Optimize for size

[engstad]

It seems that the compiler doesn't understand -Os (optimize for size):

$ rustc -C opt-level=s -Z no-landing-pads --target thumbv7em-none-eabi -g --emit obj -L libcore-thumbv7em -o main.o ./blink/src/main.rs
error: incorrect value `s` for codegen option `opt-level` - a number was expected

[steveklabnik]

As far as I know, yes, this isn't a thing.

[engstad]

I believe both GCC and Clang support the -Os option, and this is normally an embedded application's default. Support for this in Rust would follow that tradition.

[nodakai]

Give this a try:

rustc -C opt-level=3 -C lto -C link-args=-s foo.rs

I guess static linking of libstd is "to blame" for the output size rather than the optimization level.

By the way, Rust FAQ must have an item on the issue of "huge" binary size.

[engstad]

Embedded systems don't usually include libstd, only libcore. However, for embedded systems (and IoT), size is often more important than speed.

[MagaTailor]

@engstad, the idea of -Os can be safely decoupled from anything embedded or binary size in general - it used to be a rule that hot C++ code compiled for size ran faster (better fit in cache) than -O3.

So yeah, that would be a welcome addition with benchmarking in mind.

[nodakai]

@engstad Are you working on an embedded system already without libstd, and the output binary size is still that important? Well, I tend to agree that smaller binaries are faster to load...

FYI, when I compiled GNU Make with Clang 4.8 x86_64 -Ofast and -Os, the stripped binary sizes were 192 KB vs 175 KB, 10 % difference.

[engstad]

@nodakai Yes, indeed. My two chips have 4 KiB (http://www.nxp.com/products/microcontrollers-and-processors/arm-processors/lpc-cortex-m-mcus/lpc-cortex-m0-plus-m0/lpc800-series/low-cost-32-bit-microcontroller-mcu-based-on-arm-cortex-m0-plus-core:LPC810M021FN8) and 32 KiB total ROM (http://www.nxp.com/products/microcontrollers-and-processors/arm-processors/lpc-cortex-m-mcus/lpc-cortex-m0-plus-m0/lpc1100-series/32kb-flash-4kb-sram-dip28-package:LPC1114FN28), so memory size is quite important.