Andrew Trick 0976c1f76e Teach the SIL cast optimizer to handle conditional conformance. ...

Previously the cast optimizer bailed out on any conformance with
requirements.

We can now constant-propagate this:

```
protocol P {}
struct S<E> {
  var e: E
}

extension S : P where E == Int {}

func specializeMe<T>(_ t: T) {
  if let p = t as? P {
    // do fast things.
  }
}

specializeMe(S(e: 0))
```

This turns out to be as simple as calling the TypeChecker.

<rdar://problem/46375150> Inlining does not seem to handle
specialization properly for Data.

This enabled two SIL transformations required to optimize
the code above:

(1) The witness method call can be devirtualized.

(2) The allows expensive dynamic runtime checks such as:

  unconditional_checked_cast_addr Array<UInt8> in %array : $*Array<UInt8> to ContiguousBytes in %protocol : $*ContiguousBytes

Will be converted into:

  %value = init_existential_addr %existential : $*ContiguousBytes, $Array<UInt8>
  store %array to %value : $*Array<UInt8>

2018-12-11 17:37:54 -08:00

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