functions. Of course, this is fragile and would break as soon as someone puts it in a closure, but maybe it saves at least a little bit of headache. I considered a more aggressive change to make the generic fallback `reduce_first(op, x) = op(x)`, but in discussion with Jeff, this was deemed too breaking, since it would add an assumption that `op` has a one-arg form.
Keno
added a commit
that referenced
this pull request
May 21, 2025
In #58490, I said: ``` I considered a more aggressive change to make the generic fallback reduce_first(op, x) = op(x), but in discussion with Jeff, this was deemed too breaking, since it would add an assumption that op has a one-arg form. ``` However, what if we just checked whether `op` does have an appropriate method. This would generalize the fix also. For example, `reduce(+, list_of_arrays)` has the same aliasing issue that I complained about in #58490. In addition, this lets us remove the various reduce_first special cases, since the one arg reduction op has the correct behavior for all existing cases.
|
Or maybe we should do |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
I was using
reduce(vcat, list_of_arrays)and then modifying the resulting array. This worked great, except that in the case whenlength(list_of_arrays) == 1, the resulting array would alias the input, resulting in the input being modified and causing havoc down the road. Of course the proper fix for this is better built-in support for immutable arrays (and possibly memory ownership) to encode those assumptions explicitly. However, while this hack is unsatisfying, I do think it is somewhat defensible. One argumenthcatandvcatperform the copy, as do the various optimized special cases for these functions. Of course, this is fragile and would break as soon as someone puts it in a closure, but maybe it saves at least a little bit of headache.I considered a more aggressive change to make the generic fallback
reduce_first(op, x) = op(x), but in discussion with Jeff, this was deemed too breaking, since it would add an assumption thatophas a one-arg form.