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# Dependency graph for incremental compilation | ||
To learn more about how dependency tracking works in rustc, see the [rustc | ||
guide]. | ||
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This module contains the infrastructure for managing the incremental | ||
compilation dependency graph. This README aims to explain how it ought | ||
to be used. In this document, we'll first explain the overall | ||
strategy, and then share some tips for handling specific scenarios. | ||
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The high-level idea is that we want to instrument the compiler to | ||
track which parts of the AST and other IR are read/written by what. | ||
This way, when we come back later, we can look at this graph and | ||
determine what work needs to be redone. | ||
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### The dependency graph | ||
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The nodes of the graph are defined by the enum `DepNode`. They represent | ||
one of three things: | ||
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1. HIR nodes (like `Hir(DefId)`) represent the HIR input itself. | ||
2. Data nodes (like `TypeOfItem(DefId)`) represent some computed | ||
information about a particular item. | ||
3. Procedure nodes (like `CoherenceCheckTrait(DefId)`) represent some | ||
procedure that is executing. Usually this procedure is | ||
performing some kind of check for errors. You can think of them as | ||
computed values where the value being computed is `()` (and the | ||
value may fail to be computed, if an error results). | ||
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An edge `N1 -> N2` is added between two nodes if either: | ||
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- the value of `N1` is used to compute `N2`; | ||
- `N1` is read by the procedure `N2`; | ||
- the procedure `N1` writes the value `N2`. | ||
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The latter two conditions are equivalent to the first one if you think | ||
of procedures as values. | ||
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### Basic tracking | ||
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There is a very general strategy to ensure that you have a correct, if | ||
sometimes overconservative, dependency graph. The two main things you have | ||
to do are (a) identify shared state and (b) identify the current tasks. | ||
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### Identifying shared state | ||
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Identify "shared state" that will be written by one pass and read by | ||
another. In particular, we need to identify shared state that will be | ||
read "across items" -- that is, anything where changes in one item | ||
could invalidate work done for other items. So, for example: | ||
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1. The signature for a function is "shared state". | ||
2. The computed type of some expression in the body of a function is | ||
not shared state, because if it changes it does not itself | ||
invalidate other functions (though it may be that it causes new | ||
monomorphizations to occur, but that's handled independently). | ||
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Put another way: if the HIR for an item changes, we are going to | ||
recompile that item for sure. But we need the dep tracking map to tell | ||
us what *else* we have to recompile. Shared state is anything that is | ||
used to communicate results from one item to another. | ||
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### Identifying the current task, tracking reads/writes, etc | ||
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FIXME(#42293). This text needs to be rewritten for the new red-green | ||
system, which doesn't fully exist yet. | ||
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#### Dependency tracking map | ||
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`DepTrackingMap` is a particularly convenient way to correctly store | ||
shared state. A `DepTrackingMap` is a special hashmap that will add | ||
edges automatically when `get` and `insert` are called. The idea is | ||
that, when you get/insert a value for the key `K`, we will add an edge | ||
from/to the node `DepNode::Variant(K)` (for some variant specific to | ||
the map). | ||
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Each `DepTrackingMap` is parameterized by a special type `M` that | ||
implements `DepTrackingMapConfig`; this trait defines the key and value | ||
types of the map, and also defines a fn for converting from the key to | ||
a `DepNode` label. You don't usually have to muck about with this by | ||
hand, there is a macro for creating it. You can see the complete set | ||
of `DepTrackingMap` definitions in `librustc/middle/ty/maps.rs`. | ||
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As an example, let's look at the `adt_defs` map. The `adt_defs` map | ||
maps from the def-id of a struct/enum to its `AdtDef`. It is defined | ||
using this macro: | ||
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```rust | ||
dep_map_ty! { AdtDefs: ItemSignature(DefId) -> ty::AdtDefMaster<'tcx> } | ||
// ~~~~~~~ ~~~~~~~~~~~~~ ~~~~~ ~~~~~~~~~~~~~~~~~~~~~~ | ||
// | | Key type Value type | ||
// | DepNode variant | ||
// Name of map id type | ||
``` | ||
