Repository navigation
JIT: We should try to maintain full defs through async liveness #133415
Description
When we decompose stores physical promotion will often end up introducing STORE_LCL_FLDs if the destination was not fully promoted. Unfortunately these often result in async liveness thinking that the stores are partial, even though the full set is actually a full definition. This can then lead to unnecessary state being retained.
To fixes I can think of:
- Try to make async liveness more precise and track per-field state
- Instead of creating a set of
STORE_LCL_FLDon decomposition, instead createSTORE_LCL_VAR(FIELD_LIST(...))and then expand it in lowering.
I am partial to (2), it seems simpler.
Activity
dotnet-policy-service commentedon Sep 8, 2026
Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.
jakobbotsch commentedon Sep 8, 2026
I saw this problem in two places when looking at hot ASP.NET methods and asked copilot to summarize them:
ValueTask<ReadResult>constructor temporary insidePipe.ReadAsyncThe IR initially contained a whole zero-initialization followed by constructor field stores:
Physical promotion removed the whole initialization and retained only the necessary field definitions. A later whole-struct use of
V35then looked upward-exposed, so liveness carried the previous loop iteration's value.TransparentAwaitinline argumentThe inliner initially generated a complete copy:
The argument was subsequently used by:
Physical promotion replaced the whole copy with:
The
STORE_LCL_FLDnodes are partial definitions and do not contribute a full parent-local definition during non-SSA async liveness. Consequently, the later wholeV103argument use was no longer a last use, andV103appeared live through the loop and across every suspension.