Merge pull request #58888 from eeckstein/objc-string-opt

Optimizer: add the ObjCBridgingOptimization to optimize ObjectiveC bridging operations.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CMakeLists.txt

@@ -10,6 +10,7 @@ swift_compiler_sources(Optimizer
   AssumeSingleThreaded.swift
   ComputeEffects.swift
   EscapeInfoDumper.swift
+  ObjCBridgingOptimization.swift
   SILPrinter.swift
   MergeCondFails.swift
   RangeDumper.swift

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/ObjCBridgingOptimization.swift

@@ -0,0 +1,384 @@
+//===--- ObjCBridgingOptimization.swift - optimize ObjC bridging ----------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 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
+//
+//===----------------------------------------------------------------------===//
+
+import Basic
+import SIL
+
+/// Removes redundant ObjectiveC <-> Swift bridging calls.
+///
+/// Basically, if a value is bridged from ObjectiveC to Swift an then back to ObjectiveC
+/// again, then just re-use the original ObjectiveC value.
+///
+/// Things get a little bit more complicated in case of optionals (Nullable pointers).
+/// In this case both bridging calls are embedded in an `switch_enum` CFG diamond, like
+/// ```
+///   switch_enum %originalOptionalObjcValue
+/// some_bb(%1):
+///   %2 = enum #some(%1)
+///   %3 = apply %bridgeFromObjc(%2)
+///   %4 = enum #some(%3)
+///   br continue_bb(%4)
+/// none_bb:
+///   %5 = enum #none
+///   br continue_bb(%5)
+/// continue_bb(%bridgedOptionalSwiftValue):
+/// ```
+let objCBridgingOptimization = FunctionPass(name: "objc-bridging-opt", {
+  (function: Function, context: PassContext) in
+
+  if !function.hasOwnership { return }
+
+  // First try to optimize the optional -> optional case.
+  // We need to do this before handling the non-optional case to prevent
+  // sub-optimal optimization of bridging calls inside a switch_enum.
+  for block in function.blocks {
+    // Start at a block argument, which is the "result" of the switch_enum CFG diamond.
+    if !optimizeOptionalBridging(forArgumentOf: block, context) {
+      return
+    }
+  }
+
+  // Now try to optimize non-optional and optional -> non-optional bridging.
+  for block in function.blocks {
+    for inst in block.instructions {
+      if let apply = inst as? ApplyInst {
+        if !optimizeNonOptionalBridging(apply, context) {
+          return
+        }
+      }
+    }
+  }
+})
+
+//===----------------------------------------------------------------------===//
+//                      Top-level optimization functions
+//===----------------------------------------------------------------------===//
+
+/// Optimizes redundant bridging calls where both calls are within `switch_enum` diamonds.
+///
+/// For example:
+/// ```
+///   let s = returnOptionalNSString()
+///   useOptionalNSString(s)
+/// ```
+///
+/// The `block` is the continue-block of the second `switch_enum` diamond.
+/// Returns true if the pass should continue running.
+private func optimizeOptionalBridging(forArgumentOf block: BasicBlock,
+                                      _ context: PassContext) -> Bool {
+  if block.arguments.count != 1 {
+    // For simplicity only handle the common case: there is only one phi-argument which
+    // is the result of the bridging operation.
+    return true
+  }
+  // Check for the second swift -> ObjC bridging operation.
+  let finalObjCValue = block.arguments[0]
+  guard let swiftValueSwitch = isOptionalBridging(of: finalObjCValue, isBridging: isBridgeToObjcCall) else {
+    return true
+  }
+
+  // Check for the first ObjC -> swift bridging operation.
+  let swiftValue = lookThroughOwnershipInsts(swiftValueSwitch.enumOp)
+  guard let originalObjCValueSwitch = isOptionalBridging(of: swiftValue, isBridging: isBridgeToSwiftCall) else {
+    return true
+  }
+  
+  let originalObjCValue = originalObjCValueSwitch.enumOp
+  if finalObjCValue.type != originalObjCValue.type {
+    return true
+  }
+
+  if !context.continueWithNextSubpassRun(for: originalObjCValueSwitch) {
+    return false
+  }
+
+  // The second bridging operation can be in a different control region than the first one,
+  // e.g. it can be in a loop whereas the first is not in that loop. Therefore we have to
+  // copy + makeAvailable.
