add oli note

2 files changed, 6 insertions, 1 deletions

diff --git a/src/doc/rustc-dev-guide/src/appendix/glossary.md b/src/doc/rustc-dev-guide/src/appendix/glossary.md
index 5b0f7cfa2aa..d737e037f58 100644
--- a/src/doc/rustc-dev-guide/src/appendix/glossary.md
+++ b/src/doc/rustc-dev-guide/src/appendix/glossary.md
@@ -45,6 +45,7 @@ LTO                     |  Link-Time Optimizations. A set of optimizations offer
 memoize                 |  memoization is the process of storing the results of (pure) computations (such as pure function calls) to avoid having to repeat them in the future. This is typically a trade-off between execution speed and memory usage.
 MIR                     |  the Mid-level IR that is created after type-checking for use by borrowck and codegen ([see more](../mir/index.html))
 miri                    |  an interpreter for MIR used for constant evaluation ([see more](../miri.html))
+monomorphize            |  Monomorphization is the process of taking generic implementations of types and functions and producing instantiating them with concrete types. For example, in the code we might have `Vec<T>`, but in the final executable, we will have a copy of the `Vec` code for every concrete type used in the program (e.g. a copy for `Vec<usize>`, a copy for `Vec<MyStruct>`, etc).
 normalize               |  a general term for converting to a more canonical form, but in the case of rustc typically refers to [associated type normalization](../traits/associated-types.html#normalize)
 newtype                 |  a "newtype" is a wrapper around some other type (e.g., `struct Foo(T)` is a "newtype" for `T`). This is commonly used in Rust to give a stronger type for indices.
 NLL                     | [non-lexical lifetimes](../borrow_check/region_inference.html), an extension to Rust's borrowing system to make it be based on the control-flow graph.
diff --git a/src/doc/rustc-dev-guide/src/mir/optimizations.md b/src/doc/rustc-dev-guide/src/mir/optimizations.md
index fb550af4cd7..cb60af1af86 100644
--- a/src/doc/rustc-dev-guide/src/mir/optimizations.md
+++ b/src/doc/rustc-dev-guide/src/mir/optimizations.md
@@ -3,9 +3,13 @@
 MIR optimizations are optimizations run on the [MIR][mir] to produce better MIR
 before codegen. This is important for two reasons: first, it make the final
 generated executable code better, and second, it means that LLVM has less work
-to do, so compilation is faster.
+to do, so compilation is faster. Note that since MIR is generic (not
+[monomorphized][monomorph] yet), these optimizations are particularly
+effective; we can optimize the generic version, so all of the monomorphizations
+are cheaper!
 
 [mir]: https://rust-lang.github.io/rustc-guide/mir/index.html
+[monomorph]: https://rust-lang.github.io/rustc-guide/appendix/glossary.html?highlight=monomorphize#appendix-c-glossary
 
 MIR optimizations run after borrow checking. We run a series of optimization
 passes over the MIR to improve it. Some passes are required to run on all code,