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28x binary size regression in recent nightly #85519
Description
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-eabitarget - Run
cargo build size target/thumbv6m-none-eabi/debug/hello
Commits in that range: 4e3e6db...f94942d
Activity
jamesmunns commentedon May 20, 2021
This seems to have been noticed here as well: rust-osdev/bootloader#168
Edit: Fixed link to
rust-osdevrepo instead of this issueEdit 2: This also indicates you can work-around the regression by manually passing
--gc-sectionsas a linker flag.jamesmunns commentedon May 20, 2021
Also as a note, @japaric tested this with using
arm-none-eabi-ldas 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 commentedon May 20, 2021
it's very likely #85274, since adding
-C link-arg=--gc-sectionsto rustflags fixes the bloat.nagisa commentedon May 20, 2021
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 commentedon May 20, 2021
We do default to LLD, which can also garbage-collect unused sections
nagisa commentedon May 20, 2021
Yeah, I was just wondering if we should expand our logic around when we specify the
--gc-sectionsflag or just say that the targets use a gnu-ld-like linker.jonas-schievink commentedon May 20, 2021
Yeah, in practice I don't think anyone uses a non-GNU-compatible linker there
petrochenkov commentedon May 20, 2021
It may make sense to set the default for
linker_is_gnuto true becauseld-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 commentedon May 20, 2021
If the default is not flipped then
compiler\rustc_target\src\spec\thumb_base.rsneeds to addlinker_is_gnu: true+ other target specs should probably be audited as well.Auto merge of rust-lang#85531 - luqmana:flip-gc, r=petrochenkov