[FR] Enable LLVM Polly

[a1batross]

In our app, we don't write vectorized code by hand, instead we're relying on compiler autovectorization. In legacy NDK we used GCC's Graphite and now when we switched to latest NDK r20, I've noticed that Polly, LLVM's alternative to GCC Graphite, isn't enabled in NDK toolchains.

Simple way to check if it's enabled:

$ ./aarch64-linux-android21-clang -mllvm -polly -xc -c - -o test.o < /dev/null
clang (LLVM option parsing): Unknown command line argument '-polly'.  Try: 'clang (LLVM option parsing) -help'
clang (LLVM option parsing): Did you mean '-color'?
$ echo $?
1

When on Clang 8 from LLVM's APT repository it doesn't show an error:

$ clang-8 -target aarch64-linux-android21 -mllvm -polly -xc -c - -o test.o < /dev/null
$ echo $?
0

Our app is crossplatform, so testing Polly on Linux ARM machine is resulted performance boost around 10%. Is there any bugs on Android so it's not available?

[stephenhines]

This is just something that we have not had time to evaluate or test in the context of Android. It is something we can look into for future releases, but I can't commit to a specific release yet.

[a1batross]

@stephenhines thanks for answer!

Actually, I have my own research with Polly on Android as part of study of how I can use thirdparty Android NDK compiler, for example, Clang/LLVM from Ubuntu repositories, apt.llvm.org or self-built, but that set up to use NDK's sysroots.
Also, being a fan of hardfloat ABI, I also set compiler to use sysroot from older NDK with uncut hardfloat ABI support. This doesn't apply to the topic, but still was a nice try.

I did ran tests and benchmarks to ensure that this way of hack didn't generated worse code than NDK's toolchain.

So, here is results:

Toolchain	Avg FPS
NDK Clang ARMv7	77 FPS
Host Clang softfp	77 FPS
Host Clang hardfp	75.8 FPS
Host Clang softfp Polly	85 FPS
Host Clang hardfp Polly	89 FPS
NDK Clang AArch64	100 FPS
Host Clang AArch64 Polly	102 FPS

We just ended up with renderer performance on AArch64, but for ARMv7 devices that half of our audience still uses it's given a nice performance boost.

[stephenhines]

https://android-review.googlesource.com/c/toolchain/llvm_android/+/1261718 enables this for our toolchain. We'll be marking Polly support as experimental in an upcoming release of the NDK.

[DanAlbert]

I think that change missed the train for the r22 compiler, so triaging to r23. @stephenhines please correct me/move it if that's wrong.

[stephenhines]

This actually will make it for R22, since we need to take Yabin's update as well.

[DanAlbert]

@stephenhines

$ prebuilts/clang/host/linux-x86/clang-r383902c/bin/clang++ -mllvm -polly foo.cpp -c
clang (LLVM option parsing): Unknown command line argument '-polly'.  Try: 'clang (LLVM option parsing) --help'
clang (LLVM option parsing): Did you mean '--color'?

Seems this wasn't actually enabled? @nickdesaulniers as well because he was interested in using this.

[stephenhines]

Yeah, I missed that the build configuration wasn't actually picked up. We need to explicitly build polly (actually "Polly") and then bundle the necessary libraries to make this work. Let's move this to r23 instead.