+  let replacement = originalObjCValue.copy(at: originalObjCValueSwitch,
+                                           andMakeAvailableIn: block, context)
+
+  finalObjCValue.uses.replaceAll(with: replacement, context)
+  block.eraseArgument(at: 0, context)
+  
+  // The swift -> ObjC bridging call has no `readonly` attribute, therefore we have to
+  // explicitly delete it. The ObjC -> swift call has such an attribute and will be removed
+  // buy a later dead-code elimination pass.
+  removeBridgingCodeInPredecessors(of: block, context)
+  return true
+}
+
+/// Optimizes redundant bridging calls where the second call is a non-optional bridging operation,
+/// i.e. is _not_ within `switch_enum` diamond.
+///
+/// The `apply` is the second (swift -> ObjC) bridging call.
+/// Returns true if the pass should continue running.
+private func optimizeNonOptionalBridging(_ apply: ApplyInst,
+                                         _ context: PassContext) -> Bool {
+                                              
+  guard let bridgeToObjcCall = isBridgeToObjcCall(apply) else {
+    return true
+  }
+
+  let swiftValue = lookThroughOwnershipInsts(bridgeToObjcCall.arguments[0])
+
+  // Handle the first case: the ObjC -> swift bridging operation is optional and the swift -> ObjC
+  // bridging is within a test for Optional.some, e.g.
+  // ```
+  //   if let s = returnOptionalNSString() {
+  //     useNonOptionalNSString(s)
+  //   }
+  // ```
+  if let (se, someCase) = isPayloadOfSwitchEnum(swiftValue),
+     let originalObjCValueSwitch = isOptionalBridging(of: se.enumOp, isBridging: isBridgeToSwiftCall) {
+
+    if !context.continueWithNextSubpassRun(for: originalObjCValueSwitch) {
+      return false
+    }
+
+    let originalObjCValue = originalObjCValueSwitch.enumOp
+    let optionalReplacement = originalObjCValue.copy(at: originalObjCValueSwitch,
+                                                     andMakeAvailableIn: bridgeToObjcCall.block, context)
+    let builder = Builder(at: bridgeToObjcCall, context)
+    
+    // We know that it's the some-case.
+    let replacement = builder.createUncheckedEnumData(enum: optionalReplacement,
+                                                      caseIndex: someCase,
+                                                      resultType: bridgeToObjcCall.type)
+    bridgeToObjcCall.uses.replaceAll(with: replacement, context)
+    context.erase(instruction: bridgeToObjcCall)
+    return true
+  }
+
+  // Handle the second case: both bridging calls are non-optional, e.g.
+  // ```
+  //   let s = returnNonOptionalNSString()
+  //   useNonOptionalNSString(s)
+  // ```
+  guard let bridgeToSwiftCall = isBridgeToSwiftCall(swiftValue) else {
+    return true
+  }
+
+  if !context.continueWithNextSubpassRun(for: bridgeToSwiftCall) {
+    return false
+  }
+
+  let originalObjCValue = bridgeToSwiftCall.arguments[0]
+  let optionalObjCType = originalObjCValue.type
+  
+  // The bridging functions from ObjC -> Swift take an optional argument and return a
+  // non-optional Swift value. In the nil-case they return an empty (e.g. empty String,
+  // empty Array, etc.) swift value.
+  // We have to replicate that behavior here.
+
+  guard let someCase = optionalObjCType.getIndexOfEnumCase(withName: "some") else { return true }
+  guard let noneCase = optionalObjCType.getIndexOfEnumCase(withName: "none") else { return true }
+
+  // Creates a `switch_enum` on `originalObjCValue` and in the nil-case return a bridged
+  // empty value.
+  // Create the needed blocks of the `switch_enum` CFG diamond.
+  let origBlock = bridgeToSwiftCall.block
+  let someBlock = context.splitBlock(at: bridgeToSwiftCall)
+  let noneBlock = context.splitBlock(at: bridgeToSwiftCall)
+  let continueBlock = context.splitBlock(at: bridgeToSwiftCall)
+
+
+  let builder = Builder(atEndOf: origBlock, location: bridgeToSwiftCall.location, context)
+  let copiedValue = builder.createCopyValue(operand: originalObjCValue)
+  builder.createSwitchEnum(enum: copiedValue, cases: [(someCase, someBlock),
+                                                       (noneCase, noneBlock)])
+
+  // The nil case: call the ObjC -> Swift bridging function, which will return
+  // an empty swift value.
