3 ms·

I assume you're already doing this since you're on a microcontroller, but have you set [profile.release] codegen-units = 1 lto = true

samstave 3y ago
ELI5 what this does?
- hyperbrainer 3y ago
  The lto setting controls the -C lto flag which controls LLVM’s link time optimizations. LTO can produce better optimized code, using whole-program analysis, at the cost of longer linking time. The valid options are: false: Performs “thin local LTO” which performs “thin” LTO on the local crate only across its codegen units. No LTO is performed if codegen units is 1 or opt-level is 0. true or "fat": Performs “fat” LTO which attempts to perform optimizations across all crates within the dependency graph. "thin": Performs “thin” LTO. This is similar to “fat”, but takes substantially less time to run while still achieving performance gains similar to “fat”. "off": Disables LTO. Basically, increases performance at cost of compile time. Great for release builds, not so much for debug builds (because most of the time that rustc spends is on linking anyways.)
  - MallocVoidstar 3y ago
    Also, codegen-units = 1 disables some compiler parallelization which means it can find more optimizations, including for size: https://nnethercote.github.io/perf-book/build-configuration.html#codegen-units https://nnethercote.github.io/perf-book/build-configuration....
bombela 3y ago
This is what I found to reduce code size the most for the 8bit AVR MCU: [profile.release] opt-level = "z" lto = true overflow-checks = false codegen-units = 1 debug = false Also invoking cargo with: cargo build --release -Z build-std=core -Z build-std-features=panic_immediate_abort You must then implement panic handling yourself: pub mod std { #[lang = "eh_personality"] #[no_mangle] pub unsafe extern "C" fn rust_eh_personality() -> () {} // panic can be replaced by abort with the panic_immediate_abort feature. // -Z build-std-features=panic_immediate_abort // This saves space by getting rid of the unwinding. #[panic_handler] pub unsafe fn panic_impl(_info: &::core::panic::PanicInfo) -> ! { loop {} } #[no_mangle] pub unsafe extern "C" fn abort() { // blink led loop {} } } If I still have some room in the flash, I will make some led blink with a noticable pattern on abort. You can also experiment with the inlining threshold: export RUSTFLAGS+=-C inline-threshold=0
davidhyde 3y ago
Yes I’ve tried these things and more (like setting the opt-level to “z”). I don’t think that this is a show stopper, probably just a recent regression that will be fixed easily since this is all open source and the GitHub issue tracking is very active I see. If I were to hazard a guess it’s probably some small code gen change that results in mostly duplicated code for some ui primitive. But, it could also be me doing something stupid ;)

samstave 3y ago

ELI5 what this does?

hyperbrainer 3y ago
The lto setting controls the -C lto flag which controls LLVM’s link time optimizations. LTO can produce better optimized code, using whole-program analysis, at the cost of longer linking time. The valid options are: false: Performs “thin local LTO” which performs “thin” LTO on the local crate only across its codegen units. No LTO is performed if codegen units is 1 or opt-level is 0. true or "fat": Performs “fat” LTO which attempts to perform optimizations across all crates within the dependency graph. "thin": Performs “thin” LTO. This is similar to “fat”, but takes substantially less time to run while still achieving performance gains similar to “fat”. "off": Disables LTO. Basically, increases performance at cost of compile time. Great for release builds, not so much for debug builds (because most of the time that rustc spends is on linking anyways.)
- MallocVoidstar 3y ago
  Also, codegen-units = 1 disables some compiler parallelization which means it can find more optimizations, including for size: https://nnethercote.github.io/perf-book/build-configuration.html#codegen-units https://nnethercote.github.io/perf-book/build-configuration....

bombela 3y ago

This is what I found to reduce code size the most for the 8bit AVR MCU: [profile.release] opt-level = "z" lto = true overflow-checks = false codegen-units = 1 debug = false Also invoking cargo with: cargo build --release -Z build-std=core -Z build-std-features=panic_immediate_abort You must then implement panic handling yourself: pub mod std { #[lang = "eh_personality"] #[no_mangle] pub unsafe extern "C" fn rust_eh_personality() -> () {} // panic can be replaced by abort with the panic_immediate_abort feature. // -Z build-std-features=panic_immediate_abort // This saves space by getting rid of the unwinding. #[panic_handler] pub unsafe fn panic_impl(_info: &::core::panic::PanicInfo) -> ! { loop {} } #[no_mangle] pub unsafe extern "C" fn abort() { // blink led loop {} } } If I still have some room in the flash, I will make some led blink with a noticable pattern on abort. You can also experiment with the inlining threshold: export RUSTFLAGS+=-C inline-threshold=0

davidhyde 3y ago

Yes I’ve tried these things and more (like setting the opt-level to “z”). I don’t think that this is a show stopper, probably just a recent regression that will be fixed easily since this is all open source and the GitHub issue tracking is very active I see. If I were to hazard a guess it’s probably some small code gen change that results in mostly duplicated code for some ui primitive. But, it could also be me doing something stupid ;)