[AutoDiff] Enable variable debugging support for pullback functions.

- Properly clone and use debug scopes for all instructions in pullback functions.
- Emit `debug_value` instructions for adjoint values.
- Add debug locations and variable info to adjoint buffer allocations.
- Add `TangentBuilder` (a `SILBuilder` subclass) to unify and simplify special emitter utilities for tangent vector code generation. More simplifications to come.

Pullback variable inspection example:
```console
(lldb) n
Process 50984 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x0000000100003497 main`pullback of foo(x=0) at main.swift:12:11
   9   	import _Differentiation
   10
   11  	func foo(_ x: Float) -> Float {
-> 12  	  let y = sin(x)
   13  	  let z = cos(y)
   14  	  let k = tanh(z) + cos(y)
   15  	  return k
Target 0: (main) stopped.
(lldb) fr v
(Float) x = 0
(Float) k = 1
(Float) z = 0.495846391
(Float) y = -0.689988375
```

Resolves rdar://68616528 / SR-13535.

include/swift/SILOptimizer/Differentiation/ADContext.h

@@ -115,6 +115,8 @@ private:
   mutable FuncDecl *cachedPlusFn = nullptr;
   /// `AdditiveArithmetic.+=` declaration.
   mutable FuncDecl *cachedPlusEqualFn = nullptr;
+  /// `AdditiveArithmetic.zero` declaration.
+  mutable AccessorDecl *cachedZeroGetter = nullptr;
 
 public:
   /// Construct an ADContext for the given module.
@@ -201,6 +203,7 @@ public:
 
   FuncDecl *getPlusDecl() const;
   FuncDecl *getPlusEqualDecl() const;
+  AccessorDecl *getAdditiveArithmeticZeroGetter() const;
 
   /// Cleans up all the internal state.
   void cleanUp();
@@ -269,6 +272,10 @@ public:
                                 Diag<T...> diag, U &&... args);
 };
 
+raw_ostream &getADDebugStream();
+SILLocation getValidLocation(SILValue v);
+SILLocation getValidLocation(SILInstruction *inst);
+
 template <typename... T, typename... U>
 InFlightDiagnostic
 ADContext::emitNondifferentiabilityError(SILValue value,

include/swift/SILOptimizer/Differentiation/AdjointValue.h

@@ -51,29 +51,45 @@ class AdjointValueBase {
   /// The type of this value as if it were materialized as a SIL value.
   SILType type;
 
+  using DebugInfo = std::pair<SILDebugLocation, SILDebugVariable>;
+
+  /// The debug location and variable info associated with the original value.
+  Optional<DebugInfo> debugInfo;
+
   /// The underlying value.
   union Value {
-    llvm::ArrayRef<AdjointValue> aggregate;
+    unsigned numAggregateElements;
     SILValue concrete;
-    Value(llvm::ArrayRef<AdjointValue> v) : aggregate(v) {}
+    Value(unsigned numAggregateElements)
+        : numAggregateElements(numAggregateElements) {}
     Value(SILValue v) : concrete(v) {}
     Value() {}
   } value;
 
+  // Begins tail-allocated aggregate elements, if
+  // `kind == AdjointValueKind::Aggregate`.
+
   explicit AdjointValueBase(SILType type,
-                            llvm::ArrayRef<AdjointValue> aggregate)
-      : kind(AdjointValueKind::Aggregate), type(type), value(aggregate) {}
+                            llvm::ArrayRef<AdjointValue> aggregate,
+                            Optional<DebugInfo> debugInfo)
+      : kind(AdjointValueKind::Aggregate), type(type), debugInfo(debugInfo),
+        value(aggregate.size()) {
+    MutableArrayRef<AdjointValue> tailElements(
+        reinterpret_cast<AdjointValue *>(this + 1), aggregate.size());
+    std::uninitialized_copy(
+        aggregate.begin(), aggregate.end(), tailElements.begin());
+  }
 
