[Android Software/Hacking General [Developers Only]] JustArchi:
Hello dear developers.

I'd like to share with you effect of nearly 300 hours spent on trying to optimize Android and push it to the limits.

In general. You should be already experienced in setting up your buildbox, using git, building AOSP/CyanogenMod/OmniROM from source and cherry-picking things from review/gerrit. Solving git conflicts would also be nice. If you don't know how to build your own ROM from source, this is not a something you can apply to your ROM. Also, as you probably noticed, this is not a something you can apply to already prebuilt ROM (stocks), as these optimizations are applied during compilation, so only AOSP roms, self-compiled from source may use this masterpiece.

So, what is it about? As we know, Android contains a bunch of low-level C/C++ code, which is compiled and acts as a backend for our java's frontend and android apps. Unfortunately, Google didn't put their best at focusing on optimization, so as a result we're using the same old flags set back in 2006 for Android Donut or anything which existed back then. As you guess, in 2006 we didn't have as powerful devices as now, we had to sacrifice performance for smaller code size, to fit to our little devices and run well on very low amount of memory. However, this is no longer a case, and by using newest compilers and properly setting flags, we can achieve something great.

You probably may heard of some developers claiming using of "O3 Flags" in their ROMs. Well, while this may be true, they've applied only to low-level ARM code, mostly used during kernel compilation. Additionally it overwrites O2 flag, which is already fast, so as you may guess, this is more likely a placebo effect and disappears right after you change the kernel. Take a look at the most cherry-picked "O3 Flags commit". You see big "-Os" in "TARGET_thumb_CFLAGS"? This is what I'm talking about.

However, the commit I'm about to present you is not a placebo effect, as it applies flags to everything what is compiled, and mostly important - target THUMB, about 90% of an Android.

Now I'll tell you some facts. We have three interesting optimization levels. Os, O2, O3. O2 enables all optimizations that do not involve a space-speed tradeoff. Os is similar to O2, but it disables all flags that increase code size. It also performs further optimizations to reduce code size to the minimum. O3 enables all O2 optimizations and some extra optimizations that like to increase code size and may, or may not, increase performance. If you want to ask if there's something more like O4, there is - Ofast, however it breaks IEEE standard and doesn't work with Android, as i.e. sqlite3 is not compatible with Ofast's -ffast-math flag. So no go for us.

Now here comes the fun part. Android by default is compiled with O2 flag for target ARM (about 10% of Android, mostly low-level parts) and Os flag for target THUMB (about 90% of Android, nearly everything apart from low-level parfts). Some guys think that Os is better than O2 or O3 because smaller code size is faster due to better fitting in cpu cache. Unfortunately, I proven that it is a myth. Os was good back in 2006, as only with this flag Google was able to compile Dalvik and it's virtual machine while keeping good amount of free memory and space on eMMC cards. As or now, we have plenty of space, plenty of ram, plenty of CPU power and still good old Os flag for 90% of Android.

I've made countless tests to find out what is the most efficient in terms of GCC optimization, two selected tests I am about to present you right now.

As you may noticed, I compiled whetstone.c benchmark using three different optimization flags - Os, O2 and O3. I set CPU to performance, maximum frequency, and I repeated each test additional two times, just to make sure that Android doesn't lie to me. Source code of this test is available here and you may download it, compile for our beloved Android and try yourself. As you can see O3 > O2 >> Os, Os performs about 2.5x times worse than O2, and about 3.0x times worse than O3.

But, of course. Android is not a freaking benchmark, it's operating system. We can't tell if things are getting better or worse according to a simple benchmark. I kept that in mind and provided community with JustArchi's Mysterious Builds for test. I gave both mysterious builds and didn't tell my users what is the mysterious change. Both builds have been compiled with the same toolchain, same version, same commits. The one and only mysterious change was the fact that every component compiled as target thumb (major portion of an android) has been optimized for speed (O3) in build #1 (experimental), and optimized for size (Os) in build #2 (normal behaviour). Check poll yourself, 9 votes on build 1 in terms of performance, and 1 vote on build 2. I decided that this and benchmark is enough to tell that O2/O3 for target thumb is something that we want.

