remove irrelevant comments
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Niko Matsakis
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@ -32,111 +32,59 @@ MIR and then iteratively optimize it by putting it through various
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pipeline stages. This section describes those pipeline stages and how
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you can extend them.
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Here is a diagram showing the various MIR queries involved in producing
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the final `optimized_mir()` for a single def-id `D`. The arrows here
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indicate how data flows from query to query.
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To produce the `optimized_mir(D)` for a given def-id `D`, the MIR
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passes through several suites of optimizations, each represented by a
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query. Each suite consists of multiple optimizations and
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transformations. These suites represent useful intermediate points
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where we want to access the MIR for type checking or other purposes:
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```
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mir_build(D)
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-> mir_pass((0,0,D)) ---+ each suite consists of many passes
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-> ... |
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-> mir_pass((0,N,D)) |
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-> mir_suite((0,D)) ---+ ---+ there are several suites
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-> ... |
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-> mir_suite((M,D)) ---+
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-> mir_optimized(D)
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```
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The MIR transformation pipeline is organized into **suites**. When
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you ask for `mir_optimized(D)`, it will turn around and request the
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result from the final **suite** of MIR passes
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(`mir_suite((M,D))`). This will in turn (eventually) trigger the MIR
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to be build and then passes through each of the optimization suites.
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Each suite internally triggers one query for each of its passes
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(`mir_pass(...)`).
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The reason for the suites is that they represent points in the MIR
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transformation pipeline where other bits of code are interested in
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observing. For example, the `MIR_CONST` suite defines the point where
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analysis for constant rvalues and expressions can take
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place. `MIR_OPTIMIZED` naturally represents the point where we
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actually generate machine code. Nobody should ever request the result
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of an individual *pass*, at least outside of the transformation
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pipeline: this allows us to add passes into the appropriate suite
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without having to modify anything else in the compiler.
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- `mir_build(D)` -- not a query, but this constructs the initial MIR
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- `mir_const(D)` -- applies some simple transformations to make MIR ready for constant evaluation;
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- `mir_validated(D)` -- applies some more transformations, making MIR ready for borrow checking;
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- `optimized_mir(D)` -- the final state, after all optimizations have been performed.
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### Stealing
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Each of these intermediate queries yields up a `&'tcx
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Steal<Mir<'tcx>>`, allocated using `tcx.alloc_steal_mir()`. This
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indicates that the result may be **stolen** by the next pass -- this
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is an optimization to avoid cloning the MIR. Attempting to use a
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stolen result will cause a panic in the compiler. Therefore, it is
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important that you not read directly from these intermediate queries
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except as part of the MIR processing pipeline.
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The intermediate queries `mir_const()` and `mir_validated()` yield up
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a `&'tcx Steal<Mir<'tcx>>`, allocated using
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`tcx.alloc_steal_mir()`. This indicates that the result may be
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**stolen** by the next suite of optimizations -- this is an
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optimization to avoid cloning the MIR. Attempting to use a stolen
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result will cause a panic in the compiler. Therefore, it is important
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that you not read directly from these intermediate queries except as
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part of the MIR processing pipeline.
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Because of this stealing mechanism, some care must also be taken to
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ensure that, before the MIR at a particular phase in the processing
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pipeline is stolen, anyone who may want to read from it has already
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done so. Sometimes this requires **forcing** queries
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(`ty::queries::foo::force(...)`) during an optimization pass -- this
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will force a query to execute even though you don't directly require
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its result. The query can then read the MIR it needs, and -- once it
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is complete -- you can steal it.
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done so. Concretely, this means that if you have some query `foo(D)`
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that wants to access the result of `mir_const(D)` or
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`mir_validated(D)`, you need to have the successor pass either "force"
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`foo(D)` using `ty::queries::foo::force(...)`. This will force a query
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to execute even though you don't directly require its result.
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As an example, consider MIR const qualification. It wants to read the
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result produced by the `MIR_CONST` suite. However, that result will be
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**stolen** by the first pass in the next suite (that pass performs
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const promotion):
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```
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mir_suite((MIR_CONST,D)) --read-by--> mir_const_qualif(D)
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stolen-by
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v
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mir_pass((MIR_VALIDATED,0,D))
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```
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Therefore, the const promotion pass (the `mir_pass()` in the diagram)
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result produced by the `mir_const()` suite. However, that result will
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be **stolen** by the `mir_validated()` suite. If nothing was done,
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then `mir_const_qualif(D)` would succeed if it came before
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`mir_validated(D)`, but fail otherwise. Therefore, `mir_validated(D)`
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will **force** `mir_const_qualif` before it actually steals, thus
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ensuring that the reads have already happened (and the final result is
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cached).
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ensuring that the reads have already happened:
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```
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mir_const(D) --read-by--> mir_const_qualif(D)
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| ^
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stolen-by |
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| (forces)
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v |
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mir_validated(D) ------------+
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```
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### Implementing and registering a pass
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To create a new MIR pass, you have to implement one of the MIR pass
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traits. There are several traits, and you want to pick the most
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specific one that applies to your pass. They are described here in
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order of preference. Once you have implemented a trait for your type
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`Foo`, you then have to insert `Foo` into one of the suites; this is
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done in `librustc_driver/driver.rs` by invoking `push_pass()` with the
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appropriate suite.
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To create a new MIR pass, you simply implement the `MirPass` trait for
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some fresh singleton type `Foo`. Once you have implemented a trait for
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your type `Foo`, you then have to insert `Foo` into one of the suites;
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this is done in `librustc_driver/driver.rs` by invoking `push_pass(S,
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Foo)` with the appropriate suite substituted for `S`.
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**The `MirPass` trait.** For the most part, a MIR pass works by taking
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as input the MIR for a single function and mutating it imperatively to
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perform an optimization. To write such a pass, you can implement the
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`MirPass` trait, which has a single callback that takes an `&mut Mir`.
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**The `DefIdPass` trait.** When a `MirPass` trait is executed, the
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system will automatically steal the result of the previous pass and
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supply it to you. (See the section on queries and stealing below.)
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Sometimes you don't want to steal the result of the previous pass
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right away. In such cases, you can define a `DefIdPass`, which simply
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gets a callback and lets you decide when to steal the previous result.
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**The `Pass` trait.** The most primitive but flexible trait is `Pass`.
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Unlike the other pass types, it returns a `Multi` result, which means
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it scan be used for interprocedural passes which mutate more than one
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MIR at a time (e.g., `inline`).
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### The MIR Context
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All of the passes when invoked take a `MirCtxt` object. This contains
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various methods to find out (e.g.) the current pass suite and pass
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index, the def-id you are operating on, and so forth. You can also
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access the MIR for the current def-id using `read_previous_mir()`; the
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"previous" refers to the fact that this will be the MIR that was
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output by the previous pass. Finally, you can `steal_previous_mir()`
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to steal the output of the current pass (in which case you get
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ownership of the MIR).
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