swift234

Implement a narrow but effective hack to dramatically reduce retains and 
releases produced by SILGen for non-@mutating struct methods.  Before,
we would retain the entire struct (which, if it contained class pointers
would emit an actual retain operations) even when accessing only some
part of it.  Handle a special case where we just retain the accessed
field.  For example, this testcase:

class C {}

struct SomeStruct {
  var c : C
  var i = 42


  func foo() -> Int {
    return i
  }
}

used to compile into:

sil @_TFV2t210SomeStruct3foofS0_FT_Si : $@cc(method) @thin (@owned SomeStruct) -> Int64 {
bb0(%0 : $SomeStruct):
  debug_value %0 : $SomeStruct  
  // this retains "c"
  %2 = copy_value %0 : $SomeStruct  
  %3 = struct_extract %2 : $SomeStruct, #i
  // this releases "c"
  destroy_value %2 : $SomeStruct
  // free the +1 argument.
  destroy_value %0 : $SomeStruct
  return %3 : $Int64
}

now it produces:

sil @_TFV2t210SomeStruct3foofS0_FT_Si : $@cc(method) @thin (@owned SomeStruct) -> Int64 {
bb0(%0 : $SomeStruct):
  debug_value %0 : $SomeStruct  // let self       // id: %1
  %2 = struct_extract %0 : $SomeStruct, #i        // user: %4
  destroy_value %0 : $SomeStruct                  // id: %3
  return %2 : $Int64                              // id: %4
}

As mentioned, this is a narrow hack intended to specifically benefit String, a more
general approach will come as I have time to implement it.



Swift SVN r12658