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This replaces the per-shard counters with a single global counter, simplifying the JobId struct down to just a u64 and removing the need to pipe a DepKind generic through a bunch of code. The performance implications on non-parallel compilers are likely minimal (this switches to `Cell<u64>` as the backing storage over a `u64`, but the latter was already inside a `RefCell` so it's not really a significance divergence). On parallel compilers, the cost of a single global u64 counter may be more significant: it adds a serialization point in theory. On the other hand, we can imagine changing the counter to have a thread-local component if it becomes worrisome or some similar structure. The new design is sufficiently simpler that it warrants the potential for slight changes down the line if/when we get parallel compilation to be more of a default. A u64 counter, instead of u32 (the old per-shard width), is chosen to avoid possibly overflowing it and causing problems; it is effectively impossible that we would overflow a u64 counter in this context. |
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| Cargo.toml | ||
| README.md | ||
For more information about how rustc works, see the rustc dev guide.