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Add doc on performance and min builds.

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Stephen Chung 2020-04-19 13:22:37 +08:00
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README.md
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@ -87,6 +87,39 @@ Excluding unneeded functionalities can result in smaller, faster builds as well
[`no_std`]: #optional-features [`no_std`]: #optional-features
[`sync`]: #optional-features [`sync`]: #optional-features
### Performance builds
Some features are for performance. For example, using `only_i32` or `only_i64` disables all other integer types (such as `u16`).
If only a single integer type is needed in scripts - most of the time this is the case - it is best to avoid registering
lots of functions related to other integer types that will never be used. As a result, performance will improve.
If only 32-bit integers are needed - again, most of the time this is the case - using `only_i32` disables also `i64`.
On 64-bit targets this may not gain much, but on some 32-bit targets this improves performance due to 64-bit arithmetic
requiring more CPU cycles to complete.
Also, turning on `no_float`, and `only_i32` makes the key [`Dynamic`] data type only 8 bytes small on 32-bit targets
while normally it can be up to 16 bytes (e.g. on x86/x64 CPU's) in order to hold an `i64` or `f64`.
Making [`Dynamic`] small helps performance due to more caching efficiency.
### Minimal builds
In order to compile a _minimal_build - i.e. a build optimized for size - perhaps for embedded targets, it is essential that
the correct linker flags are used in `cargo.toml`:
```toml
[profile.release]
opt-level = "z" # optimize for size
```
Opt out of as many features as possible, if they are not needed, to reduce code size because, remember, by default
all code is compiled in as what a script requires cannot be predicted. If a language feature is not needed,
omitting them via special features is a prudent strategy to optimize the build for size.
Start by using [`Engine::new_raw`](#raw-engine) to create a _raw_ engine which does not register the standard library of utility
functions. Secondly, omitting arrays (`no_index`) yields the most code-size savings, followed by floating-point support
(`no_float`), checked arithmetic (`unchecked`) and finally object maps and custom types (`no_object`). Disable script-defined
functions (`no_function`) only when the feature is not needed because code size savings is minimal.
Related Related
------- -------
@ -380,7 +413,7 @@ The default integer type is `i64`. If other integer types are not needed, it is
smaller build with the [`only_i64`] feature. smaller build with the [`only_i64`] feature.
If only 32-bit integers are needed, enabling the [`only_i32`] feature will remove support for all integer types other than `i32`, including `i64`. If only 32-bit integers are needed, enabling the [`only_i32`] feature will remove support for all integer types other than `i32`, including `i64`.
This is useful on some 32-bit systems where using 64-bit integers incurs a performance penalty. This is useful on some 32-bit targets where using 64-bit integers incur a performance penalty.
If no floating-point is needed or supported, use the [`no_float`] feature to remove it. If no floating-point is needed or supported, use the [`no_float`] feature to remove it.