relates to:

• SR-7277 -Onone should inline @inline(__always)

Issue Description:

Wrapping an always-inline (or @transparent) call in a no-escape closure should have no impact on the generated SIL at -Onone.

In particular, the stdlib should be able to rely on

defer { _fixLifetime(x) }

to have identical behavior to a direct call to _fixLifetime, even at -Onone.

When a no-escape closure is called directly, as in `defer`, inlining it reduces code size. There's no way to mark the closure itself as always-inline, but that could be inferred from simple patterns. If each captured variable is directly passed to an always-inline function like _fixLifetime, the users certainly wants the closure to be inlined.

Example:

public func testFixLifetime(obj1: AnyObject, obj2: AnyObject) \{
 defer \{ _fixLifetime(obj1); _fixLifetime(obj2) }
 let bits1 = unsafeBitCast(obj1, to: Int.self)
 let bits2 = unsafeBitCast(obj2, to: Int.self)
 checkBits(bits1 | bits2)
}
{

This could be done a couple different ways

1. Recognize certain noescape closures patterns and mark them always-inline before mandatory inlining.

2. More generally recognize noescape closure patterns for inlining during or after MandatoryInlining. For example, a closure with only fix_lifetime instructions should be recognized as zero-cost, regardless of multiple callers. Furthermore, we may just want to inline all no-escape closures with only one caller.