Enable Bitcode on libmobilecoin, reduces downloadable size by ~25%

[the-real-adammork]

Soundtrack of this PR: SW2 & Friends - For the People

Motivation

Add support for Apple's Bitcode and reduce the size of our SDK & its dependencies. With this enabled the compressed "downloadable" size shrinks by ~25%.

ENABLE_BITCODE = YES

App Thinning Size Report for All Variants of Example
App + On Demand Resources size: 7.2 MB compressed, 19.7 MB uncompressed
App size: 7.2 MB compressed, 19.7 MB uncompressed

ENABLE_BITCODE = NO

App Thinning Size Report for All Variants of Example
App + On Demand Resources size: 9.6 MB compressed, 35.6 MB uncompressed
App size: 9.6 MB compressed, 35.6 MB uncompressed

In this PR

rust-bitcode

Add a submodule rust-bitcode to compile rustup toolchains that include LLVM compatible with Apple's Xcode build system.

No more symbol stripping

Remove "symbol" stripping step from the Makefile because it's not compatible with enabling Bitcode. While this step had large impact on the size of the archive .a (10mb vs 55mb) it has a small impact on the "downloadable" size (~0.2mb). This is because un-linked symbols are not included in final binary regardless of whether or not they get stripped.

Remove build-target x86_64-apple-darwin

To my knowledge MobileCoin and Signal are not using this target, and it does not support Bitcode.

Future Work

Technically there may be a way to strip and keep the Bitcode but the size savings would be small ~0.2mb compressed. This can be left as a future task if needed.

Linked PRs

libmobilecoin-ios-artifacts - mobilecoinofficial/libmobilecoin#4
MobileCoin-Swift - mobilecoinofficial/MobileCoin-Swift#80

[cbeck88]

that is pretty cool!

so our CI is now testing this thing with the toolchain being installed from the makefile?

[the-real-adammork]

that is pretty cool!

so our CI is now testing this thing with the toolchain being installed from the makefile?

Not sure about the CI, Ill look into that.

[the-real-adammork]

that is pretty cool!

so our CI is now testing this thing with the toolchain being installed from the makefile?

Update on CI

The CI workflow uses this cargo command: cargo build --frozen --target "$HOST_TARGET_TRIPLE" -p libmobilecoin which won't hit the Makefile or my new changes.

I have a WIP branch, using the Makefile, but it takes ~2hrs to complete (rust + llvm = slow). It can be refined to be faster but would take me some time. Im gonna make it as a future work ticket.

Legacy Build process

I added a recipe make legacy that uses the previous rustup toolchain setup & build flow.

[cbeck88]

that sounds good -- i'm happy with either approach to CI, whatever you are comfortable with. i would hate to break your build by accident but if it's not practical to test it in CI that's too bad. ideally we could put the custom toolchain in a docker container for CI to just fetch and run or something, but that might not work on an apple target?

[the-real-adammork]

that sounds good -- i'm happy with either approach to CI, whatever you are comfortable with. i would hate to break your build by accident but if it's not practical to test it in CI that's too bad. ideally we could put the custom toolchain in a docker container for CI to just fetch and run or something, but that might not work on an apple target?

ya this is the type of refinement I was looking at. We're already doing cache-ing in a few places I could implement that for the rust-toolchains. When we update the rust nightly, we update the caches

[the-real-adammork]

@jcape changed to using a shallow clone of rust-bitcode, and added the stripping code back into the legacy recipe.