Skip to content

[6.3] LoopInvariantCodeMotion: don't reuse existing instructions in the loop pre-header #85640

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom
Open
wants to merge 1 commit into
base: release/6.3
Choose a base branch
from
Open

[6.3] LoopInvariantCodeMotion: don't reuse existing instructions in the loop pre-header #85640

wants to merge 1 commit into from
+39 −24

Conversation

@eeckstein
Copy link
Contributor

@eeckstein eeckstein commented Nov 21, 2025

  • Explanation: Fixes a bug in the loop-invariant-code-motion pass which causes a miscompile. The fix is to remove a small peephole optimization which tried to re-use existing instructions in a loop pre-header block when hoisting instructions out of the loop. This is wrong for load instructions because the loaded value could be overridden by another instruction in the pre-header block.
  • Risk: Low. The only change is to remove a small peephole optimization
  • Testing: Tested by a lit test.
  • Issue: rdar://164034503
  • Reviewer: @aidan-hall
  • Main branch PR: LoopInvariantCodeMotion: don't reuse existing instructions in the loop pre-header #85569

@eeckstein
LoopInvariantCodeMotion: don't reuse existing instructions in the loo…
b5a2962 
…p pre-header

This is wrong for hoisted load instructions because we don't check for aliasing in the pre-header.
And for side-effect-free instructions it's not really necessary, because that can cleanup CSE afterwards.

Fixes a miscompile
rdar://164034503
@eeckstein eeckstein requested a review from a team as a code owner November 21, 2025 06:58
@eeckstein
Copy link
Contributor Author

eeckstein commented Nov 21, 2025

@swift-ci test

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Reviewers

@tbkka tbkka tbkka approved these changes

Assignees

No one assigned

Labels

swift 6.3

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

2 participants

@eeckstein @tbkka