cranelift: Move VReg aliases to VRegAllocator #8493
Merged
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
Now that bytecodealliance#8486 landed, allowing us to resolve aliases in machine instructions, we have ensured that all VReg aliases are resolved by the time we're done building the VCode. Therefore we only need to keep track of the aliases map before that. The VReg allocator is also dropped when we finish building the VCode, and it makes sense to track aliases there. This lets us maintain an invariant, that PCC facts are only stored on VRegs which are not aliased, while only reasoning locally within VRegAllocator. I've changed the trace-log output to print the VCode immediately before it's finalized, along with key details in the VRegAllocator. This allows seeing the instructions before aliases are rewritten, although they're in reverse order at that point. There's another trace-log message somewhere else which logs the finalized VCode, so you can see both. Previously, the initial capacity of the vreg_aliases map was set to ten times the number of basic blocks in the function. However we can make a better estimate based on the number of SSA values in the function, and use that to preallocate storage for other things in VRegAllocator too. Keeping the aliases outside the VCode fixes previous borrow-checker challenges, which is a nice bonus.
github-actions
bot
added
cranelift
cfallin
approved these changes
Apr 29, 2024
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Outdated suggestions cannot be applied.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.
Now that #8486 landed, allowing us to resolve aliases in machine instructions, we have ensured that all VReg aliases are resolved by the time we're done building the VCode. Therefore we only need to keep track of the aliases map before that.
The VReg allocator is also dropped when we finish building the VCode, and it makes sense to track aliases there. This lets us maintain an invariant, that PCC facts are only stored on VRegs which are not aliased, while only reasoning locally within VRegAllocator.
I've changed the trace-log output to print the VCode immediately before it's finalized, along with key details in the VRegAllocator. This allows seeing the instructions before aliases are rewritten, although they're in reverse order at that point. There's another trace-log message somewhere else which logs the finalized VCode, so you can see both.
Previously, the initial capacity of the vreg_aliases map was set to ten times the number of basic blocks in the function. However we can make a better estimate based on the number of SSA values in the function, and use that to preallocate storage for other things in VRegAllocator too.
Keeping the aliases outside the VCode fixes previous borrow-checker challenges, which is a nice bonus.