swift234

Progress on <rdar://problem/15867140> [string perf] member_ref_expr on an rvalue producing unnecessary retains/releases

- Remove my previous local hack.
- Add a new flag to SGFContext indicating that clients are ok with +0 rvalues.
- Teach emitRValueForPropertyLoad and emitRValueForDecl how to work with +0 rvalues.

This allows us to avoid retaining bases in arbitrarily nested struct rvalue
member_ref_expr's.  For example, this:

class SomeClass {}

struct AnotherStruct {
  var x : Int
  var c : SomeClass
}

struct StructMemberTest {
  var c1 : SomeClass, c2 : SomeClass
  var s : AnotherStruct

  func testRecursiveStruct() -> Int {
    return s.x
  }
}

used to compile to:

sil @_TFV1t16StructMemberTest19testRecursiveStructfS0_FT_Si : $@cc(method) @thin (@owned StructMemberTest) -> Int64 {
bb0(%0 : $StructMemberTest):
  debug_value %0 : $StructMemberTest  // let self // id: %1
  %2 = struct_extract %0 : $StructMemberTest, #s  // user: %3
  %3 = copy_value %2 : $AnotherStruct             // users: %5, %4
  %4 = struct_extract %3 : $AnotherStruct, #x     // user: %7
  destroy_value %3 : $AnotherStruct               // id: %5
  destroy_value %0 : $StructMemberTest            // id: %6
  return %4 : $Int64                              // id: %7
}

and now it compiles to:

sil @_TFV1t16StructMemberTest19testRecursiveStructfS0_FT_Si : $@cc(method) @thin (@owned StructMemberTest) -> Int64 {
bb0(%0 : $StructMemberTest):
  debug_value %0 : $StructMemberTest  // let self // id: %1
  %2 = struct_extract %0 : $StructMemberTest, #s  // user: %3
  %3 = struct_extract %2 : $AnotherStruct, #x     // user: %5
  destroy_value %0 : $StructMemberTest            // id: %4
  return %3 : $Int64                              // id: %5
}

There is more that can come from this, but it is a start.  This cuts out 50 retain/release pairs from the stdlib.



Swift SVN r12857