WIP: Integrate Polly and make it available via `@polly` macro

[MatthiasJReisinger]

This is a first attempt to integrate the polyhedral optimizer Polly into Julia in the context of my GSoC project. Since (for now) it is not yet clear whether it's beneficial to turn on Polly by default, it adds a macro @polly which can be used to explicitly annotate functions that should be optimized via Polly. The @polly macro relies on the :meta mechanism to instruct Julia's LLVM code-generator whether to apply Polly to a particular function or not. Since Polly is simply a collection of LLVM passes this involves to selectively "turn on" these passes for the respective function. Therefore, in emit_function, where the lowering to LLVM IR happens, I check whether the given function is marked with the special :meta-symbol :polly and accordingly mark the generated LLVM function with an attribute PollySkipFnAttr (this attribute is already provided by Polly and allows to tell Polly to skip its optimizations for a certain function).

I do not know how I can "automatically" test whether Polly is executed for a certain function or not, so for now I had to manually verify that Polly is invoked for a function when @polly is present.

Since these changes also regard the build process of Julia I added a build option USE_POLLY which can be used to enable/disable these features (which is 0 by default to ensure the standard build is not influenced). When USE_POLLY is set to 1, Julia will be linked against Polly's static libraries libPolly.a and libPollyISL.a. For now, the build expects these libraries in the lib directory of the used LLVM installation, which basically means that LLVM has to be built manually together with Polly (like described here). So to successfully build Julia to enable these features, it would be necessary to have a Make.user that contains the following configurations, where YOUR_LLVM_BUILD_DIR points to the according LLVM installation containing Polly:

USE_SYSTEM_LLVM:=1
USE_POLLY:=1
LLVM_CONFIG:=$(YOUR_LLVM_BUILD_DIR)/bin/llvm-config

So to make the build a little more "user-friendly" it would be nice to have Polly be downloaded and built automatically which I'll try to solve next. Basically, this will require changing deps/Makefile to download Polly from its svn/git repository.

Please feel free to post your thoughts and suggestions on this.

[tobiasgrosser]

Hi Matthias,

thanks for pushing this ahead. From the Polly side, the architecture of this patch looks right. It connects all of Polly to Julia, while being minimal-intrusive. With this, we can start to increase the coverage and work on first optimizations.

I am interested to hear what the Julia side thinks of this patch.

[tkelman]

Surprisingly non-disruptive, a nice start. Will polly.jl get much larger? If not, maybe just put the macro definition inline in a different file, somewhere near wherever inbounds, simd, inline etc get defined?

[timholy]

As far as testing goes, perhaps something like

function looporder1(A::Matrix)
    s = zero(eltype(A))
    @inbounds for i = 1:size(A,1), j = 1:size(A,2) # deliberately in cache-inefficient order
        s += A[i,j]
    end
end

@polly function looporder2(A::Matrix)
    ...   # same function body as above
end

and then test that looporder2 is faster (by some substantial factor) than looporder1 for a sufficiently large matrix?

[MatthiasJReisinger]

Thank you for the feedback. For now, I don't expect any additions to polly.jl - I've inlined it to expr.jl (seemed to be an appropriate place, inline and noinline are also defined there).

[MatthiasJReisinger]

I now extended this to enable the build to automatically retrieve Polly, like mentioned above. So, if one is interested in trying these features it's sufficient to have the following in Make.user:

LLVM_VER:=svn
USE_POLLY:=1

[tkelman]

This is only because you haven't hooked up the tarball yet, right? I would hold off on that and make it a separate PR.

pulling from polly should probably be added to the update-llvm rule at the very end of this file though

[MatthiasJReisinger]

You are right, this will probably be better off on a separate pull request. Yes, since my changes on the Polly side will only be available in the new version (therefore not yet available as tarball), I don't know if it's meaningful to download older versions here. (and thank you for the hint with the update-llvm rule).

[MatthiasJReisinger]

@tkelman Just to make sure that I didn't get you wrong: do you think I should just wait with this functionality until the tarball is available, or is it okay if I already open a new PR for this functionality?

[tkelman]

Go ahead I'd say. It may be some time before Julia uses LLVM 3.9 by default. As long as your options are off by default initially they can be a bit unreliable or temporary to start with.

[MatthiasJReisinger]

Great, thank you for the clarification.

[tkelman]

I'm now going to think "Scop" is a typo every time I see it. "Static control part," apparently? https://github.com/llvm-mirror/polly/blob/master/include/polly/ScopDetection.h

[MatthiasJReisinger]

Yes, that's correct