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add mir-opt section for optimization fuel

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cjkenn 2020-11-20 12:09:37 -05:00 committed by Joshua Nelson
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@ -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`. current level using `tcx.sess.opts.debugging_opts.mir_opt_level`.
[compiler MCP]: https://github.com/rust-lang/compiler-team/issues/319 [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