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Aaron Hill 2021-12-23 13:44:04 -05:00
parent 28f19f62c7
commit 27ed52c0a2
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1 changed files with 13 additions and 8 deletions
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21
compiler/rustc_query_system/src/dep_graph/graph.rs
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@ -185,20 +185,20 @@ impl<K: DepKind> DepGraph<K> {
///
/// `A -> B -> C`
///
/// Suppose that decoding the result of query `B` required invoking
/// a query `D`. If we did not create a fresh `TaskDeps` when
/// decoding `B`, we might would still be using the `TaskDeps` for query `A`
/// Suppose that decoding the result of query `B` required re-computing
/// the query `C`. If we did not create a fresh `TaskDeps` when
/// decoding `B`, we would still be using the `TaskDeps` for query `A`
/// (if we needed to re-execute `A`). This would cause us to create
/// a new edge `A -> D`. If this edge did not previously
/// a new edge `A -> C`. If this edge did not previously
/// exist in the `DepGraph`, then we could end up with a different
/// `DepGraph` at the end of compilation, even if there were no
/// meaningful changes to the overall program (e.g. a newline was added).
/// In addition, this edge might cause a subsequent compilation run
/// to try to force `D` before marking other necessary nodes green. If
/// `D` did not exist in the new compilation session, then we might
/// to try to force `C` before marking other necessary nodes green. If
/// `C` did not exist in the new compilation session, then we could
/// get an ICE. Normally, we would have tried (and failed) to mark
/// some other query green (e.g. `item_children`) which was used
/// to obtain `D`, which would prevent us from ever trying to force
/// to obtain `C`, which would prevent us from ever trying to force
/// a non-existent `D`.
///
/// It might be possible to enforce that all `DepNode`s read during
@ -208,7 +208,12 @@ impl<K: DepKind> DepGraph<K> {
/// of `B`, this would result in an edge `B -> D`. If that edge already
/// existed (with the same `DepPathHash`es), then it should be correct
/// to allow the invocation of the query to proceed during deserialization
/// of a query result. However, this would require additional complexity
/// of a query result. We would merely assert that the dep-graph fragment
/// that would have been added by invoking `C` while decoding `B`
/// is equivalent to the dep-graph fragment that we already instantiated for B
/// (at the point where we successfully marked B as green).
///
/// However, this would require additional complexity
/// in the query infrastructure, and is not currently needed by the
/// decoding of any query results. Should the need arise in the future,
/// we should consider extending the query system with this functionality.