Update doc to add a section for LTO (#937)

This PR adds a section in MMTk users guide to describe LTO.

Author: qinsoon

docs/userguide/src/portingguide/SUMMARY.md

@@ -7,3 +7,5 @@
 - [How to Undertake a Port](./howto/prefix.md)
     - [NoGC](./howto/nogc.md)
     - [Next Steps](./howto/next_steps.md)
+- [Performance Tuning]()
+    - [Link Time Optimization](./perf_tuning/lto.md)

docs/userguide/src/portingguide/perf_tuning/lto.md

@@ -0,0 +1,38 @@
+# Enabling Link Time Optimization (LTO) with MMTk
+
+MMTk's API is designed with an assumption that LTO will be enabled for a performant build.
+It is essential to allow the Rust compiler to optimize across the crate boundary between the binding crate and mmtk-core.
+LTO allows inlining for both directions (from mmtk-core to the binding, and from the binding to mmtk-core),
+and allows further optimization such as specializing and constant folding for the `VMBinding` trait.
+
+We suggest enabling LTO for the release build in the binding's manifest (`Cargo.toml`) by adding a profile for the release build,
+so LTO is always enabled for a release build.
+
+```toml
+[profile.release]
+lto = true
+```
+
+If your binding project is a Rust binary (e.g. the VM is written in Rust), this should be enough. However, if your binding project
+is a library, there are some limitations with cargo that you should be aware of.
+
+
+## Binding as a library
+
+Cargo only allows LTO for certain crate types. You will need to specify the crate type properly, otherwise cargo may skip LTO without
+any warning or error.
+
+```toml
+[lib]
+...
+# be careful - LTO is only allowed for certain crate types
+crate-type = ["cdylib"]
+```
+
+At the time of writing, cargo has some limitations about LTO with different crate types:
+1. LTO is only allowed with `cdylib` and `staticlib` (other than `bin`).
+Check the code of [`can_lto`](https://github.com/rust-lang/cargo/blob/5f40a97e5c85affecfbc4fde67fc06bf188c07db/src/cargo/core/compiler/crate_type.rs#L33)
+for your Rust version to clarify.
+2. If the `crate-type` field includes any type that LTO is not allowed, LTO will be skipped for all the libraries generated (https://github.com/rust-lang/rust/issues/51009).
+For example, if you have `crate-type = ["cdylib", "rlib"]` and cargo cannot do LTO for `rlib`, LTO will be skipped for `cdylib` as well.
+So only keep the crate type that you actually need in the `crate-type` field.