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this indicates that a map id type `AdtDefs` will be created. The key | ||
of the map will be a `DefId` and value will be | ||
`ty::AdtDefMaster<'tcx>`. The `DepNode` will be created by | ||
`DepNode::ItemSignature(K)` for a given key. | ||
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Once that is done, you can just use the `DepTrackingMap` like any | ||
other map: | ||
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```rust | ||
let mut map: DepTrackingMap<M> = DepTrackingMap::new(dep_graph); | ||
map.insert(key, value); // registers dep_graph.write | ||
map.get(key; // registers dep_graph.read | ||
``` | ||
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#### Memoization | ||
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One particularly interesting case is memoization. If you have some | ||
shared state that you compute in a memoized fashion, the correct thing | ||
to do is to define a `RefCell<DepTrackingMap>` for it and use the | ||
`memoize` helper: | ||
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```rust | ||
map.memoize(key, || /* compute value */) | ||
``` | ||
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This will create a graph that looks like | ||
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... -> MapVariant(key) -> CurrentTask | ||
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where `MapVariant` is the `DepNode` variant that the map is associated with, | ||
and `...` are whatever edges the `/* compute value */` closure creates. | ||
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In particular, using the memoize helper is much better than writing | ||
the obvious code yourself: | ||
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```rust | ||
if let Some(result) = map.get(key) { | ||
return result; | ||
} | ||
let value = /* compute value */; | ||
map.insert(key, value); | ||
``` | ||
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If you write that code manually, the dependency graph you get will | ||
include artificial edges that are not necessary. For example, imagine that | ||
two tasks, A and B, both invoke the manual memoization code, but A happens | ||
to go first. The resulting graph will be: | ||
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... -> A -> MapVariant(key) -> B | ||
~~~~~~~~~~~~~~~~~~~~~~~~~~~ // caused by A writing to MapVariant(key) | ||
~~~~~~~~~~~~~~~~~~~~ // caused by B reading from MapVariant(key) | ||
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This graph is not *wrong*, but it encodes a path from A to B that | ||
should not exist. In contrast, using the memoized helper, you get: | ||
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... -> MapVariant(key) -> A | ||
| | ||
+----------> B | ||
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which is much cleaner. | ||
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**Be aware though that the closure is executed with `MapVariant(key)` | ||
pushed onto the stack as the current task!** That means that you must | ||
add explicit `read` calls for any shared state that it accesses | ||
implicitly from its environment. See the section on "explicit calls to | ||
read and write when starting a new subtask" above for more details. | ||
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### How to decide where to introduce a new task | ||
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Certainly, you need at least one task on the stack: any attempt to | ||
`read` or `write` shared state will panic if there is no current | ||
task. But where does it make sense to introduce subtasks? The basic | ||
rule is that a subtask makes sense for any discrete unit of work you | ||
may want to skip in the future. Adding a subtask separates out the | ||
reads/writes from *that particular subtask* versus the larger | ||
context. An example: you might have a 'meta' task for all of borrow | ||
checking, and then subtasks for borrow checking individual fns. (Seen | ||
in this light, memoized computations are just a special case where we | ||
may want to avoid redoing the work even within the context of one | ||
compilation.) | ||
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The other case where you might want a subtask is to help with refining | ||
the reads/writes for some later bit of work that needs to be memoized. | ||
For example, we create a subtask for type-checking the body of each | ||
fn. However, in the initial version of incr. comp. at least, we do | ||
not expect to actually *SKIP* type-checking -- we only expect to skip | ||
trans. However, it's still useful to create subtasks for type-checking | ||
individual items, because, otherwise, if a fn sig changes, we won't | ||
know which callers are affected -- in fact, because the graph would be | ||
so coarse, we'd just have to retrans everything, since we can't | ||
distinguish which fns used which fn sigs. | ||
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### Testing the dependency graph | ||
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There are various ways to write tests against the dependency graph. | ||
The simplest mechanism are the | ||
`#[rustc_if_this_changed]` and `#[rustc_then_this_would_need]` | ||
annotations. These are used in compile-fail tests to test whether the | ||
expected set of paths exist in the dependency graph. As an example, | ||
see `src/test/compile-fail/dep-graph-caller-callee.rs`. | ||
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The idea is that you can annotate a test like: | ||
