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Link to the -Zmir-opt-level proposal 

I was confused about what the different levels mean. This PR adds a paragraph that shortly explains the rationale behind different mir-opt-levels and links to the current proposal where there's some information about them.

Let me know if I wrote something unreasonable or incorrect.

Co-authored-by: Joshua Nelson <joshua@yottadb.com>
This commit is contained in:
Dániel Buga 2020-09-23 22:29:17 +02:00 committed by Joshua Nelson
parent 7609eb11ee
commit 727a536ba9
1 changed files with 14 additions and 1 deletions
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src/mir/optimizations.md
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@ -47,3 +47,16 @@ You can see the ["Implementors" section of the `MirPass` rustdocs][impl] for mor
[impl]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/trait.MirPass.html#implementors [impl]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/transform/trait.MirPass.html#implementors
[constprop]: https://en.wikipedia.org/wiki/Constant_folding#Constant_propagation [constprop]: https://en.wikipedia.org/wiki/Constant_folding#Constant_propagation
## MIR optimization levels
MIR optimizations can come in various levels of readiness. Experimental
optimizations may cause miscompilations, or slow down compile times.
These passes are still included in nightly builds to gather feedback and make it easier to modify
the pass. To enable working with slow or otherwise experimental optimization passes,
you can specify the `-Z mir-opt-level` debug flag. You can find the
definitions of the levels in the [compiler MCP]. If you are developing a MIR pass and
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