+  let noneBuilder = Builder(atEndOf: noneBlock, location: bridgeToSwiftCall.location, context)
+  let subst = bridgeToObjcCall.substitutionMap
+  let emptySwiftValue = noneBuilder.createApply(
+        function: bridgeToSwiftCall.callee,
+        bridgeToSwiftCall.substitutionMap, arguments: Array(bridgeToSwiftCall.arguments))
+  // ... and bridge that to ObjectiveC.
+  let emptyObjCValue = noneBuilder.createApply(
+        function: noneBuilder.createFunctionRef(bridgeToObjcCall.referencedFunction!),
+        subst, arguments: [emptySwiftValue])
+  noneBuilder.createDestroyValue(operand: emptySwiftValue)
+  noneBuilder.createBranch(to: continueBlock, arguments: [emptyObjCValue])
+
+  // In the some-case just forward the original NSString.
+  let objCType = emptyObjCValue.type
+  let forwardedValue = someBlock.addBlockArgument(type: objCType, ownership: .owned, context)
+  let someBuilder = Builder(atEndOf: someBlock, location: bridgeToSwiftCall.location, context)
+  someBuilder.createBranch(to: continueBlock, arguments: [forwardedValue])
+
+  let s = continueBlock.addBlockArgument(type: objCType, ownership: .owned, context)
+  
+  // Now replace the bridged value with the original value in the destination block.
+  let replacement = s.makeAvailable(in: bridgeToObjcCall.block, context)
+  bridgeToObjcCall.uses.replaceAll(with: replacement, context)
+  context.erase(instruction: bridgeToObjcCall)
+  return true
+}
+
+//===----------------------------------------------------------------------===//
+//                               Utility functions
+//===----------------------------------------------------------------------===//
+
+/// Removes `enum` instructions and bridging calls in all predecessors of `block`.
+private func removeBridgingCodeInPredecessors(of block: BasicBlock, _ context: PassContext) {
+  for pred in block.predecessors {
+    let branch = pred.terminator as! BranchInst
+    let builder = Builder(after: branch, context)
+    builder.createBranch(to: block)
+    
+    let en = branch.operands[0].value as! EnumInst
+    context.erase(instruction: branch)
+    let op = en.operand
+    context.erase(instruction: en)
+    if let bridgingCall = op {
+      context.erase(instruction: bridgingCall as! ApplyInst)
+    }
+  }
+}
+
+private func lookThroughOwnershipInsts(_ value: Value) -> Value {
+  // Looks like it's sufficient to support begin_borrow for now.
+  // TODO: add copy_value if needed.
+  if let bbi = value as? BeginBorrowInst {
+    return bbi.operand
+  }
+  return value
+}
+
+/// Checks for an optional bridging `switch_enum` diamond.
+///
+/// ```
+///   switch_enum %0             // returned instruction
+/// some_bb(%1):
+///   %2 = enum #some(%1)        // only in case of ObjC -> Swift briding
+///   %3 = apply %bridging(%2)   // returned by `isBridging`
+///   %4 = enum #some(%3)
+///   br continue_bb(%4)
+/// none_bb:
+///   %5 = enum #none
+///   br continue_bb(%5)
+/// continue_bb(%value):         // passed value
+/// ```
+private func isOptionalBridging(of value: Value, isBridging: (Value) -> ApplyInst?) -> SwitchEnumInst? {
+  guard let arg = value as? BlockArgument,
+        arg.isPhiArgument else {
+    return nil
+  }
+  
+  var noneSwitch: SwitchEnumInst?
+  var someSwitch: SwitchEnumInst?
+  
+  // Check if one incoming value is the none-case and the other is the some-case.
+  for incomingVal in arg.incomingPhiValues {
+    // In both branches, the result must be an `enum` which is passed to the
+    // continue_bb's phi-argument.