-  explicit AdjointValueBase(SILValue v)
-      : kind(AdjointValueKind::Concrete), type(v->getType()), value(v) {}
+  explicit AdjointValueBase(SILValue v, Optional<DebugInfo> debugInfo)
+      : kind(AdjointValueKind::Concrete), type(v->getType()),
+        debugInfo(debugInfo), value(v) {}
 
-  explicit AdjointValueBase(SILType type)
-      : kind(AdjointValueKind::Zero), type(type) {}
+  explicit AdjointValueBase(SILType type, Optional<DebugInfo> debugInfo)
+      : kind(AdjointValueKind::Zero), type(type), debugInfo(debugInfo) {}
 };
 
-/// A symbolic adjoint value that is capable of representing zero value 0 and
-/// 1, in addition to a materialized SILValue. This is expected to be passed
-/// around by value in most cases, as it's two words long.
+/// A symbolic adjoint value that wraps a `SILValue`, a zero, or an aggregate
+/// thereof.
 class AdjointValue final {
 
 private:
@@ -85,26 +101,37 @@ public:
   AdjointValueBase *operator->() const { return base; }
   AdjointValueBase &operator*() const { return *base; }
 
-  static AdjointValue createConcrete(llvm::BumpPtrAllocator &allocator,
-                                     SILValue value) {
-    return new (allocator.Allocate<AdjointValueBase>()) AdjointValueBase(value);
+  using DebugInfo = AdjointValueBase::DebugInfo;
+
+  static AdjointValue createConcrete(
+      llvm::BumpPtrAllocator &allocator, SILValue value,
+      Optional<DebugInfo> debugInfo = None) {
+    auto *buf = allocator.Allocate<AdjointValueBase>();
+    return new (buf) AdjointValueBase(value, debugInfo);
   }
 
-  static AdjointValue createZero(llvm::BumpPtrAllocator &allocator,
-                                 SILType type) {
-    return new (allocator.Allocate<AdjointValueBase>()) AdjointValueBase(type);
+  static AdjointValue createZero(
+      llvm::BumpPtrAllocator &allocator, SILType type,
+      Optional<DebugInfo> debugInfo = None) {
+    auto *buf = allocator.Allocate<AdjointValueBase>();
+    return new (buf) AdjointValueBase(type, debugInfo);
   }
 
-  static AdjointValue createAggregate(llvm::BumpPtrAllocator &allocator,
-                                      SILType type,
-                                      llvm::ArrayRef<AdjointValue> aggregate) {
-    return new (allocator.Allocate<AdjointValueBase>())
-        AdjointValueBase(type, aggregate);
+  static AdjointValue createAggregate(
+      llvm::BumpPtrAllocator &allocator, SILType type,
+      ArrayRef<AdjointValue> elements,
+      Optional<DebugInfo> debugInfo = None) {
+    AdjointValue *buf = reinterpret_cast<AdjointValue *>(allocator.Allocate(
+        sizeof(AdjointValueBase) + elements.size() * sizeof(AdjointValue),
+        alignof(AdjointValueBase)));
+    return new (buf) AdjointValueBase(type, elements, debugInfo);
   }
 
   AdjointValueKind getKind() const { return base->kind; }
   SILType getType() const { return base->type; }
   CanType getSwiftType() const { return getType().getASTType(); }
+  Optional<DebugInfo> getDebugInfo() const { return base->debugInfo; }
+  void setDebugInfo(DebugInfo debugInfo) const { base->debugInfo = debugInfo; }
 
   NominalTypeDecl *getAnyNominal() const {
     return getSwiftType()->getAnyNominal();
@@ -116,16 +143,18 @@ public:
 
   unsigned getNumAggregateElements() const {
     assert(isAggregate());
-    return base->value.aggregate.size();
+    return base->value.numAggregateElements;
   }
 