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```rust | ||
#[rustc_if_this_changed] | ||
fn foo() { } | ||
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#[rustc_then_this_would_need(TypeckTables)] //~ ERROR OK | ||
fn bar() { foo(); } | ||
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#[rustc_then_this_would_need(TypeckTables)] //~ ERROR no path | ||
fn baz() { } | ||
``` | ||
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This will check whether there is a path in the dependency graph from | ||
`Hir(foo)` to `TypeckTables(bar)`. An error is reported for each | ||
`#[rustc_then_this_would_need]` annotation that indicates whether a | ||
path exists. `//~ ERROR` annotations can then be used to test if a | ||
path is found (as demonstrated above). | ||
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### Debugging the dependency graph | ||
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#### Dumping the graph | ||
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The compiler is also capable of dumping the dependency graph for your | ||
debugging pleasure. To do so, pass the `-Z dump-dep-graph` flag. The | ||
graph will be dumped to `dep_graph.{txt,dot}` in the current | ||
directory. You can override the filename with the `RUST_DEP_GRAPH` | ||
environment variable. | ||
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Frequently, though, the full dep graph is quite overwhelming and not | ||
particularly helpful. Therefore, the compiler also allows you to filter | ||
the graph. You can filter in three ways: | ||
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1. All edges originating in a particular set of nodes (usually a single node). | ||
2. All edges reaching a particular set of nodes. | ||
3. All edges that lie between given start and end nodes. | ||
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To filter, use the `RUST_DEP_GRAPH_FILTER` environment variable, which should | ||
look like one of the following: | ||
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``` | ||
source_filter // nodes originating from source_filter | ||
-> target_filter // nodes that can reach target_filter | ||
source_filter -> target_filter // nodes in between source_filter and target_filter | ||
``` | ||
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`source_filter` and `target_filter` are a `&`-separated list of strings. | ||
A node is considered to match a filter if all of those strings appear in its | ||
label. So, for example: | ||
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``` | ||
RUST_DEP_GRAPH_FILTER='-> TypeckTables' | ||
``` | ||
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would select the predecessors of all `TypeckTables` nodes. Usually though you | ||
want the `TypeckTables` node for some particular fn, so you might write: | ||
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``` | ||
RUST_DEP_GRAPH_FILTER='-> TypeckTables & bar' | ||
``` | ||
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This will select only the `TypeckTables` nodes for fns with `bar` in their name. | ||
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Perhaps you are finding that when you change `foo` you need to re-type-check `bar`, | ||
but you don't think you should have to. In that case, you might do: | ||
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``` | ||
RUST_DEP_GRAPH_FILTER='Hir&foo -> TypeckTables & bar' | ||
``` | ||
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This will dump out all the nodes that lead from `Hir(foo)` to | ||
`TypeckTables(bar)`, from which you can (hopefully) see the source | ||
of the erroneous edge. | ||
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#### Tracking down incorrect edges | ||
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Sometimes, after you dump the dependency graph, you will find some | ||
path that should not exist, but you will not be quite sure how it came | ||
to be. **When the compiler is built with debug assertions,** it can | ||
help you track that down. Simply set the `RUST_FORBID_DEP_GRAPH_EDGE` | ||
environment variable to a filter. Every edge created in the dep-graph | ||
will be tested against that filter -- if it matches, a `bug!` is | ||
reported, so you can easily see the backtrace (`RUST_BACKTRACE=1`). | ||
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The syntax for these filters is the same as described in the previous | ||
section. However, note that this filter is applied to every **edge** | ||
and doesn't handle longer paths in the graph, unlike the previous | ||
section. | ||
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Example: | ||
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You find that there is a path from the `Hir` of `foo` to the type | ||
check of `bar` and you don't think there should be. You dump the | ||
dep-graph as described in the previous section and open `dep-graph.txt` | ||
to see something like: | ||
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Hir(foo) -> Collect(bar) | ||
Collect(bar) -> TypeckTables(bar) | ||
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That first edge looks suspicious to you. So you set | ||
`RUST_FORBID_DEP_GRAPH_EDGE` to `Hir&foo -> Collect&bar`, re-run, and | ||
then observe the backtrace. Voila, bug fixed! | ||
[rustc guide]: https://rust-lang-nursery.github.io/rustc-guide/query.html |
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