[LLVMdev] Vectorization: Next Steps

[Hal Finkel]

On Tue, 2012-02-07 at 00:47 +0100, Tobias Grosser wrote:
> On 02/06/2012 11:28 PM, Chris Lattner wrote:
> > On Feb 6, 2012, at 1:56 PM, Hal Finkel wrote:
> >>> If you do not want to use polly, you could use ISL
> >>> http://freecode.com/projects/isl to set up the dependence problem and
> >>> use ISL's ILP to solve it.
> >>
> >> isl is an LGPL project. It is not clear to me what the general consensus
> >> would be on having a core analysis pass carry an LGPL dependency.
> >
> > This is fine for something that wants to live out of tree or be a secondary subproject, but isn't acceptable for something that wants to go into the mainline llvm repository and be a default part of the compiler.
> 
> If there is consensus that parts of Polly should be included in the core 
> compiler, I am sure there are ways to solve these issues. Polly itself 
> is BSD anyways, ISL and GMP are LGPL. As isl is using just a tiny subset 
> of GMP I am pretty sure we can either rewrite this subset ourselves or 
> extend the existing arbitrary (but fixed) width integer functionality in 
> LLVM.
> 
> This leaves us with ISL and CLooG. For both the copyright situation is 
> pretty clear. The more complex library here is ISL. To my knowledge it 
> is the only competitive open source integer set library. Rewriting it 
> will be hard*. However, it was developed by one person (Sven 
> Verdoolaege) and some smaller patches of me. AFAIK the copyrights are 
> hold by the academic institutions he was working for. This means the 
> copyright situation is clear. It cannot be changed overnight, but the 
> institutions that need to be addressed are known.

In my experience, it is better to investigate these kinds of things
sooner rather than later. The more contributors a project acquires the
harder it is to get the license changed.

 -Hal

> 
> The main reason I did not look into this earlier is that I prioritized 
> the development of Polly itself and I postponed copyright issues to the 
> point where they actually need to be solved. This was not the case for 
> Polly as a pure research project, but if parts of it may also be used in 
> production that is obviously different.
> 
> If there is a consensus that people want isl being used within core 
> LLVM, I am very glad to investigate this further. (Otherwise it is still 
> on my TODO list, but not highest priority). For now I must admit I am 
> very comfortable having Polly as a side project, but I can also see that 
> it might be useful to move more mature parts of it into LLVM.
> 
> Cheers
> Tobi
> 
> * Developing an equivalent library from scratch within LLVM will be at 
> least as hard.

-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory