swift234

[+0 self] Teach SILGen how to avoid emitting extra rr pairs when emitting RValues with Nominal Types and Tuples.

Specifically this patch makes the following changes:

1. We properly propagate down the SGFContext of a tuple to its elements
when extracting from a tuple. There was a typo that caused us to use
data from the newly created default initialized copy instead of from the
actual SGFContext of the parent tuple.

2. If we are accessing a member ref of self in a guaranteed context:

  a. If self is a struct, regardless of whether or not the field is a
     var or a let we no longer emit a retain. This is b/c self in a
     guaranteed context is immutable. This implies even a var in the
     struct can not be reassigned to.

  b. If self is a class, if the field is a let, we no longer emit an
     extra retain.

This makes it so that the only rr traffic in IntTreeNode::find is in the
block of code where we return self.

class IntTreeNode {
 let value : Int
 let left, right : IntTreeNode

 init() {} // not needed for -emit-silgen

 func find(v : Int) -> IntTreeNode {
   if v == value { return self }
   if v < value { return left.find(v) }
   return right.find(v)
 }
}

One gets the same effect using a struct for IntTreeNode and a generic
box class, i.e.:

class Box<T> {
  let value: T
  init(newValue: T) { value = newValue }
}

class Kraken {}
struct IntTreeNode {
  let k : Kraken // Just to make IntTreeNode non-trivial for rr purposes.
  let left, right : Box<IntTreeNode>

  init() {} // not needed for -emit-silgen

  func find(v : Int) -> IntTreeNode {
    if v == value { return self }
    if v < value { return left.value.find(v) }
    return right.value.find(v)
  }
}

There is more work that can be done here by applying similar logic to
enums, i.e. switching on self in an enum should not generate any rr
traffic over the switch. Also it seems that there are some places in SILGen
where isGuaranteedValid is not being propagated to SILGenFunction::emitLoad. But
that is for another day.

I am going to gather data over night and send an email to the list.

rdar://15729033

Swift SVN r27632