| Issue |
165780
|
| Summary |
[LifetimeSafety] Optimize joins by separating block-local origins from cross-block persistent origins
|
| Labels |
clang:temporal-safety
|
| Assignees |
usx95
|
| Reporter |
usx95
|
Prevent block-local origins from participating in Join operations
The lifetime analysis currently performs join operations on all origins at every
block boundary, which is expensive. However, many origins are block-local: they
are created and destroyed within a single block, existing only to propagate loans
between persistent origins across temporary declarations.
Proposed optimization:
1. Classify origins into two categories during analysis:
- Block-local origins: start and die within the same block
- Persistent origins: visible across multiple blocks
2. During join operations at block boundaries, only join the persistent origins
and skip block-local ones entirely
Benefits:
- Significantly reduces join complexity, especially in functions with many
temporary locals
- Performance improvement scales with the ratio of temporary to persistent origins
Considerations:
- Need a pre-pass to correctly classify origins
- Loan propagation for block-local origins must still work correctly within
their block
Implementation:
Split the loan propagation lattice into two separate maps:
1. Persistent origins map: for origins visible across multiple blocks
2. Block-local origins map: for origins confined to a single block
At block entries (after joining), the block-local map would be empty. During
block execution, both maps are populated and used for loan propagation. At block
boundaries, only the persistent map participates in join operations.
Block-local map naturally empties at block entries.
Credits to @Xazax-hun for this idea.
![]()
_______________________________________________
llvm-bugs mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
