[LLVMdev] [RFC] Heuristic for complete loop unrolling

[Michael Zolotukhin]

Hi devs,

Recently I came across an interesting testcase that LLVM failed to optimize well. The test does some image processing, and as a part of it, it traverses all the pixels and computes some value basing on the adjacent pixels. So, the hot part looks like this:

for(y = 0..height) {
 for (x = 0..width) {
   val = 0
   for (j = 0..5) {
     for (i = 0..5) {
       val += img[x+i,y+j] * weight[i,j]
     }
   }
 }
}

And ‘weight' is just a constant matrix with some coefficients.

If we unroll the two internal loops (with tripcount 5), then we can replace weight[i,j] with concrete constant values. In this particular case, many of the coefficients are actually 0 or 1, which enables huge code simplifications later on. But currently we unroll only the innermost one, because unrolling both of them will exceed the threshold.

When deciding whether to unroll or not, we currently look only at the instruction count of the loop. My proposal is to, on top of that, check if we can enable any later optimizations by unrolling - in this case by replacing a load with a constant. Similar to what we do in inlining heuristics, we can estimate how many instructions would be potentially eliminated after unrolling and adjust our threshold with this value.

I can imagine that it might be also useful for computations, involving sparse constant matrixes (like identity matrix).

The attached patch implements this feature, and with it we handle the original testcase well.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: complete-unroll.patch
Type: application/octet-stream
Size: 4398 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150123/85bd7256/attachment.obj>
-------------- next part --------------


Does it look good? Of course, any ideas, suggestions and other feedback are welcome!


Thanks,
Michael