stdlib code size increase in nightly-2024-09-01 for aarch64-linux-android

[jrose-signal]

We keep track of our code size on Android to keep our overall download size down. In updating our pinned nightly compiler (from nightly-2024-06-21; we're due for it), our CI turned up a code size increase of ~120KB in a library that's overall about 4.6MB—not huge, but not expected either. Bisecting on Rust toolchains turned up that the regression was introduced in nightly-2024-09-01 (as in, nightly-2024-08-30 does not have this extra code size, and there was no nightly-2024-08-31), and moreover I discovered that using -Zbuild-std recovered the additional code size. So something changed in how the prebuilt standard library is built, in a way that may not have been intended.

Unfortunately, our Android build process does a whole bunch of work at once, so I don't have a minimal example for you. My reproduction has been

1. Install the Android NDK (27.0.12077973 is the one we're currently using)
2. Check out /~https://github.com/signalapp/libsignal/releases/tag/v0.57.1
3. ANDROID_NDK_HOME=path/to/ndk/27.0.12077973 java/build_jni.sh android-arm64
4. Save the resulting library, then repeat with RUSTUP_TOOLCHAIN=nightly-2024-09-01.
5. Compare the libraries' post-stripped sizes, or use a tool like bloaty to measure VM size only

It's possible this change was expected, in which case please close the issue; I understand code sizes go up sometimes!

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[RalfJung]

Thanks for the report!

Any chance you could try to bisect this further using cargo-bisect-rustc? Since this is recent enough, that should be able to narrow it down to a single PR.

[jrose-signal]

It took a bit to get it right, and I'm still not sure I got it entirely correctly, but here's the tail end of the output!

% cargo bisect-rustc --start=2024-08-30 --end=2024-09-01 --target aarch64-linux-android --by-commit --script ./test.sh
...
searched toolchains 0d634185dfddefe09047881175f35c65d68dcff1 through a7399ba69d37b019677a9c47fe89ceb8dd82db2d

********************************************************************************
Regression in 9649706eada1b2c68cf6504356efb058f68ad739
********************************************************************************

Attempting to search unrolled perf builds
Found commits ["c8a538a7", "c5f6a66b", "52b51b8a", "970b7b00", "32892549", "8c930915", "16f4069d", "cb679928", "fc177c6c", "d7b7d7de", "d101b3fa", "1b1ad99a", "702679cd", "2ab629b2", "19653419"]
installing c8a538a7032ba3d35c90642bc6c47d985282af8d
uninstalling c8a538a7032ba3d35c90642bc6c47d985282af8d
installing c5f6a66bd472f75e77a7d9ed1ee83454b63d5057
uninstalling c5f6a66bd472f75e77a7d9ed1ee83454b63d5057
installing 52b51b8a3e8716b491fdf82c075c303f79cce65e
uninstalling 52b51b8a3e8716b491fdf82c075c303f79cce65e
installing 970b7b00bc353065c23d1e2d8e22c0a2b8260da7
uninstalling 970b7b00bc353065c23d1e2d8e22c0a2b8260da7
installing 32892549aa22df7399226f3281b94b32cd2ec7c3
uninstalling 32892549aa22df7399226f3281b94b32cd2ec7c3
installing 8c930915fe4ca02780c9ff2fec9612723ca9c203
uninstalling 8c930915fe4ca02780c9ff2fec9612723ca9c203
installing 16f4069d874ebc66e07259afddae546ccb536ca1
uninstalling 16f4069d874ebc66e07259afddae546ccb536ca1
installing cb6799285d407dab26843816f9b98d515eecadc9
uninstalling cb6799285d407dab26843816f9b98d515eecadc9
installing fc177c6c435e97b84e3d7ec9ed19870c4dfbfd3b
uninstalling fc177c6c435e97b84e3d7ec9ed19870c4dfbfd3b
installing d7b7d7deaeb75804926dc0d2529f064a1ef0d1fb
uninstalling d7b7d7deaeb75804926dc0d2529f064a1ef0d1fb
installing d101b3fa61198f4a193e77477e77d95dd9889fec
uninstalling d101b3fa61198f4a193e77477e77d95dd9889fec
installing 1b1ad99aff96a560ae4405daf63996c023f43801
uninstalling 1b1ad99aff96a560ae4405daf63996c023f43801
installing 702679cd9f0236ce8997c93aa01ee8c57d7a30e6
uninstalling 702679cd9f0236ce8997c93aa01ee8c57d7a30e6
installing 2ab629b2fff152427b0f45f5ade6c44cefe50d50
uninstalling 2ab629b2fff152427b0f45f5ade6c44cefe50d50
installing 196534199a7aba5e0b5195084a5bf1ca72cca969
uninstalling 196534199a7aba5e0b5195084a5bf1ca72cca969
ERROR: none of the toolchains satisfied the predicate

Of the commits in 9649706, #129640 is my personal bet…

My detection script could also be off, I used test "$(du "$lib"| cut -f1)" -lt 9300 as the "good" case (the full jump was 9168 du-blocks to 9408).

[workingjubilee]

@jrose-signal Does this affect the size of any completed binaries you use?

[jrose-signal]

Yes, the .so being measured here is the aarch64 slice of libsignal, as shipped in our Android app.

[workingjubilee]

Oh, okay!

@jrose-signal Actually, I think the commit at fault is likely to be #129642 because now Android is getting the backtrace code linked in All The Time.

It's either that, the PR you noticed, or #129366 which changes linkage.

[jrose-signal]

Ah, backtraces. >< That might explain why -Zbuild-std helps so much, last time I measured the most expensive part of backtrace support is resolving symbol names, which is useless for us because we strip debug info for our release builds.

[workingjubilee]

Yeah, we really ought to come up with a better way to support decent backtraces than stuffing a DWARF parser in every single binary.

[workingjubilee]

That or at least make the DWARF parser a lot smaller. Really, I thought they were supposed to be shorter than both Men and Elves, but I've seen an ELF or two in my time that were massively outweighed by their DWARF...