Kernel Size

[tyler-potyondy]

I'm opening this issue to track some of the explorations into decreasing the ents kernel's binary size.

Starting Kernel Size 82Kb

Starting Point

Size as of current upstream (f4f86b4)

  Finished `release` profile [optimized + debuginfo] target(s) in 0.04s
   text	   data	    bss	    dec	    hex	filename
  81964	      0	  17620	  99584	  18500	tock/target/thumbv7em-none-eabi/release/lora_e5_mini

Removing some debug/io module

Removing the functionality to print output from panics, specifically:

• remove import of io mod
• replace panic handler with:

/// todo
#[panic_handler]
pub unsafe fn panic_fmt(info: &core::panic::PanicInfo) -> ! {
    loop {}
}

This results in 15kb of code size savings!

  Finished `release` profile [optimized + debuginfo] target(s) in 0.04s
   text	   data	    bss	    dec	    hex	filename
  65580	      0	  17620	  83200	  14500	tock/target/thumbv7em-none-eabi/release/lora_e5_mini

Removing the console / process printer:

This gets rid of another ~20Kb!

  Finished `release` profile [optimized + debuginfo] target(s) in 0.04s
   text	   data	    bss	    dec	    hex	filename
  47148	      0	  15520	  62668	   f4cc	tock/target/thumbv7em-none-eabi/release/lora_e5_mini

Update debug!() macro to replace with noop - ()

2Kb decrease

  Finished `release` profile [optimized + debuginfo] target(s) in 1.15s
   text	   data	    bss	    dec	    hex	filename
  45100	      0	  13320	  58420	   e434	tock/target/thumbv7em-none-eabi/release/lora_e5_mini

Lowest Possible Kernel Size: 45K

Decreasing the stack size

Going down from 0x2000 to 0x1000 for the kernel stack size further shaves down the bss by 4kb. This helps us for the RAM limits on our board. We can likely only shrink this by this much if we aren't doing debugs statements.

    Finished `release` profile [optimized + debuginfo] target(s) in 1.16s
   text	   data	    bss	    dec	    hex	filename
  45100	      0	   9224	  54324	   d434	tock/target/thumbv7em-none-eabi/release/lora_e5_mini