[llvm-dev] SCEV LoopTripCount

[Michael Zolotukhin via llvm-dev]

Hi,

> On Aug 9, 2016, at 6:39 PM, Alexandre Isoard via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> 
> I was doing some experiments with SCEV and especially the loop trip count.
> Sorry for the dumb question, what is the easiest way to dump SCEV analysis results on a .bc file?
opt -analyze -scalar-evolution < file.bc

> 
> On a side note, I wanted to see if we could optimize this function:
> 
> unsigned long kernel(long w, long h, long d) {
> 	unsigned long count = 0;
> 	for(int i = 0; i < w; ++i)
> 		for(int j = i; j < h; ++j)
> 			for(int k = j; k < d; ++k)
> 				++count;
> 	return count;
> }
> 
> into this (it might not take into account overflows, it was generated using the barvinok library):
Looks like we can not, but I haven’t examined why. Overflows might indeed get into the way.

Michael

> unsigned long kernel2(long w, long h, long d) {
> 	return
> 		(-1 + w >= 0 && -1 - w + h >= 0 && -1 - h + d >= 0)
> 		? (((((2 * w - 3 * w*w + w*w*w) + 3 * w * h + -3 * w * h*h) + ((3 * w - 3 * w*w) + 6 * w * h) * d))/6)
> 		: (-1 + w >= 0 && -1 - w + d >= 0 && h - d >= 0)
> 		? ((((2 * w - 3 * w*w + w*w*w) + (6 * w - 3 * w*w) * d + 3 * w * d*d))/6)
> 		: (-1 + h >= 0 && w - h >= 0 && -1 - h + d >= 0)
> 		? ((((2 * h - 2 * h*h*h) + (3 * h + 3 * h*h) * d))/6)
> 		: (-1 + d >= 0 && h - d >= 0 && w - d >= 0)
> 		? (((2 * d + 3 * d*d + d*d*d))/6)
> 		: 0;
> }
> 
> I am not sure how advanced are SCEV-based trip counts.
> 
> Best regards.
> 
> -- 
> Alexandre Isoard
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