[AutoDiff] Move stdlib sources to stdlib/public/core/Differentiation.

Move differentiable-programming-related stdlib sources to
`stdlib/public/core/Differentiation`.

Related master branch PR: swiftlang#27511.
master branch will have a `_Differentiation` support library containing
the `Differentiable` protocol and related APIs.

On tensorflow branch, the Swift source files in
`stdlib/public/core/Differentiation` will be directly built as part of
swiftCore for simplicity. Swift for TensorFlow users can continue to use
the `Differentiable` protocol and related APIs without adding
`import _Differentiation`.

Rename `protocol _Differentiable` back to `protocol Differentiable`.

Author: dan-zheng

include/swift/AST/KnownIdentifiers.def

@@ -53,6 +53,7 @@ IDENTIFIER(decode)
 IDENTIFIER(decodeIfPresent)
 IDENTIFIER(Decoder)
 IDENTIFIER(decoder)
+IDENTIFIER_(Differentiation)
 IDENTIFIER(dynamicallyCall)
 IDENTIFIER(dynamicMember)
 IDENTIFIER(Element)

include/swift/AST/KnownProtocols.def

@@ -93,7 +93,7 @@ PROTOCOL_(DestructorSafeContainer)
 
 PROTOCOL(StringInterpolationProtocol)
 
-PROTOCOL_(Differentiable)
+PROTOCOL(Differentiable)
 
 EXPRESSIBLE_BY_LITERAL_PROTOCOL(ExpressibleByArrayLiteral, "Array", false)
 EXPRESSIBLE_BY_LITERAL_PROTOCOL(ExpressibleByBooleanLiteral, "BooleanLiteralType", true)

stdlib/public/Differentiation/CMakeLists.txt

@@ -0,0 +1,25 @@
+#===--- CMakeLists.txt - Differentiable programming support library ------===#
+#
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See https://swift.org/LICENSE.txt for license information
+# See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+#
+#===----------------------------------------------------------------------===#
+
+# SWIFT_ENABLE_TENSORFLOW
+# NOTE: This CMakeLists.txt file is currently used only on master branch, not
+# on tensorflow branch. Instead, on tensorflow branch, the Swift source files
+# in this directory are directly built as part of swiftCore: see
+# stdlib/public/core/CMakeLists.txt.
+# SWIFT_ENABLE_TENSORFLOW END
+
+add_swift_target_library(swift_Differentiation ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
+  Differentiable.swift
+
+  SWIFT_COMPILE_FLAGS ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+  LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
+  INSTALL_IN_COMPONENT stdlib)

stdlib/public/Differentiation/Differentiable.swift

@@ -0,0 +1,70 @@
+//===--- Differentiable.swift ---------------------------------*- swift -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the Differentiable protocol, used by the experimental
+// differentiable programming project. This API is not stable and subject to
+// change.
+//
+// Please see forum discussion for more information about the differentiable
+// programming project:
+// https://forums.swift.org/t/differentiable-programming-mega-proposal/28547
+//
+//===----------------------------------------------------------------------===//
+
+/// A type that mathematically represents a differentiable manifold whose
+/// tangent spaces are finite-dimensional.
+public protocol Differentiable {
+  /// A type representing a differentiable value's derivatives.
+  ///
+  /// Mathematically, this is equivalent to the tangent bundle of the
+  /// differentiable manifold represented by the differentiable type.
+  associatedtype TangentVector: Differentiable & AdditiveArithmetic
+    where TangentVector.TangentVector == TangentVector
+
+  /// Moves `self` along the given direction. In Riemannian geometry, this is
+  /// equivalent to exponential map, which moves `self` on the geodesic surface
+  /// along the given tangent vector.
+  mutating func move(along direction: TangentVector)
+
+  // SWIFT_ENABLE_TENSORFLOW
+  /// A tangent vector such that `move(along: zeroTangentVector)` will not
+  /// modify `self`.
+  /// - Note: `zeroTangentVector` can be `TangentVector.zero` in most cases,
+  ///   but types whose tangent vectors depend on instance properties of `self`
+  ///   need to provide a different implementation. For example, the tangent
+  ///   vector of an `Array` depends on the array's `count`.
+  @available(*, deprecated, message: """
+      `zeroTangentVector` derivation has not been implemented; do not use \
+      this property
+      """)
+  var zeroTangentVector: TangentVector { get }
+  // SWIFT_ENABLE_TENSORFLOW END
+}
+
+public extension Differentiable where TangentVector == Self {
+  @_alwaysEmitIntoClient
+  mutating func move(along direction: TangentVector) {
+    self += direction
+  }
+}
+
+// SWIFT_ENABLE_TENSORFLOW
+public extension Differentiable {
+  // This is a temporary solution that allows us to add `zeroTangentVector`
+  // without implementing derived conformances. This property is marked
+  // unavailable because it will produce incorrect results when tangent vectors
+  // depend on instance properties of `self`.
+  // FIXME: Implement derived conformance and remove this default
+  // implementation.
+  var zeroTangentVector: TangentVector { .zero }
+}
+// SWIFT_ENABLE_TENSORFLOW END

stdlib/public/core/DifferentiationSupport.swift renamed to stdlib/public/Differentiation/DifferentiationSupport.swift

@@ -197,15 +197,16 @@ set(SWIFTLIB_ESSENTIAL_GYB_SOURCES
   UnsafeRawBufferPointer.swift.gyb
   )
 
