Teach unrolling to exploit conditions in enclosing ifs

[abadams]

Fixes #7968

This also changes the interface to simplify() to accept a vector of boolean Exprs that can be assumed to be true, which is a change I've been meaning to make for a while. The simplifier supports it internally, but it wasn't exposed in the API.

[vksnk]

I think a similar optimization can be done in VectorizeLoops, but I guess it's far less common (useful) for vectorize to be used like this in schedules.

[abadams]

Maybe a better design is to have a pass that forces loops that need to have a constant extent to have a constant extent and run it prior to unrolling and vectorization. Those two passes run back-to-back anyway.

[abadams]

Changed to that strategy. As a bonus this now permits vectorization of loops with a constant upper bound on their extent, by injecting a predicate, just like with unrolling. This means you can write slightly simpler schedules when you have something compute_at a GuardWithIf loop. Whereas previously you had to say:

g(x) = f(x) * 2;

g.vectorize(x, 8, TailStrategy::GuardWithIf);
f.compute_at(g, x).vectorize(x, 8, TailStrategy::GuardWithIf);

Now you can just say:

g(x) = f(x) * 2;

g.vectorize(x, 8, TailStrategy::GuardWithIf);
f.compute_at(g, x).vectorize(x);

This makes GuardWithIf schedules a little more similar to ShiftInwards or RoundUp schedules.

[abadams]

Adding release notes tag so we remember to mention that you can now vectorize loops that merely have a constant upper bound on their extent.

[vksnk]

Changed to that strategy. As a bonus this now permits vectorization of loops with a constant upper bound on their extent, by injecting a predicate, just like with unrolling. This means you can write slightly simpler schedules when you have something compute_at a GuardWithIf loop. Whereas previously you had to say:

g(x) = f(x) * 2;

g.vectorize(x, 8, TailStrategy::GuardWithIf);
f.compute_at(g, x).vectorize(x, 8, TailStrategy::GuardWithIf);

Now you can just say:

g(x) = f(x) * 2;

g.vectorize(x, 8, TailStrategy::GuardWithIf);
f.compute_at(g, x).vectorize(x);

This makes GuardWithIf schedules a little more similar to ShiftInwards or RoundUp schedules.

Ohhh, that's really nice!

[steven-johnson]

Ready to land?