src/doc/rustc-guide/src/mir/optimizations.md

   1 # MIR optimizations
   2
   3 MIR optimizations are optimizations run on the [MIR][mir] to produce better MIR
   4 before codegen. This is important for two reasons: first, it makes the final
   5 generated executable code better, and second, it means that LLVM has less work
   6 to do, so compilation is faster. Note that since MIR is generic (not
   7 [monomorphized][monomorph] yet), these optimizations are particularly
   8 effective; we can optimize the generic version, so all of the monomorphizations
   9 are cheaper!
  10
  11 [mir]: https://rust-lang.github.io/rustc-guide/mir/index.html
  12 [monomorph]: https://rust-lang.github.io/rustc-guide/appendix/glossary.html?highlight=monomorphize#appendix-c-glossary
  13
  14 MIR optimizations run after borrow checking. We run a series of optimization
  15 passes over the MIR to improve it. Some passes are required to run on all code,
  16 some passes don't actually do optimizations but only check stuff, and some
  17 passes are only turned on in `release` mode.
  18
  19 The [`optimized_mir`][optmir] [query] is called to produce the optimized MIR
  20 for a given [`DefId`][defid]. This query makes sure that the borrow checker has
  21 run and that some validation has occurred. Then, it [steals][steal] the MIR
  22 optimizes it, and returns the improved MIR.
  23
  24 [optmir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/fn.optimized_mir.html
  25 [query]: https://rust-lang.github.io/rustc-guide/query.html
  26 [defid]: https://rust-lang.github.io/rustc-guide/appendix/glossary.html?highlight=DefId#appendix-c-glossary
  27 [steal]: https://rust-lang.github.io/rustc-guide/mir/passes.html?highlight=steal#stealing
  28
  29 ## Defining optimization passes
  30
  31 The list of passes run and the order in which they are run is defined by the
  32 [`run_optimization_passes`][rop] function. It contains an array of passes to
  33 run.  Each pass in the array is a struct that implements the [`MirPass`] trait.
  34 The array is an array of `&dyn MirPass` trait objects. Typically, a pass is
  35 implemented in its own submodule of the [`rustc_mir::transform`][trans] module.
  36
  37 [rop]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/fn.run_optimization_passes.html
  38 [`MirPass`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/trait.MirPass.html
  39 [trans]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/index.html
  40
  41 Some examples of passes are:
  42 - `CleanupNonCodegenStatements`: remove some of the info that is only needed for
  43   analyses, rather than codegen.
  44 - `ConstProp`: Does [constant propagation][constprop]
  45
  46 You can see the ["Implementors" section of the `MirPass` rustdocs][impl] for more examples.
  47
  48 [impl]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/trait.MirPass.html#implementors
  49 [constprop]: https://en.wikipedia.org/wiki/Constant_folding#Constant_propagation