   AdjointValue getAggregateElement(unsigned i) const {
-    assert(isAggregate());
-    return base->value.aggregate[i];
+    return getAggregateElements()[i];
   }
 
   llvm::ArrayRef<AdjointValue> getAggregateElements() const {
-    return base->value.aggregate;
+    assert(isAggregate());
+    return {
+        reinterpret_cast<const AdjointValue *>(base + 1),
+        getNumAggregateElements()};
   }
 
   SILValue getConcreteValue() const {
@@ -143,7 +172,7 @@ public:
       if (auto *decl =
               getType().getASTType()->getStructOrBoundGenericStruct()) {
         interleave(
-            llvm::zip(decl->getStoredProperties(), base->value.aggregate),
+            llvm::zip(decl->getStoredProperties(), getAggregateElements()),
             [&s](std::tuple<VarDecl *, const AdjointValue &> elt) {
               s << std::get<0>(elt)->getName() << ": ";
               std::get<1>(elt).print(s);
@@ -151,7 +180,7 @@ public:
             [&s] { s << ", "; });
       } else if (getType().is<TupleType>()) {
         interleave(
-            base->value.aggregate,
+            getAggregateElements(),
             [&s](const AdjointValue &elt) { elt.print(s); },
             [&s] { s << ", "; });
       } else {

include/swift/SILOptimizer/Differentiation/Common.h

@@ -27,7 +27,9 @@
 #include "swift/SIL/TypeSubstCloner.h"
 #include "swift/SILOptimizer/Analysis/ArraySemantic.h"
 #include "swift/SILOptimizer/Analysis/DifferentiableActivityAnalysis.h"
+#include "swift/SILOptimizer/Differentiation/ADContext.h"
 #include "swift/SILOptimizer/Differentiation/DifferentiationInvoker.h"
+#include "swift/SILOptimizer/Differentiation/TangentBuilder.h"
 
 namespace swift {
 
@@ -142,6 +144,9 @@ template <class Inst> Inst *peerThroughFunctionConversions(SILValue value) {
   return nullptr;
 }
 
+Optional<std::pair<SILDebugLocation, SILDebugVariable>>
+findDebugLocationAndVariable(SILValue originalValue);
+
 //===----------------------------------------------------------------------===//
 // Diagnostic utilities
 //===----------------------------------------------------------------------===//
@@ -190,12 +195,6 @@ SILValue joinElements(ArrayRef<SILValue> elements, SILBuilder &builder,
 void extractAllElements(SILValue value, SILBuilder &builder,
                         SmallVectorImpl<SILValue> &results);
 
-/// Emit a zero value into the given buffer access by calling
-/// `AdditiveArithmetic.zero`. The given type must conform to
-/// `AdditiveArithmetic`.
-void emitZeroIntoBuffer(SILBuilder &builder, CanType type,
-                        SILValue bufferAccess, SILLocation loc);
-
 /// Emit a `Builtin.Word` value that represents the given type's memory layout
 /// size.
 SILValue emitMemoryLayoutSize(

