28x binary size regression in recent nightly

[jonas-schievink]

There seems to be a significant regression in binary size in the most recent nightly:

nightly-2021-05-19

$ size target/thumbv6m-none-eabi/debug/hello
   text	   data	    bss	    dec	    hex	filename
   6056	     48	   1032	   7136	   1be0	target/thumbv6m-none-eabi/debug/hello

nightly-2021-05-20

$ size target/thumbv6m-none-eabi/debug/hello
   text	   data	    bss	    dec	    hex	filename
 201372	     48	   1032	 202452	  316d4	target/thumbv6m-none-eabi/debug/hello

Steps to reproduce:

• Clone https://github.com/jonas-schievink/size-regression
• Install thumbv6m-none-eabi target
• Run cargo build
• size target/thumbv6m-none-eabi/debug/hello

Commits in that range: 4e3e6db...f94942d

#85274 looks suspicious (cc @luqmana)

[jamesmunns]

This seems to have been noticed here as well: rust-osdev/bootloader#168

Edit: Fixed link to rust-osdev repo instead of this issue

Edit 2: This also indicates you can work-around the regression by manually passing --gc-sections as a linker flag.

[jamesmunns]

Also as a note, @japaric tested this with using arm-none-eabi-ld as the linker instead of the default rust-lld, and this issue remained (referring to #85274). He can confirm whether he set the "linker flavor" or similar as well.

[Dirbaio]

it's very likely #85274, since adding -C link-arg=--gc-sections to rustflags fixes the bloat.

[nagisa]

Is the linker for this target guaranteed to be gnu ld? Doesn't sound like something that we can claim, can we? It does look like we default to lld.

[jonas-schievink]

We do default to LLD, which can also garbage-collect unused sections

[nagisa]

Yeah, I was just wondering if we should expand our logic around when we specify the --gc-sections flag or just say that the targets use a gnu-ld-like linker.

[jonas-schievink]

Yeah, in practice I don't think anyone uses a non-GNU-compatible linker there

[petrochenkov]

It may make sense to set the default for linker_is_gnu to true because ld-style linkers are usually gnu, and it's easier to explicitly list exceptions than to not forget setting it to true for all random embedded targets.

[petrochenkov]

If the default is not flipped then compiler\rustc_target\src\spec\thumb_base.rs needs to add linker_is_gnu: true + other target specs should probably be audited as well.