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# Incremental Compilation in detail # Incremental compilation in detail
<!-- toc --> <!-- toc -->
@ -66,7 +66,7 @@ because it reads the up-to-date version of `Hir(bar)`. Also, we re-run
`type_check_item(bar)` because result of `type_of(bar)` might have changed. `type_check_item(bar)` because result of `type_of(bar)` might have changed.
## The Problem With The Basic Algorithm: False Positives ## The problem with the basic algorithm: false positives
If you read the previous paragraph carefully you'll notice that it says that If you read the previous paragraph carefully you'll notice that it says that
`type_of(bar)` *might* have changed because one of its inputs has changed. `type_of(bar)` *might* have changed because one of its inputs has changed.
@ -93,7 +93,7 @@ of examples like this and small changes to the input often potentially affect
very large parts of the output binaries. As a consequence, we had to make the very large parts of the output binaries. As a consequence, we had to make the
change detection system smarter and more accurate. change detection system smarter and more accurate.
## Improving Accuracy: The red-green Algorithm ## Improving accuracy: the red-green algorithm
The "false positives" problem can be solved by interleaving change detection The "false positives" problem can be solved by interleaving change detection
and query re-evaluation. Instead of walking the graph all the way to the and query re-evaluation. Instead of walking the graph all the way to the
@ -191,7 +191,7 @@ then itself involve recursively invoking more queries, which can mean we come ba
to the `try_mark_green()` algorithm for the dependencies recursively. to the `try_mark_green()` algorithm for the dependencies recursively.
## The Real World: How Persistence Makes Everything Complicated ## The real world: how persistence makes everything complicated
The sections above described the underlying algorithm for incremental The sections above described the underlying algorithm for incremental
compilation but because the compiler process exits after being finished and compilation but because the compiler process exits after being finished and
@ -258,7 +258,7 @@ the `LocalId`s within it are still the same.
### Checking Query Results For Changes: HashStable And Fingerprints ### Checking query results for changes: `HashStable` and `Fingerprint`s
In order to do red-green-marking we often need to check if the result of a In order to do red-green-marking we often need to check if the result of a
query has changed compared to the result it had during the previous query has changed compared to the result it had during the previous
@ -306,7 +306,7 @@ This approach works rather well but it's not without flaws:
their stable equivalents while doing the hashing. their stable equivalents while doing the hashing.
### A Tale Of Two DepGraphs: The Old And The New ### A tale of two `DepGraph`s: the old and the new
The initial description of dependency tracking glosses over a few details The initial description of dependency tracking glosses over a few details
that quickly become a head scratcher when actually trying to implement things. that quickly become a head scratcher when actually trying to implement things.
@ -344,7 +344,7 @@ new graph is serialized out to disk, alongside the query result cache, and can
act as the previous dep-graph in a subsequent compilation session. act as the previous dep-graph in a subsequent compilation session.
### Didn't You Forget Something?: Cache Promotion ### Didn't you forget something?: cache promotion
The system described so far has a somewhat subtle property: If all inputs of a The system described so far has a somewhat subtle property: If all inputs of a
dep-node are green then the dep-node itself can be marked as green without dep-node are green then the dep-node itself can be marked as green without
@ -374,7 +374,7 @@ the result cache doesn't unnecessarily shrink again.
# Incremental Compilation and the Compiler Backend # Incremental compilation and the compiler backend
The compiler backend, the part involving LLVM, is using the query system but The compiler backend, the part involving LLVM, is using the query system but
it is not implemented in terms of queries itself. As a consequence it does not it is not implemented in terms of queries itself. As a consequence it does not
@ -406,7 +406,7 @@ would save.
## Query Modifiers ## Query modifiers
The query system allows for applying [modifiers][mod] to queries. These The query system allows for applying [modifiers][mod] to queries. These
modifiers affect certain aspects of how the system treats the query with modifiers affect certain aspects of how the system treats the query with
@ -472,7 +472,7 @@ respect to incremental compilation:
[mod]: ../query.html#adding-a-new-kind-of-query [mod]: ../query.html#adding-a-new-kind-of-query
## The Projection Query Pattern ## The projection query pattern
It's interesting to note that `eval_always` and `no_hash` can be used together It's interesting to note that `eval_always` and `no_hash` can be used together
in the so-called "projection query" pattern. It is often the case that there is in the so-called "projection query" pattern. It is often the case that there is
@ -516,7 +516,7 @@ because we have the projections to take care of keeping things green as much
as possible. as possible.
# Shortcomings of the Current System # Shortcomings of the current system
There are many things that still can be improved. There are many things that still can be improved.