[AutoDiff] Bump-pointer allocate pullback structs in loops. (#34886)

In derivatives of loops, no longer allocate boxes for indirect case payloads. Instead, use a custom pullback context in the runtime which contains a bump-pointer allocator.

When a function contains a differentiated loop, the closure context is a `Builtin.NativeObject`, which contains a `swift::AutoDiffLinearMapContext` and a tail-allocated top-level linear map struct (which represents the linear map struct that was previously directly partial-applied into the pullback). In branching trace enums, the payloads of previously indirect cases will be allocated by `swift::AutoDiffLinearMapContext::allocate` and stored as a `Builtin.RawPointer`.

stdlib/public/runtime/AutoDiffSupport.cpp

@@ -0,0 +1,72 @@
+//===--- AutoDiffSupport.cpp ----------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "AutoDiffSupport.h"
+#include "swift/ABI/Metadata.h"
+#include "swift/Runtime/HeapObject.h"
+
+using namespace swift;
+using namespace llvm;
+
+SWIFT_CC(swift)
+static void destroyLinearMapContext(SWIFT_CONTEXT HeapObject *obj) {
+  free(obj);
+}
+
+/// Heap metadata for a linear map context.
+static FullMetadata<HeapMetadata> linearMapContextHeapMetadata = {
+  {
+    {
+      &destroyLinearMapContext
+    },
+    {
+      /*value witness table*/ nullptr
+    }
+  },
+  {
+    MetadataKind::Opaque
+  }
+};
+
+AutoDiffLinearMapContext::AutoDiffLinearMapContext()
+    : HeapObject(&linearMapContextHeapMetadata) {
+}
+
+void *AutoDiffLinearMapContext::projectTopLevelSubcontext() const {
+  auto offset = alignTo(
+      sizeof(AutoDiffLinearMapContext), alignof(AutoDiffLinearMapContext));
+  return const_cast<uint8_t *>(
+      reinterpret_cast<const uint8_t *>(this) + offset);
+}
+
+void *AutoDiffLinearMapContext::allocate(size_t size) {
+  return allocator.Allocate(size, alignof(AutoDiffLinearMapContext));
+}
+
+AutoDiffLinearMapContext *swift::swift_autoDiffCreateLinearMapContext(
+    size_t topLevelLinearMapStructSize) {
+  auto allocationSize = alignTo(
+      sizeof(AutoDiffLinearMapContext), alignof(AutoDiffLinearMapContext))
+      + topLevelLinearMapStructSize;
+  auto *buffer = (AutoDiffLinearMapContext *)malloc(allocationSize);
+  return new (buffer) AutoDiffLinearMapContext;
+}
+
+void *swift::swift_autoDiffProjectTopLevelSubcontext(
+    AutoDiffLinearMapContext *allocator) {
+  return allocator->projectTopLevelSubcontext();
+}
+
+void *swift::swift_autoDiffAllocateSubcontext(
+    AutoDiffLinearMapContext *allocator, size_t size) {
+  return allocator->allocate(size);
+}