Now it doesn't matter that match if you wish to use O2 or O3, but here is some comparison:

1. Kernel compiled with O2 has 4902 KB, with O3 4944 KB, so O3 is 42 KB bigger.

2. ROM compiled with O3 is 3 MB larger than O2 after zip compression. Fast overview: 97 binaries in /system/bin and 2 binaries in /system/xbin + 283 libraries in /system/lib and other files, about 400 files in total. 3 MB / 400 = 7,5 KB per file size increase.

3. It's unlikely that code working properly with O2 level might break on O3 level, most issues are on the Os <-> O2 part.

4. If it doesn't cause any issues, and speeds up a binary by a little bit, why not use it?

5. The only real reason to not use O3 is potential higher memory usage due to oversized binaries.

In general, I doubt that this extra chunk of code may cause any significant memory usage or slower performance. I suggest to use O3 if it doesn't cause any issues to you compared to O2, but older devices may use O2 purely for saving on code size, similar way Google did it back in 2006 using Os flag.

[SIZE="+1"]Now let's get down to business[/SIZE].

Here is a list of important improvements:

Sound cool, doesn't it? Head over to my ArchiDroid project and see yourself how people react after switching to my ROM. Take a look at just one small example, or another one :). No bullsh*t guys, this is not a placebo.

However, please read my commit carefully before you decide to cherry-pick it. You must understand that Google's flags weren't touched since 7 years and nobody can assure you that they will work properly for your ROM and your device. You may experiment with them a bit to find out if they're not causing conflicts or other issues.

I can assure you that my ArchiDroid based on CM compiles fine with suggested steps written in the commit itself. Just don't forget to clean ccache (rm -rf /home/youruser/.ccache or rm -rf /root/.ccache) and make clean/clobber.

You can use, modify and share my commit anyway you want, just please keep proper credits in changelogs and in the repo itself. If you feel generous, you may also buy me a coke for massive amount of hours put into those experiments.

Now go ahead and show your users how things should be done :cool:.

Cherry-picking time!

Android "Lollipop" (5.1.1 & 5.0.2 tested)

JustArchi's ArchiDroid Optimizations V4.1 for CyanogenMod (latest)

A set of commits you may want to pick to fix O3-related issues:

external_bluetooth_bluedroid | hardware_qcom_display | libcore | frameworks_av #1 | frameworks_av #2

Older entries are provided for reference only. I suggest using only latest commit above.

Android "Lollipop" (5.1.1 & 5.0.2 tested)

JustArchi's ArchiDroid Optimizations V4 for CyanogenMod

Android "Kitkat" 4.4.4:

JustArchi's ArchiDroid Optimizations V3 for CyanogenMod

JustArchi's ArchiDroid Optimizations V3 for OmniROM

JustArchi's ArchiDroid Optimizations V2

JustArchi's ArchiDroid Optimizations V1

AFTER applying above commit and AFTER EVERY CHANGE regarding flags, ALWAYS make clean/clobber AND empty ccache (rm -rf ~/.ccache)

Q: How to properly change toolchains used in local manifest?

Open from your source rootdir .repo/local_manifests/roomservice.xml (or create one). Here is a sample manifest that replaces default 4.8 toolchain (both eabi and androideabi) with 4.8 SaberMod and 4.9 ArchiToolchain:

<?xml version="1.0" encoding="UTF-8"?>
<manifest>
<remove-project name="platform/prebuilts/gcc/linux-x86/arm/arm-eabi-4.8" />
<project name="ArchiDroid/Toolchain" path="prebuilts/gcc/linux-x86/arm/arm-eabi-4.8" remote="github" revision="architoolchain-5.2-arm-linux-gnueabihf" />
<remove-project name="platform/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8" />
<project name="ArchiDroid/Toolchain" path="prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8" remote="github" revision="uber-4.9-arm-linux-androideabi" />
</manifest>

This is only an example, you should use the toolchains that suit you best. My ArchiDroid/Toolchain github repo is a good start to test various different toolchains and decide which one you like the most, or which one causes the least problems for you. I do not suggest any other magic tricks to include custom toolchains, putting your selected one in proper path is enough, avoid magic android_build modifications.

