Rewrite StackNesting to use a single-pass algorithm
#84688
+638
−522
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
|
@swift-ci Please test |
eeckstein
reviewed
Oct 6, 2025
|
|
||
| dealloc_stack %the_array: $*Array<Int> | ||
| %24 = tuple () | ||
| return %24 : $() |
There was a problem hiding this comment.
In the past we used stack promotion to test fixStackNesting. However, today we can write unit tests using FunctionTest. It would be good to add such tests for this new StackNesting implementation.
The previous algorithm was doing an iterative forward data flow analysis followed by a reverse data flow analysis. I suspect the history here is that it was a reverse analysis, and that didn't really work for infinite loops, and so complexity kindof accumulated. The new algorithm is quite straightforward and relies on the allocations being properly jointly post-dominated, just not nested. We simply walk forward through the blocks in consistent-with-dominance order, maintaining the stack of active allocations and deferring deallocations that are improperly nested until we deallocate the allocations above it. The reason I'm doing this, besides it just being a simpler, faster algorithm, is that modeling some of the uses of the async stack allocator properly requires builtins that cannot just be semantically reordered. That should be somewhat easier to handle with the new approach, although really (1) we should not have runtime functions that need this and (2) we're going to need a conservatively-correct solution that's different from this anyway because hoisting allocations is *also* limited in its own way. The test cases that changed are... I don't think the new output is wrong under the current rules that are being enforced, but really we should be enforcing different rules, because it's not really okay to have broken stack nesting in blocks just because they don't lead to an exit. But it's broken coming into StackNesting, and I don't think the rewrite actually makes it worse, so... The thing that concerns me most about the rewritten pass is that it isn't actually validating joint post-dominance on input, so if you give it bad input, it might be a little mystifying to debug the verifier failures.
rjmccall
force-pushed
the
stack-nesting-2
branch
from
72b44bd to
19b34ee
Compare
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.
The previous algorithm was doing an iterative forward data flow analysis followed by a reverse data flow analysis. I suspect the history here is that it was a reverse analysis, and that didn't really work for infinite loops, and so complexity kindof accumulated.
The new algorithm is quite straightforward and relies on the allocations being properly jointly post-dominated, just not nested. We simply walk forward through the blocks in consistent-with-dominance order, maintaining the stack of active allocations and deferring deallocations that are improperly nested until we deallocate the allocations above it.
The reason I'm doing this, besides it just being a simpler, faster algorithm, is that modeling some of the uses of the async stack allocator properly requires builtins that cannot just be semantically reordered. That should be somewhat easier to handle with the new approach, although really (1) we should not have runtime functions that need this and (2) we're going to need a conservatively-correct solution that's different from this anyway because hoisting allocations is also limited in its own way.
The test cases that changed are... I don't think the new output is wrong under the current rules that are being enforced, but really we should be enforcing different rules, because it's not really okay to have broken stack nesting in blocks just because they don't lead to an exit. But it's broken coming into StackNesting, and I don't think the rewrite actually makes it worse, so...
The thing that concerns me most about the rewritten pass is that it isn't actually validating joint post-dominance on input, so if you give it bad input, it might be a little mystifying to debug the verifier failures.