Incremental compilation

The incremental compilation scheme is, in essence, a surprisingly simple extension to the overall query system. We'll start by describing a slightly simplified variant of the real thing – the "basic algorithm" – and then describe some possible improvements.

The basic algorithm

The basic algorithm is called the red-green algorithm1. The high-level idea is that, after each run of the compiler, we will save the results of all the queries that we do, as well as the query DAG. The query DAG is a DAG that indexes which queries executed which other queries. So, for example, there would be an edge from a query Q1 to another query Q2 if computing Q1 required computing Q2 (note that because queries cannot depend on themselves, this results in a DAG and not a general graph).

On the next run of the compiler, then, we can sometimes reuse these query results to avoid re-executing a query. We do this by assigning every query a color:

  • If a query is colored red, that means that its result during this compilation has changed from the previous compilation.
  • If a query is colored green, that means that its result is the same as the previous compilation.

There are two key insights here:

  • First, if all the inputs to query Q are colored green, then the query Q must result in the same value as last time and hence need not be re-executed (or else the compiler is not deterministic).
  • Second, even if some inputs to a query changes, it may be that it still produces the same result as the previous compilation. In particular, the query may only use part of its input.
    • Therefore, after executing a query, we always check whether it produced the same result as the previous time. If it did, we can still mark the query as green, and hence avoid re-executing dependent queries.

The try-mark-green algorithm

At the core of incremental compilation is an algorithm called "try-mark-green". It has the job of determining the color of a given query Q (which must not have yet been executed). In cases where Q has red inputs, determining Q's color may involve re-executing Q so that we can compare its output, but if all of Q's inputs are green, then we can conclude that Q must be green without re-executing it or inspecting its value at all. In the compiler, this allows us to avoid deserializing the result from disk when we don't need it, and in fact enables us to sometimes skip serializing the result as well (see the refinements section below).

Try-mark-green works as follows:

  • First check if the query Q was executed during the previous compilation.
    • If not, we can just re-execute the query as normal, and assign it the color of red.
  • If yes, then load the 'dependent queries' of Q.
  • If there is a saved result, then we load the reads(Q) vector from the query DAG. The "reads" is the set of queries that Q executed during its execution.
    • For each query R in reads(Q), we recursively demand the color of R using try-mark-green.
      • Note: it is important that we visit each node in reads(Q) in same order as they occurred in the original compilation. See the section on the query DAG below.
      • If any of the nodes in reads(Q) wind up colored red, then Q is dirty.
        • We re-execute Q and compare the hash of its result to the hash of the result from the previous compilation.
        • If the hash has not changed, we can mark Q as green and return.
      • Otherwise, all of the nodes in reads(Q) must be green. In that case, we can color Q as green and return.

The query DAG

The query DAG code is stored in src/librustc/dep_graph. Construction of the DAG is done by instrumenting the query execution.

One key point is that the query DAG also tracks ordering; that is, for each query Q, we not only track the queries that Q reads, we track the order in which they were read. This allows try-mark-green to walk those queries back in the same order. This is important because once a subquery comes back as red, we can no longer be sure that Q will continue along the same path as before. That is, imagine a query like this:

fn main_query(tcx) {
    if tcx.subquery1() {
        tcx.subquery2()
    } else {
        tcx.subquery3()
    }
}

Now imagine that in the first compilation, main_query starts by executing subquery1, and this returns true. In that case, the next query main_query executes will be subquery2, and subquery3 will not be executed at all.

But now imagine that in the next compilation, the input has changed such that subquery1 returns false. In this case, subquery2 would never execute. If try-mark-green were to visit reads(main_query) out of order, however, it might visit subquery2 before subquery1, and hence execute it. This can lead to ICEs and other problems in the compiler.

Improvements to the basic algorithm

In the description of the basic algorithm, we said that at the end of compilation we would save the results of all the queries that were performed. In practice, this can be quite wasteful – many of those results are very cheap to recompute, and serializing and deserializing them is not a particular win. In practice, what we would do is to save the hashes of all the subqueries that we performed. Then, in select cases, we also save the results.

This is why the incremental algorithm separates computing the color of a node, which often does not require its value, from computing the result of a node. Computing the result is done via a simple algorithm like so:

  • Check if a saved result for Q is available. If so, compute the color of Q. If Q is green, deserialize and return the saved result.
  • Otherwise, execute Q.
    • We can then compare the hash of the result and color Q as green if it did not change.

Footnotes

1

I have long wanted to rename it to the Salsa algorithm, but it never caught on. -@nikomatsakis