[semantic-arc-opts] load [copy] -> load_borrow if copy is completely enclosed in certain kinds of exclusive access scopes.

[gottesmm]

Specifically if we have a begin_access [read] or a begin_access [modify] that is
never actually written to. In that case if we can prove that the load [copy] is
completely within the exclusive access region, we will load_borrow. Example:

  %0 = begin_access [read] ...
  %1 = load [copy] %0
  ...
  destroy_value %1
  end_access %0

=>

  %0 = begin_access [read] ...
  %1 = load_borrow %0
  ...
  end_borrow %1
  end_access %0

rdar://60064692

[gottesmm]

@swift-ci test

[gottesmm]

@swift-ci benchmark

[gottesmm]

I am not expecting this to hit that much stuff in the benchmarks necessarily b/c today when you access a var of a class, you have a very small begin_access scope. But this will hit cases where due to @_transparent we have a long begin_access scope like:

func myMethod() {
   let this = self
   func inoutUser(_ x: inout X) { ... }
   inoutUser(&this.myX)
}

[gottesmm]

If we eventually move a run of semantic arc opts later than inlining, this will be needed as well in general.

[gottesmm]

@swift-ci test source compatibility

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubFromNSDateRef	3780	4160	+10.1%	0.91x (?)

Code size: -O

Performance: -Osize

Code size: -Osize

Performance: -Onone

Code size: -swiftlibs

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 16 GB

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 0aecebd

[gottesmm]

@swift-ci test os x platform