Allan Shortlidge cad1ee75d5 AST: Optimize the layout of AvailabilityRange. ...

`AvailabilityRange` is now being used as a currency type in more of the
compiler, and some of those uses are in permanent `ASTContext` allocations. The
class wraps the `VersionRange` utility, which is itself a wrapper around
`llvm::VersionTuple` with some additional storage for representing sentinel
values. Even though the two sentinel values can be be represented with just a
single bit of additional storage on top of the 16 bytes required to represent
`VersionTuple`, because of alignment requirements the sentinel values end up
bloating the layout of `VersionRange` by many bytes.

To make `AvailabilityRange` and `VersionRange` more efficient to store, we can
instead reserve two unlikely `llvm::VersionTuple` bit patterns as the sentinel
values instead. The values chosen are the same ones LLVM uses to represent
version tuple tombstones and empty keys in a `DenseMap`.

2025-03-05 07:15:15 -08:00

5.7 KiB

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