[LLVMdev] [Polly] Comionpile-time of Polly's code generation

[Tobias Grosser]

On 09/01/2013 08:02 PM, Star Tan wrote:
> Hi all,
>
>
> It seems that Polly's code generation can leads to high compile-time overhead, especially for PolyBench applications such as 2mm, 3mm, gemm, syrk, etc. Some basic evaluation and analysis for Polly's code generation can be referred to  http://llvm.org/bugs/show_bug.cgi?id=16898.
>
>
> Currently, we can choose to run -polly-code-generator=cloog or -polly-code-generator=isl for code generation, but both of them lead to almost double compile-time overhead for the 2mm benchmark. Unfortunately, both Cloog and ISL can not improve the execution time compared with -polly-code-generator=none.  I think if we could identify it will not improve execution time in advance, then we can skip the expensive Cloog and ISL code generator.
>
>
> Can any one provide some suggestions or hints on this problem?

OK. I think in this case the problem is actually to figure out why Polly 
does not give a speedup in terms of execution time, because we have seen 
large speedups for 2mm and 3mm.

Here is what I see:

2mm$ polly-clang 2mm.c -O3 -I ../../../utilities/ -DPOLYBENCH_TIME 
-DPOLYBENCH_USE_SCALAR_LB -mllvm -polly-ignore-aliasing
2mm$ time ./a.out
18.217128

real	0m18.256s
user	0m18.128s
sys	0m0.064s
2mm$ polly-clang 2mm.c -O3 -I ../../../utilities/ -DPOLYBENCH_TIME 
-DPOLYBENCH_USE_SCALAR_LB -mllvm -polly-ignore-aliasing -mllvm -polly
2mm$ time ./a.out
4.986877

real	0m5.036s
user	0m4.940s
sys	0m0.068s

So the reason this does not work is that the polybench kernels in the 
test suite do not annotate the functions called with the 'restrict' 
keyword (that's whe we need the ignore-aliasing) as well as that the 
size of the arrays is given as scalars but the corresponding loop bounds 
are not. It would be great to fix up those issues.

The first issue can be fixed by adding run-time alias analysis checks.
Adding those checks now became very easy with the new isl code 
generation. The basic idea is that we ask isl to generate the necessary 
run-time check and add it into the condition created by 
executeScopConditionally(). In case you are interested in looking into 
this, this would be a great help!

Cheers,
Tobias