[abadams]

Sliding window has numerous bugs relating to the interaction between the two kinds of sliding (loop unwinding vs adding selects). One known one was #8140 , but there were more. This PR adds a fuzzer for sliding window schedules and rewrites the sliding window pass to pass the fuzz tests. For review it's probably easier to just read the new version of the file, rather than looking at diffs. Several rounds of refactoring were done to try to make it easier to understand.

Separately, sliding window has long had an annoying limitation. When sliding and folding are combined, you get modulos due to the circular buffers. One might like to remove these modulos by unrolling the loop by the same factor, but if you do this you just get a different modulo, because sliding and folding can only slide over either the outer loop or the inner loop, not the loop you just split. This PR adds Func::slide, which lets you slide over a named Var, even if that Var has been split. For example, this is legal:

consumer.split(x, xo, xi, 8, TailStrategy::RoundUp).split(xo, xoo, xoi, 8, TailStrategy::RoundUp);
producer.store_root().compute_at(consumer, xi).slide(consumer, xo);

The producer can slide over xo, which is a transient loop that only existed in the middle of the chain of splits. It does this by jointly considering the loops it was split into - xoo and xoi. These loops must still appear in the schedule in order with no other loops in between. More commonly you would slide the producer over x - one of the original Vars. Using this directive lets you slide over x and also unroll the consumer's loop over x without changing the meaning of the slide.

This change is necessary for inductive Funcs (#8878) to be truly useful, because those rely critically on sliding window working well.

I can break this into two PRs if helpful. However Func::slide required a rewrite on top of the rewrite already done to fix the bugs, and I don't want to ask reviewers to review two rewrites of the same code. I would construct the stack by deleting the new feature from this branch, which might make some of the changes look pointless.

EDIT: Broken into multiple PRs. This one now just adds the slide directive and the ability to slide over spit loops. It is stacked on the rewrite and fuzzer.

[mcourteaux]

Can this address #9375 ?

[codecov]

Codecov Report

❌ Patch coverage is 86.54709% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.10%. Comparing base (76a9d57) to head (75ca864).

Files with missing lines	Patch %	Lines
src/StorageFolding.cpp	75.00%	13 Missing and 12 partials ⚠️
src/SlidingWindow.cpp	91.30%	4 Missing and 14 partials ⚠️
src/ScheduleFunctions.cpp	85.34%	10 Missing and 7 partials ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           abadams/sliding_window_rewrite    #9633      +/-   ##
==================================================================
+ Coverage                           72.84%   73.10%   +0.25%     
==================================================================
  Files                                 266      266              
  Lines                               83414    83762     +348     
  Branches                            20496    20585      +89     
==================================================================
+ Hits                                60765    61232     +467     
+ Misses                              16175    16149      -26     
+ Partials                             6474     6381      -93     

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