include/swift/SILOptimizer/Differentiation/TangentBuilder.h

@@ -0,0 +1,78 @@
+//===--- TangentBuilder.h - Tangent SIL builder --------------*- C++ -*----===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2019 - 2020 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 a helper class for emitting tangent code for automatic
+// differentiation.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SILOPTIMIZER_UTILS_DIFFERENTIATION_TANGENTBUILDER_H
+#define SWIFT_SILOPTIMIZER_UTILS_DIFFERENTIATION_TANGENTBUILDER_H
+
+#include "swift/SIL/SILBuilder.h"
+
+namespace swift {
+namespace autodiff {
+
+class ADContext;
+
+class TangentBuilder: public SILBuilder {
+private:
+  ADContext &adContext;
+
+public:
+  TangentBuilder(SILFunction &fn, ADContext &adContext)
+      : SILBuilder(fn), adContext(adContext) {}
+  TangentBuilder(SILBasicBlock *bb, ADContext &adContext)
+      : SILBuilder(bb), adContext(adContext) {}
+  TangentBuilder(SILBasicBlock::iterator insertionPt, ADContext &adContext)
+      : SILBuilder(insertionPt), adContext(adContext) {}
+  TangentBuilder(SILBasicBlock *bb, SILBasicBlock::iterator insertionPt,
+                 ADContext &adContext)
+      : SILBuilder(bb, insertionPt), adContext(adContext) {}
+
+  /// Emits an `AdditiveArithmetic.zero` into the given buffer. If it is not an
+  /// initialization (`isInit`), a `destroy_addr` will be emitted on the buffer
+  /// first. The buffer must have a type that conforms to `AdditiveArithmetic`
+  /// or be a tuple thereof.
+  void emitZeroIntoBuffer(SILLocation loc, SILValue buffer,
+                          IsInitialization_t isInit);
+
+  /// Emits an `AdditiveArithmetic.zero` of the given type. The type must be a
+  /// loadable type, and must conform to `AddditiveArithmetic` or be a tuple
+  /// thereof.
+  SILValue emitZero(SILLocation loc, CanType type);
+
+  /// Emits an `AdditiveArithmetic.+=` for the given destination buffer and
+  /// operand. The type of the buffer and the operand must conform to
+  /// `AddditiveArithmetic` or be a tuple thereof. The operand will not be
+  /// consumed.
+  void emitInPlaceAdd(SILLocation loc, SILValue destinationBuffer,
+                      SILValue operand);
+
+  /// Emits an `AdditiveArithmetic.+` for the given operands. The type of the
+  /// operands must conform to `AddditiveArithmetic` or be a tuple thereof. The
+  /// operands will not be consumed.
+  void emitAddIntoBuffer(SILLocation loc, SILValue destinationBuffer,
+                         SILValue lhsAddress, SILValue rhsAddress);
+
+  /// Emits an `AdditiveArithmetic.+` for the given operands. The type of the
+  /// operands must be a loadable type, and must conform to
+  /// `AddditiveArithmetic` or be a tuple thereof. The operands will not be
+  /// consumed.
+  SILValue emitAdd(SILLocation loc, SILValue lhs, SILValue rhs);
+};
+
+} // end namespace autodiff
+} // end namespace swift
+
+#endif /* SWIFT_SILOPTIMIZER_UTILS_DIFFERENTIATION_TANGENTBUILDER_H */

include/swift/SILOptimizer/Differentiation/Thunk.h

@@ -38,6 +38,8 @@ class ArchetypeType;
 
 namespace autodiff {
 
+class ADContext;
+
 //===----------------------------------------------------------------------===//
 // Thunk helpers
 //===----------------------------------------------------------------------===//
@@ -107,7 +109,7 @@ std::pair<SILFunction *, SubstitutionMap>
 getOrCreateSubsetParametersThunkForDerivativeFunction(
     SILOptFunctionBuilder &fb, SILValue origFnOperand, SILValue derivativeFn,
     AutoDiffDerivativeFunctionKind kind, AutoDiffConfig desiredConfig,
-    AutoDiffConfig actualConfig);
+    AutoDiffConfig actualConfig, ADContext &adContext);
 
 /// Get or create a derivative function parameter index subset thunk from
 /// `actualIndices` to `desiredIndices` for the given associated function
@@ -119,7 +121,8 @@ getOrCreateSubsetParametersThunkForLinearMap(
     SILOptFunctionBuilder &fb, SILFunction *assocFn,
     CanSILFunctionType origFnType, CanSILFunctionType linearMapType,
     CanSILFunctionType targetType, AutoDiffDerivativeFunctionKind kind,
-    AutoDiffConfig desiredConfig, AutoDiffConfig actualConfig);
+    AutoDiffConfig desiredConfig, AutoDiffConfig actualConfig,
+    ADContext &adContext);
 
 } // end namespace autodiff