Make Halide not blow up for novice users who don't write a schedule

[jrk]

Options:

• Raise a warning (least appealing)
• Have in a linter
• Throw an error (require at least one explicit .inline() or output.compute_root() to override)
• Have a built-in simple autoscheduler that's -O0 or -O1
  • Should always be trivial to run on any pipeline in any context with no extra work
    • Don't use the complex autoscheduler plugin interface
    • Don't require estimates
    • Don't require loading modules
    • Don't require generators
    • Test aggressively to be sure it is robust to any input
  • Call it something different than "autoscheduler" to set clear expectations
    • Pipeline::trivial_schedule (or schedule_trivially)
    • Pipeline::default_schedule

Discussed with @abadams, @BachiLi, @derek-gerstmann, @TH3CHARLie. I lean towards combining options (3) & (4): error, plus an easy default way to get past it by calling output.trivial_schedule().

[steven-johnson]

Generally agreed.

Having the default be an -O0 autoscheduler will likely result in code shipping that way :-/

[jrk]

1. It could still be opt-in, not a silent default. That's what I meant to suggest. (The default if you do nothing would be an error, this is just the simplest "do something" that will be non-pathological.)
2. Yes, some slower-than-necessary code will ship, but the current situation is lots of people never get close to shipping any Halide code because it blows up in their face. And a trivial schedule that still does inner loop vectorization and elementwise fusion will be similar to or faster than most people's reasonable baseline C code in most cases. Also, this baseline is probably better than most novice hand-written schedules.

[steven-johnson]

Oh, I'm fine with slower-than-necessary, I just want it to have some clue in the source (e.g. .trivial_schedule()) rather than being a silent default. Nomenclature for the directive should be suggestive of adequate-but-suboptimal... .basic_schedule(), .dumb_schedule(), I dunno. .trivial_schedule() seems possible.

[jrk]

I totally agree.