[size=+1]Troubleshooting[/size]

Q: Compiler errror:

(...)/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8/bin/../libexec/gcc/arm-linux-androideabi/4.8.x-sabermod/cc1: error while loading shared libraries: libcloog-isl.so.4: cannot open shared object file: No such file or directory

This error can be fixed by installing missing library. libcloog-isl.so.4 is provided by libcloog-isl4 package, so on debian-like OSes, you should be able to fix it with:

apt-get install libcloog-isl4

Q: Compiler errror:

(...)/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8/bin/../libexec/gcc/arm-linux-androideabi/4.8.x-sabermod/cc1: error while loading shared libraries: libisl.so.13: cannot open shared object file: No such file or directory

This error is very similar to above, but considers other shared library. libisl.so.13 is provided by libisl13 package. Now the problem is that this package is in testing/sid, so we'll need to install it from there.

Add to your /etc/apt/sources.list following entries:

deb http://ftp.debian.org/debian testing main contrib non-free
deb-src http://ftp.debian.org/debian testing main contrib non-free

Then apt-get update && apt-get install libisl13.

Issues below are for older commits and should be used for reference only

Kitkat THUMB O2+ errors?

These are the most common issues.

* Change -O3 flag from TARGET_thumb_CFLAGS back to -Os, make clean/clobber, empty ccache and try again. This fixes most of the issues.

* RIL problems for for the Exynos 4210 family? Add -fno-tree-vectorize to TARGET_thumb_CFLAGS.

* Broken exFAT -> https://github.com/JustArchi/android_external_fuse/commit/78ebbc4404de260862dca5f0454bffccee650e0d

Errors caused by toolchain?

1. Try Google's GCC 4.8 if you used Linaro 4.8 or SaberMod 4.8

2. Fallback to Google's GCC 4.7 if above didn't help (change TARGET_GCC_VERSION back to 4.7)

Errors caused by GCC 4.8+?

* ART Fix (bootloop) -> https://github.com/JustArchi/android_art/commit/71a0ca3057cc3865bd8e41dcb94443998d028407

* Not booting kernel -> https://github.com/JustArchi/androi...mmit/9262707f4ea471acf40baa43ffe4bfb3cff64de9 and https://github.com/JustArchi/androi...mmit/41a70abcdad746d9415f3ee40f90528feb0c9bdd

Errors caused by GCC 4.9+?

* Graphical glitches in PlayStore -> https://github.com/JustArchi/android_external_webp/commit/36c6201fbb108d6b757f860e2cd57f3982191662

Errors caused by Linaro?

* error: unknown CPU architecture -> https://github.com/JustArchi/androi...mmit/5e5e158a7c147725beae1eeb6785174baacecb03 (Keep in mind that this is a sample fix for smdk4412 kernel, you may need to use similar solution in your own case. Also, this error happens only with Linaro toolchain, doesn't happen with Google's GCC)

Other errors?

* error: undefined reference to 'memmove' -> https://github.com/XperiaSTE/androi...mmit/679a4e571ef77f47892a785e852d8219c1e6807a

[size=+1]Credits[/size]

[MENTION=4203241]IAmTheOneTheyCallNeo[/MENTION] - For inspiration and first steps

[MENTION=4328678]metalspring[/MENTION] - For some nice commits

[MENTION=2394329]sparksco[/MENTION] - For SaberMod, some nice commits and support for the optimization idea