+# SWIFT_ENABLE_TENSORFLOW
 # Compile differentiable programming sources only if enabled.
 set(SWIFTLIB_DIFFERENTIABLE_PROGRAMMING_SOURCES)
 if(SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING)
   list(APPEND SWIFTLIB_DIFFERENTIABLE_PROGRAMMING_SOURCES
-    Differentiable.swift
-    # SWIFT_ENABLE_TENSORFLOW
-    DifferentiationSupport.swift)
+    ../Differentiation/Differentiable.swift
+    ../Differentiation/DifferentiationSupport.swift)
   message(STATUS "Differentiable programming standard library additions enabled.")
 endif()
+# SWIFT_ENABLE_TENSORFLOW END
 
 # The complete list of sources in the core standard library.  Includes
 # all the essential sources listed above.
@@ -225,7 +226,9 @@ set(SWIFTLIB_SOURCES
   VarArgs.swift
   Zip.swift
   "${SWIFT_SOURCE_DIR}/stdlib/linker-support/magic-symbols-for-install-name.c"
+  # SWIFT_ENABLE_TENSORFLOW
   ${SWIFTLIB_DIFFERENTIABLE_PROGRAMMING_SOURCES}
+  # SWIFT_ENABLE_TENSORFLOW END
   )
 
 set(SWIFTLIB_GYB_SOURCES

stdlib/public/core/CMakeLists.txt

@@ -134,9 +134,11 @@ normalize_boolean_spelling(SWIFT_AST_VERIFIER)
 normalize_boolean_spelling(SWIFT_ASAN_BUILD)
 normalize_boolean_spelling(SWIFT_BUILD_SYNTAXPARSERLIB)
 normalize_boolean_spelling(SWIFT_ENABLE_SOURCEKIT_TESTS)
+normalize_boolean_spelling(SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING)
 # SWIFT_ENABLE_TENSORFLOW
 normalize_boolean_spelling(SWIFT_ENABLE_TENSORFLOW)
 normalize_boolean_spelling(SWIFT_ENABLE_TENSORFLOW_GPU)
+# SWIFT_ENABLE_TENSORFLOW END
 is_build_type_optimized("${SWIFT_STDLIB_BUILD_TYPE}" SWIFT_OPTIMIZED)
 
 set(profdata_merge_worker
@@ -317,6 +319,10 @@ _Block_release(void) { }\n")
 
       list(APPEND LIT_ARGS "--xunit-xml-output=${SWIFT_TEST_RESULTS_DIR}/lit-tests.xml")
 
+      if(SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING)
+        list(APPEND LIT_ARGS "--param" "differentiable_programming")
+      endif()
+
       foreach(test_subset ${TEST_SUBSETS})
         set(directories)
         set(dependencies ${test_dependencies})

test/CMakeLists.txt

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift
+// REQUIRES: differentiable_programming
+
+import _Differentiation
+
+// Test conformances.
+
+struct Wrapper<T> {
+  var value: T
+}
+extension Wrapper: Equatable where T: Equatable {}
+extension Wrapper: AdditiveArithmetic where T: AdditiveArithmetic {
+  static var zero: Self {
+    Wrapper(value: T.zero)
+  }
+  static func + (lhs: Self, rhs: Self) -> Self {
+    return Wrapper(value: lhs.value + rhs.value)
+  }
+  static func - (lhs: Self, rhs: Self) -> Self {
+    return Wrapper(value: lhs.value + rhs.value)
+  }
+}
+extension Wrapper: Differentiable where T: Differentiable {
+  typealias TangentVector = Wrapper<T.TangentVector>
+  mutating func move(along direction: TangentVector) {
+    value.move(along: direction.value)
+  }
+}
+
+// Test conformances for standard library types.
+
+extension Float: Differentiable {
+  public typealias TangentVector = Self
+}

test/Differentiation/differentiable_protocol.swift

@@ -345,6 +345,10 @@ swift_version = lit_config.params.get('swift-version',
 lit_config.note('Compiling with -swift-version ' + swift_version)
 config.swift_test_options = '-swift-version ' + swift_version
 
+differentiable_programming = lit_config.params.get('differentiable_programming', None)
+if differentiable_programming is not None:
+    config.available_features.add('differentiable_programming')
+
 # SWIFT_ENABLE_TENSORFLOW
 swift_tensorflow_test_run_extra_options = ''
 swift_tensorflow_extra_options = ''
@@ -363,6 +367,7 @@ else:
     config.substitutions.append( ('%filecheck-tensorflow-extra-options', '') )
 config.substitutions.append( ('%swift-tensorflow-extra-options', swift_tensorflow_extra_options) )
 config.substitutions.append( ('%swift-tensorflow-test-run-extra-options', swift_tensorflow_test_run_extra_options) )
+# SWIFT_ENABLE_TENSORFLOW END
 
 test_options = os.environ.get('SWIFT_TEST_OPTIONS')
 if test_options:

test/lit.cfg

@@ -126,6 +126,9 @@ if '@SWIFT_INCLUDE_TOOLS@' == 'TRUE':
 if "@SWIFT_SOURCEKIT_USE_INPROC_LIBRARY@" == "TRUE":
     config.available_features.add('sourcekit_use_inproc_library')
 
+if "@SWIFT_ENABLE_EXPERIMENTAL_DIFFERENTIABLE_PROGRAMMING@" == "TRUE":
+    config.available_features.add('differentiable_programming')
+
 # Let the main config do the real work.
 if config.test_exec_root is None:
     config.test_exec_root = os.path.dirname(os.path.realpath(__file__))