[llvm-dev] [Proposal][RFC] Epilog loop vectorization

[Zaks, Ayal via llvm-dev]

On 02/27/2017 12:41 PM, Michael Kuperstein wrote:
There's another issue with re-running the vectorizer (which I support, btw - I'm just saying there are more problems to solve on the way :-) )

Historically, we haven't even tried to evaluate the cost of the "constant" (not per-iteration) vectorization overhead - things like alias checks. Instead, we have hard bounds - we won't perform alias checks that are "too expensive", and, more importantly, we don't even try to vectorize loops with known low iteration counts. The bound right now is 16, IIRC. That means we don't have a good way to evaluate whether vectorizing a loop with a low iteration count is profitable or not.

We should really improve this as well.

@Michael: OTOH, we should reach the same decision again (i.e., that of performing the alias checks) when encountering the remainder loop as we did with the original loop, given that hard bounds are used ;-).
But agreed, it is better to evaluate the cost of these bounds along with the overall estimated cost instead.

This also makes me wary of the "we can clean up redundant alias checks later" approach. When trying to decide whether to vectorize by 4 a loop that has no more than 8 iterations (because we just vectorized by 8 and it's the remainder loop), we really want to know if the alias checks we're introducing are going to survive a not.

It occurs to me that, if SCEV's known-predicate logic were smart enough, it would seem practical to not introduce redundant checks in the first place (although it would imply some gymnastics when examining the control flow around the loop and then restructuring things when we generate the code for the loop).

The scalar remainder loop, when reached from the vectorized loop, is already known to be vectorizable to a VF larger than EpilogVF. No need to introduce again any potential aliasing, wrapping or whatnot checks, even if this redundancy can later be eliminated, if instead this vectorizability property could be recorded somehow. Similar to having annotated the remainder loop with “#pragma clang loop vectorize(assume_safety)”, except that this vectorizability property does not hold when reaching the remainder loop along the other path – that which fails these checks for the main loop...

Ayal.

 -Hal



Michael

On Mon, Feb 27, 2017 at 10:11 AM, Hal Finkel <hfinkel at anl.gov<mailto:hfinkel at anl.gov>> wrote:


On 02/27/2017 11:47 AM, Adam Nemet wrote:

On Feb 27, 2017, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org<mailto:dberlin at dberlin.org>> wrote:



On Mon, Feb 27, 2017 at 9:29 AM, Adam Nemet <anemet at apple.com<mailto:anemet at apple.com>> wrote:

On Feb 27, 2017, at 7:27 AM, Hal Finkel <hfinkel at anl.gov<mailto:hfinkel at anl.gov>> wrote:


On 02/27/2017 06:29 AM, Nema, Ashutosh wrote:
Thanks for looking into this.

1) Issues with re running vectorizer:
Vectorizer might generate redundant alias checks while vectorizing epilog loop.
Redundant alias checks are expensive, we like to reuse the results of already computed alias checks.
With metadata we can limit the width of epilog loop, but not sure about reusing alias check result.
Any thoughts on rerunning vectorizer with reusing the alias check result ?

One way of looking at this is: Reusing the alias-check result is really just a conditional propagation problem; if we don't already have an optimization that can combine these after the fact, then we should.

+Danny

Isn’t Extended SSA supposed to help with this?

Yes, it will solve this with no issue already.  GVN probably does already too.

even if if you have

if (a == b)
if (a == c)
 if (a == d)
 if (a == e)
 if (a == g)


and  we can prove a ... g equivalent, newgvn will eliminate them all and set all the branches true.

If you need a simpler clean up pass, we could run it on sub-graphs.

Yes we probably don’t want to run a full GVN after the “loop-scheduling” passes.

FWIW, we could, just without the memory-dependence analysis enabled (i.e. set the NoLoads constructor parameter to true). GVN is pretty fast in that mode.

 -Hal



I guess the pipeline to experiment with for now is opt -loop-vectorize -loop-vectorize -newgvn.

Adam


The only thing you'd have to do is write some code to set "live on entry" subgraph variables in their own congruence classes.
We already do this for incoming arguments.

Otherwise, it's trivial to make it only walk things in the subgraph.






--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory




--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory
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