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Switch QueryJobId to a single global counter

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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.
This commit is contained in:
Mark Rousskov 2022-02-07 11:03:51 -05:00
parent 88fb06a1f3
commit e240783a4d
7 changed files with 116 additions and 178 deletions
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10
compiler/rustc_query_system/src/query/mod.rs
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@ -117,10 +117,12 @@ impl QuerySideEffects {
}
pub trait QueryContext: HasDepContext {
/// Get the query information from the TLS context.
fn current_query_job(&self) -> Option<QueryJobId<Self::DepKind>>;
fn next_job_id(&self) -> QueryJobId;
fn try_collect_active_jobs(&self) -> Option<QueryMap<Self::DepKind>>;
/// Get the query information from the TLS context.
fn current_query_job(&self) -> Option<QueryJobId>;
fn try_collect_active_jobs(&self) -> Option<QueryMap>;
/// Load side effects associated to the node in the previous session.
fn load_side_effects(&self, prev_dep_node_index: SerializedDepNodeIndex) -> QuerySideEffects;
@ -140,7 +142,7 @@ pub trait QueryContext: HasDepContext {
/// captured during execution and the actual result.
fn start_query<R>(
&self,
token: QueryJobId<Self::DepKind>,
token: QueryJobId,
diagnostics: Option<&Lock<ThinVec<Diagnostic>>>,
compute: impl FnOnce() -> R,
) -> R;