Session ID: HKG18-215 Session Name: HKG18-215 - AOSP Toolchains: Where we are with building kernels, bootloaders and AOSP with Clang Speaker: Bernhard ""Bero"" Rosenkränzer,Minseong Kim Track: Mobile ★ Session Summary ★ Linaro has been working on making AOSP build with Clang for a long time - and our upstreaming of those patches has contributed to the fact that AOSP userland is officially built with clang these days. It is working out so well that AOSP wants to go a step further and use the clang based toolchain for everything including kernels and bootloaders - something we're beginning to tackle. This presentation gives an update on where we are, where we're facing problems, and our continued efforts to keep AOSP userland building with newer versions of clang than what upstream is using. --------------------------------------------------- ★ Resources ★ Event Page: http://connect.linaro.org/resource/hkg18/hkg18-215/ Presentation: http://connect.linaro.org.s3.amazonaws.com/hkg18/presentations/hkg18-215.pdf Video: http://connect.linaro.org.s3.amazonaws.com/hkg18/videos/hkg18-215.mp4 --------------------------------------------------- ★ Event Details ★ Linaro Connect Hong Kong 2018 (HKG18) 19-23 March 2018 Regal Airport Hotel Hong Kong --------------------------------------------------- Keyword: Mobile 'http://www.linaro.org' 'http://connect.linaro.org' --------------------------------------------------- Follow us on Social Media https://www.facebook.com/LinaroOrg https://www.youtube.com/user/linaroorg?sub_confirmation=1 https://www.linkedin.com/company/1026961

1. HKG18-215: Building AOSP master with
Clang master
MinSeong Kim

2. Building AOSP master with Clang master
● Prebuilt clang in AOSP lag behind in time 3 to 6 months
○ Purpose
■ Check clang master’s performance and size impact on AOSP
■ Help clang master to soft-land on AOSP when the time comes
● CI 1: Clang master build
○ https://ci.linaro.org/job/android-clang-toolchain/
○ Build command: toolchain/llvm_android/build.py
● CI 2: AOSP master build using clang from CI 1
○ https://ci.linaro.org/job/android-master-clang/
○ Minor patches to AOSP: tuning compiler flags to bypass newly added errors/warnings
○ If bugs are found, fix/report them
● CI jobs often fail as two moving targets → requires constant maintenance
● Upon success, boot-up test on hikey using LAVA

3. Performance and Size comparisons
● Size
○ Not much: Roughly 0.1% difference in size (even only ELF files)
● Performance
○ Sometimes, no visible performance increase compared to prebuilt clang
○ Some other times,
■ Quadrant Pro: 12649 → 12990 (2.7% up)
■ Antutu 6.0: 31605 → 32661 (3.3% up)
■ Geekbench 3: 2597 → 2597 (0%)
■ Andebench Pro 2015: 3344 → 3831 (13% up)
○ Fortunately, haven’t seen cases with performance and size regression so far

4. Clang master with new techniques for AOSP
● LLD (LLVM Linker: a drop-in replacement for GNU linkers)
○ Current default linker for aarch64 AOSP: gold
○ Tried to replace gold to LLD for linking AOSP
○ Found a few issues (3 missing options and some compatibility issues)
○ system.img size increases by roughly 7 or 8%
● Souper (Peep-hole optimizer on LLVM IR)
○ Performs missed simplifications on the final LLVM IR.
○ Applied to bionic and benchmarked - no visible performance increase
● Polly (Loop and data-locality optimizer)
○ 3 major options to turn on its optimizations
■ Scalar, OpenMP, and Vectorizer
○ No visible performance increase (e.g Antutu 6.3)

5. Building kernels, bootloaders and TEEs
with Clang
Bernhard “Bero” Rosenkränzer

6. Building kernels with clang
● Mainline kernels are looking good… Except for some rarely used modules
(exofs, gcc code analysis plugins), kernels can build with clang 6.0 on aarch64
and x86_64.
armv7 is a little behind due to more inline assembly that clang’s internal
assembler doesn’t like, but isn’t far from working.
● Relevant patches have been backported to most android-common trees that
are still relevant
● Right now, building the kernel with gcc is getting a lot more testing; for
production builds, it is probably still a good idea to use gcc. Latest version
from TCWG is recommended over the outdated 4.x based AOSP gcc
toolchain.

7. Building bootloaders with clang
● U-boot still contains many gcc hardcodes, from compiler naming conventions
($(CROSS_COMPILE)-gcc) to compiler flags (-ffixed-r9) to inline assembly
(incorrect arguments to msr/mrs)
● It’s clearly possible to fix, but will take some more time.
● Are there other bootloaders we should look into? Tianocore?

8. Building TEEs with clang
● OP-TEE Makefiles hardcode gcc-isms, and OP-TEE includes some code
(Linux 4.12, grub, …) that can’t currently be built with clang.
Many other components contained in OP-TEE work great when built with clang
already.
● Trusty Makefiles also hardcode gcc-isms, the lk kernel isn’t ready to be built
with clang, other than that a lot of code is shared with AOSP (where it builds
with clang perfectly).
● It should be possible to get both TEEs to be buildable with clang, but it will be
a bit of an effort.

9. Thank You
Questions/Comments:
minseong.kim@linaro.org, bero@linaro.org
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