+    guard let enumInst = incomingVal as? EnumInst,
+          let singleEnumUse = enumInst.uses.singleUse,
+          singleEnumUse.instruction is BranchInst else {
+      return nil
+    }
+    if let enumOp = enumInst.operand {
+      // The some-case
+      if someSwitch != nil { return nil }
+      guard let bridgingCall = isBridging(enumOp),
+            bridgingCall.uses.isSingleUse else {
+        return nil
+      }
+      let callArgument = bridgingCall.arguments[0]
+      
+      // If it's an ObjC -> Swift bridging call the argument is wrapped into an optional enum.
+      if callArgument.type.isEnum {
+        guard let sourceEnum = callArgument as? EnumInst,
+              let sourceEnumOp = sourceEnum.operand,
+              let (se, someCase) = isPayloadOfSwitchEnum(sourceEnumOp),
+              enumInst.caseIndex == someCase,
+              sourceEnum.caseIndex == someCase,
+              sourceEnum.type == se.enumOp.type else {
+          return nil
+        }
+        someSwitch = se
+      } else {
+        guard let (se, someCase) = isPayloadOfSwitchEnum(callArgument),
+              enumInst.caseIndex == someCase else {
+          return nil
+        }
+        someSwitch = se
+      }
+    } else {
+      // The none-case
+      if noneSwitch != nil { return nil }
+      guard let singlePred = enumInst.block.singlePredecessor,
+            let se = singlePred.terminator as? SwitchEnumInst,
+            se.getUniqueSuccessor(forCaseIndex: enumInst.caseIndex) === enumInst.block else {
+        return nil
+      }
+      noneSwitch = se
+    }
+  }
+  guard let noneSwitch = noneSwitch,
+        let someSwitch = someSwitch,
+        noneSwitch == someSwitch else {
+    return nil
+  }
+  return someSwitch
+}
+
+/// Returns the `switch_enum` together with the enum case index, if `value` is
+/// the payload block argument of the `switch_enum`.
+private func isPayloadOfSwitchEnum(_ value: Value) -> (SwitchEnumInst, case: Int)? {
+  if let payloadArg = value as? BlockArgument,
+     let pred = payloadArg.block.singlePredecessor,
+     let se = pred.terminator as? SwitchEnumInst,
+     let caseIdx = se.getUniqueCase(forSuccessor: payloadArg.block) {
+    return (se, caseIdx)
+  }
+  return nil
+}
+
+/// Returns the apply instruction if `value` is an ObjC -> Swift bridging call.
+func isBridgeToSwiftCall(_ value: Value) -> ApplyInst? {
+  guard let bridgingCall = value as? ApplyInst,
+        let bridgingFunc = bridgingCall.referencedFunction else {
+    return nil
+  }
+  let funcName = bridgingFunc.name
+  guard  bridgingFunc.hasSemanticsAttribute("bridgeFromObjectiveC") ||
+         // Currently the semantics attribute is not used, so test for specific functions, too.
+         // TODO: remove those checks once the briding functions are annotate with "bridgeFromObjectiveC"
+         //       in Foundation.
+         //
+         // String._unconditionallyBridgeFromObjectiveC(_:)
+         funcName == "$sSS10FoundationE36_unconditionallyBridgeFromObjectiveCySSSo8NSStringCSgFZ" ||
+         // Array._unconditionallyBridgeFromObjectiveC(_:)
+         funcName == "$sSa10FoundationE36_unconditionallyBridgeFromObjectiveCySayxGSo7NSArrayCSgFZ" else {
+    return nil
+  }
+  guard bridgingCall.arguments.count == 2,
+        bridgingCall.getArgumentConvention(calleeArgIndex: 0) == .directGuaranteed else {
+    return nil
+  }
+  return bridgingCall
+}
+
+/// Returns the apply instruction if `value` is a Swift -> ObjC bridging call.
+func isBridgeToObjcCall(_ value: Value) -> ApplyInst? {
+  guard let bridgingCall = value as? ApplyInst,
+        let bridgingFunc = bridgingCall.referencedFunction,
+        bridgingFunc.hasSemanticsAttribute("convertToObjectiveC"),
+        bridgingCall.arguments.count == 1,
+        bridgingCall.getArgumentConvention(calleeArgIndex: 0) == .directGuaranteed else {
+    return nil
+  }
+  return bridgingCall
+}