AArch64: fix sbcs/ngcs flags when destination is xzr #6446
Conversation
This file contains 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
ryanmkurtz
added
Feature: Processor/AARCH64
Status: Triage
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Outdated suggestions cannot be applied.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.
As part of a research project testing the accuracy of the sleigh specifications compared to real hardware, we observed an unexpected behaviour in the sbcs and ngcs instructions for AARCH64. According to Section C6.2.266 and C6.2.237, the expected behaviour is to update the status flags based on the result of the subtract/negate. While the current behaviour instead updates the flags based on the value in the destination register. When the destination register is xzr, the result is discarded, and zero is used.