[LAA] Support assumptions with non-constant deref sizes. #156529
Closed
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
Update evaluatePtrAddrecAtMaxBTCWillNotWrap to support non-constant sizes in dereferenceable assumptions. Note that with that we will need to add a constant to a backedge-taken-count, so expressions like (-1 + %n) + 1. While adding something to -1 technically wraps in the unsigned sense, in practice the result should not consider overflowing in addSCEVNoOverflow. I am not sure if there's a more general place for this kind of reasoning?
Collaborator
I don't see any obvious reason it's safe to conclude the computation can't overflow. Loops with BTC of "n-1" usually have a trip count of "n", but you can easily write code to violate that, like |
Contributor
Author
|
Argh yes, this was a stale change and should be taken care with info from loop guards. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
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Outdated suggestions cannot be applied.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.
Update evaluatePtrAddrecAtMaxBTCWillNotWrap to support non-constant sizes in dereferenceable assumptions.
Note that with that we will need to add a constant to a backedge-taken-count, so expressions like (-1 + %n) + 1. While adding something to -1 technically wraps in the unsigned sense, in practice the result should not consider overflowing in addSCEVNoOverflow. I am not sure if there's a more general place for this kind of reasoning?