Optimizations: the speed size tradeoff
Everyone wants their program to be super fast and super small but it's usually
not possible to have both characteristics. This section discusses the
different optimization levels that
rustc provides and how they affect the
execution time and binary size of a program.
This is the default. When you call
cargo build you use the development (AKA
dev) profile. This profile is optimized for debugging so it enables debug
information and does not enable any optimizations, i.e. it uses
-C opt-level = 0.
At least for bare metal development, debuginfo is zero cost in the sense that it won't occupy space in Flash / ROM so we actually recommend that you enable debuginfo in the release profile -- it is disabled by default. That will let you use breakpoints when debugging release builds.
[profile.release] # symbols are nice and they don't increase the size on Flash debug = true
No optimizations is great for debugging because stepping through the code feels
like you are executing the program statement by statement, plus you can
$0 = <value optimized out> being printed.
The biggest downside of the
dev profile is that the resulting binary will be
huge and slow. The size is usually more of a problem because unoptimized
binaries can occupy dozens of KiB of Flash, which your target device may not
have -- the result: your unoptimized binary doesn't fit in your device!
Can we have smaller, debugger friendly binaries? Yes, there's a trick.
There's a Cargo feature named
profile-overrides that lets you
override the optimization level of dependencies. You can use that feature to
optimize all dependencies for size while keeping the top crate unoptimized and
Here's an example:
# Cargo.toml [package] name = "app" # .. [profile.dev.package."*"] # + opt-level = "z" # +
Without the override:
$ cargo size --bin app -- -A app : section size addr .vector_table 1024 0x8000000 .text 9060 0x8000400 .rodata 1708 0x8002780 .data 0 0x20000000 .bss 4 0x20000000
With the override:
$ cargo size --bin app -- -A app : section size addr .vector_table 1024 0x8000000 .text 3490 0x8000400 .rodata 1100 0x80011c0 .data 0 0x20000000 .bss 4 0x20000000
That's a 6 KiB reduction in Flash usage without any loss in the debuggability of
the top crate. If you step into a dependency then you'll start seeing those
<value optimized out> messages again but it's usually the case that you want
to debug the top crate and not the dependencies. And if you do need to debug a
dependency then you can use the
profile-overrides feature to exclude a
particular dependency from being optimized. See example below:
# .. # don't optimize the `cortex-m-rt` crate [profile.dev.package.cortex-m-rt] # + opt-level = 0 # + # but do optimize all the other dependencies [profile.dev.package."*"] codegen-units = 1 # better optimizations opt-level = "z"
Now the top crate and
cortex-m-rt are debugger friendly!
Optimize for speed
As of 2018-09-18
rustc supports three "optimize for speed" levels:
opt-level = 1,
3. When you run
cargo build --release you are using the release
profile which defaults to
opt-level = 3.
opt-level = 2 and
3 optimize for speed at the expense of binary size,
3 does more vectorization and inlining than level
particular, you'll see that at
opt-level equal to or greater than
2 LLVM will
unroll loops. Loop unrolling has a rather high cost in terms of Flash / ROM
(e.g. from 26 bytes to 194 for a zero this array loop) but can also halve the
execution time given the right conditions (e.g. number of iterations is big
Currently there's no way to disable loop unrolling in
opt-level = 2 and
if you can't afford its cost you should optimize your program for size.
Optimize for size
As of 2018-09-18
rustc supports two "optimize for size" levels:
opt-level = "s" and
"z". These names were inherited from clang / LLVM and are not too
"z" is meant to give the idea that it produces smaller
If you want your release binaries to be optimized for size then change the
profile.release.opt-level setting in
Cargo.toml as shown below.
[profile.release] # or "z" opt-level = "s"
These two optimization levels greatly reduce LLVM's inline threshold, a metric
used to decide whether to inline a function or not. One of Rust principles are
zero cost abstractions; these abstractions tend to use a lot of newtypes and
small functions to hold invariants (e.g. functions that borrow an inner value
as_ref) so a low inline threshold can make LLVM miss
optimization opportunities (e.g. eliminate dead branches, inline calls to
When optimizing for size you may want to try increasing the inline threshold to
see if that has any effect on the binary size. The recommended way to change the
inline threshold is to append the
-C inline-threshold flag to the other
# .cargo/config.toml # this assumes that you are using the cortex-m-quickstart template [target.'cfg(all(target_arch = "arm", target_os = "none"))'] rustflags = [ # .. "-C", "inline-threshold=123", # + ]
What value to use? As of 1.29.0 these are the inline thresholds that the different optimization levels use:
opt-level = 3uses 275
opt-level = 2uses 225
opt-level = "s"uses 75
opt-level = "z"uses 25
You should try
275 when optimizing for size.