Rematerialization
Nobody has claimed this yet.
Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
Research direction
Start by tracing the Function trait and Edit enum described in the issue, along with the existing VReg, PReg, spill, and reload handling. Determine whether the simple constant-rematerialization scope can be designed without supporting complex rematerialization; done would include the metadata interface, rematerialization edits, and coverage for spill and reload behavior.
Written by the indexing model from the issue text.
Description
As an alternative to spilling and reloading, a vreg value can sometimes be recomputed from other available values. There are two ways this can be supported in regalloc2:
Simple rematerialization
Supporting rematerialization for constants, including runtime constants which only depend on pinned registers (e.g. VMContext), is relatively straightforward. The client needs to track rematerialization metadata for each VReg, after which we can elide spills for this vreg (unless there is a stack use) and replace reloads with rematerializations.
trait Function {
fn can_rematerialize(&self, vreg: VReg) -> bool;
}
enum Edit {
/// The code sequence for rematerialization is only allowed to
/// use `dest` and the dedicated scratch register.
Rematerialize {
vreg: VReg,
// This is not an Allocation because rematerializing
// into a stack slot doesn't make sense.
dest: PReg,
}
}
Complex rematerialization
Rematerialization involving values in other vregs is more complex and probably not worth the effort of implementing. It has been done before as part of a research project on V8 but AFAIK this has not made it into the main V8 codebase.
- Dominant language
- Rust
- Stars
- 266
- Forks
- 54
- PR merge metrics
- No merged PRs in 30d
Contributor guide
No contributing guide indexed for this repository
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
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