The Query Evaluation Model in Detail

This chapter provides a deeper dive into the abstract model queries are built on. It does not go into implementation details but tries to explain the underlying logic. The examples here, therefore, have been stripped down and simplified and don't directly reflect the compilers internal APIs.

What is a query?

Abstractly we view the compiler's knowledge about a given crate as a "database" and queries are the way of asking the compiler questions about it, i.e. we "query" the compiler's "database" for facts.

However, there's something special to this compiler database: It starts out empty and is filled on-demand when queries are executed. Consequently, a query must know how to compute its result if the database does not contain it yet. For doing so, it can access other queries and certain input values that the database is pre-filled with on creation.

A query thus consists of the following things:

A name that identifies the query
A "key" that specifies what we want to look up
A result type that specifies what kind of result it yields
A "provider" which is a function that specifies how the result is to be computed if it isn't already present in the database.

As an example, the name of the type_of query is type_of, its query key is a DefId identifying the item we want to know the type of, the result type is Ty<'tcx>, and the provider is a function that, given the query key and access to the rest of the database, can compute the type of the item identified by the key.

So in some sense a query is just a function that maps the query key to the corresponding result. However, we have to apply some restrictions in order for this to be sound:

The key and result must be immutable values.
The provider function must be a pure function, that is, for the same key it must always yield the same result.
The only parameters a provider function takes are the key and a reference to the "query context" (which provides access to rest of the "database").

The database is built up lazily by invoking queries. The query providers will invoke other queries, for which the result is either already cached or computed by calling another query provider. These query provider invocations conceptually form a directed acyclic graph (DAG) at the leaves of which are input values that are already known when the query context is created.

Caching/Memoization

Results of query invocations are "memoized" which means that the query context will cache the result in an internal table and, when the query is invoked with the same query key again, will return the result from the cache instead of running the provider again.

This caching is crucial for making the query engine efficient. Without memoization the system would still be sound (that is, it would yield the same results) but the same computations would be done over and over again.

Memoization is one of the main reasons why query providers have to be pure functions. If calling a provider function could yield different results for each invocation (because it accesses some global mutable state) then we could not memoize the result.

Input data

When the query context is created, it is still empty: No queries have been executed, no results are cached. But the context already provides access to "input" data, i.e. pieces of immutable data that where computed before the context was created and that queries can access to do their computations. Currently this input data consists mainly of the HIR map and the command-line options the compiler was invoked with. In the future, inputs will just consist of command-line options and a list of source files -- the HIR map will itself be provided by a query which processes these source files.

Without inputs, queries would live in a void without anything to compute their result from (remember, query providers only have access to other queries and the context but not any other outside state or information).

For a query provider, input data and results of other queries look exactly the same: It just tells the context "give me the value of X". Because input data is immutable, the provider can rely on it being the same across different query invocations, just as is the case for query results.

An example execution trace of some queries

How does this DAG of query invocations come into existence? At some point the compiler driver will create the, as yet empty, query context. It will then, from outside of the query system, invoke the queries it needs to perform its task. This looks something like the following:

fn compile_crate() {
    let cli_options = ...;
    let hir_map = ...;

    // Create the query context `tcx`
    let tcx = TyCtxt::new(cli_options, hir_map);

    // Do type checking by invoking the type check query
    tcx.type_check_crate();
}

The type_check_crate query provider would look something like the following:

fn type_check_crate_provider(tcx, _key: ()) {
    let list_of_items = tcx.hir_map.list_of_items();

    for item_def_id in list_of_hir_items {
        tcx.type_check_item(item_def_id);
    }
}

We see that the type_check_crate query accesses input data (tcx.hir_map.list_of_items()) and invokes other queries (type_check_item). The type_check_item invocations will themselves access input data and/or invoke other queries, so that in the end the DAG of query invocations will be built up backwards from the node that was initially executed:

         (2)                                                 (1)
  list_of_all_hir_items <----------------------------- type_check_crate()
                                                               |
    (5)             (4)                  (3)                   |
  Hir(foo) <--- type_of(foo) <--- type_check_item(foo) <-------+
                                      |                        |
                    +-----------------+                        |
                    |                                          |
    (7)             v  (6)                  (8)                |
  Hir(bar) <--- type_of(bar) <--- type_check_item(bar) <-------+

// (x) denotes invocation order

We also see that often a query result can be read from the cache: type_of(bar) was computed for type_check_item(foo) so when type_check_item(bar) needs it, it is already in the cache.

Query results stay cached in the query context as long as the context lives. So if the compiler driver invoked another query later on, the above graph would still exist and already executed queries would not have to be re-done.

Cycles

Earlier we stated that query invocations form a DAG. However, it would be easy form a cyclic graph by, for example, having a query provider like the following:

fn cyclic_query_provider(tcx, key) -> u32 {
  // Invoke the same query with the same key again
  tcx.cyclic_query(key)
}

Since query providers are regular functions, this would behave much as expected: Evaluation would get stuck in an infinite recursion. A query like this would not be very useful either. However, sometimes certain kinds of invalid user input can result in queries being called in a cyclic way. The query engine includes a check for cyclic invocations and, because cycles are an irrecoverable error, will abort execution with a "cycle error" messages that tries to be human readable.

At some point the compiler had a notion of "cycle recovery", that is, one could "try" to execute a query and if it ended up causing a cycle, proceed in some other fashion. However, this was later removed because it is not entirely clear what the theoretical consequences of this are, especially regarding incremental compilation.

"Steal" Queries

Some queries have their result wrapped in a Steal<T> struct. These queries behave exactly the same as regular with one exception: Their result is expected to be "stolen" out of the cache at some point, meaning some other part of the program is taking ownership of it and the result cannot be accessed anymore.

This stealing mechanism exists purely as a performance optimization because some result values are too costly to clone (e.g. the MIR of a function). It seems like result stealing would violate the condition that query results must be immutable (after all we are moving the result value out of the cache) but it is OK as long as the mutation is not observable. This is achieved by two things:

Before a result is stolen, we make sure to eagerly run all queries that might ever need to read that result. This has to be done manually by calling those queries.
Whenever a query tries to access a stolen result, we make the compiler ICE so that such a condition cannot go unnoticed.

This is not an ideal setup because of the manual intervention needed, so it should be used sparingly and only when it is well known which queries might access a given result. In practice, however, stealing has not turned out to be much of a maintenance burden.

To summarize: "Steal queries" break some of the rules in a controlled way. There are checks in place that make sure that nothing can go silently wrong.

Parallel Query Execution

The query model has some properties that make it actually feasible to evaluate multiple queries in parallel without too much of an effort:

All data a query provider can access is accessed via the query context, so the query context can take care of synchronizing access.
Query results are required to be immutable so they can safely be used by different threads concurrently.

The nightly compiler already implements parallel query evaluation as follows:

When a query foo is evaluated, the cache table for foo is locked.

If there already is a result, we can clone it,release the lock and we are done.
If there is no cache entry and no other active query invocation computing the same result, we mark the key as being "in progress", release the lock and start evaluating.
If there is another query invocation for the same key in progress, we release the lock, and just block the thread until the other invocation has computed the result we are waiting for. This cannot deadlock because, as mentioned before, query invocations form a DAG. Some thread will always make progress.

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