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Diffstat (limited to 'src/doc/rustc-dev-guide')
| -rw-r--r-- | src/doc/rustc-dev-guide/src/mir/optimizations.md | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/src/doc/rustc-dev-guide/src/mir/optimizations.md b/src/doc/rustc-dev-guide/src/mir/optimizations.md index 65323ed52e4..b34520f353d 100644 --- a/src/doc/rustc-dev-guide/src/mir/optimizations.md +++ b/src/doc/rustc-dev-guide/src/mir/optimizations.md @@ -60,3 +60,24 @@ want to query whether your optimization pass should run, you can check the current level using `tcx.sess.opts.debugging_opts.mir_opt_level`. [compiler MCP]: https://github.com/rust-lang/compiler-team/issues/319 + +## Optimization fuel + +Optimization fuel is a compiler option (`-Z fuel=<crate>=<value>`) that allows for fine grained +control over which optimizations can be applied during compilation: each optimization reduces +fuel by 1, and when fuel reaches 0 no more optimizations are applied. This can help with debugging +and identifying problems with optimizations. + +MIR optimizations respect fuel, and in general each pass should check fuel by calling +[`tcx.consider_optimizing`][consideroptimizing] and skipping the optimization if fuel +is empty. There are a few considerations: + +1. If the pass is considered "guaranteed" (for example, it should always be run because it is +needed for correctness), then fuel should not be used. An example of this is `PromoteTemps`. +2. In some cases, an initial pass is performed to gather candidates, which are then iterated to +perform optimizations. In these situations, we should allow for the initial gathering pass +and then check fuel as close to the mutation as possible. This allows for the best +debugging experience, because we can determine where in the list of candidates an optimization +may have been misapplied. Examples of this are `InstCombine` and `ConstantPropagation`. + +[consideroptimizing]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.consider_optimizing |