Refactor Span<T> to ease implementation of JIT intrinsics (dotnet/cor…

…eclr#8497)

- Introduce internal ByReference<T> type for byref fields and change Span to use it
- Generalize handling of byref-like types in the type loader
- Make DangerousGetPinnableReference public while I was on it

Commit migrated from dotnet/coreclr@64c2ad1

src/coreclr/src/inc/dacvars.h

@@ -219,8 +219,7 @@ DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pArrayClass, ::g_pArrayClass)
 DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pSZArrayHelperClass, ::g_pSZArrayHelperClass)
 DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pNullableClass, ::g_pNullableClass)
 #ifdef FEATURE_SPAN_OF_T
-DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pSpanClass, ::g_pSpanClass)
-DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pReadOnlySpanClass, ::g_pReadOnlySpanClass)
+DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pByReferenceClass, ::g_pByReferenceClass)
 #endif
 DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pExceptionClass, ::g_pExceptionClass)
 DEFINE_DACVAR(ULONG, UNKNOWN_POINTER_TYPE, dac__g_pThreadAbortExceptionClass, ::g_pThreadAbortExceptionClass)

src/coreclr/src/mscorlib/model.xml

@@ -12406,6 +12406,7 @@
  <Member Name="#ctor(T[],System.Int32)" />
  <Member Name="#ctor(T[],System.Int32,System.Int32)" />
  <Member Name="#ctor(System.Void*,System.Int32)" />
+ <Member Name="DangerousGetPinnableReference" />
  <Member Name="op_Implicit(T[])" ReturnType="System.Span&lt;T&gt;" />
  <Member Name="op_Implicit(System.ArraySegment&lt;T&gt;)" ReturnType="System.Span&lt;T&gt;" />
  <Member Name="get_Length" />
@@ -12425,6 +12426,7 @@
  <Member Name="#ctor(T[],System.Int32)" />
  <Member Name="#ctor(T[],System.Int32,System.Int32)" />
  <Member Name="#ctor(System.Void*,System.Int32)" />
+ <Member Name="DangerousGetPinnableReference" />
  <Member Name="op_Implicit(System.Span&lt;T&gt;)" ReturnType="System.ReadOnlySpan&lt;T&gt;" />
  <Member Name="op_Implicit(T[])" ReturnType="System.ReadOnlySpan&lt;T&gt;" />
  <Member Name="op_Implicit(System.ArraySegment&lt;T&gt;)" ReturnType="System.ReadOnlySpan&lt;T&gt;" />

src/coreclr/src/mscorlib/mscorlib.shared.sources.props

@@ -438,6 +438,7 @@
  <SystemSources Condition="'$(FeatureCominterop)' == 'true'" Include="$(BclSourcesRoot)\System\__ComObject.cs" />
  <SystemSources Condition="'$(FeatureCominterop)' == 'true'" Include="$(BclSourcesRoot)\System\Variant.cs" />
  <SystemSources Condition="'$(FeatureClassicCominterop)' == 'true'" Include="$(BclSourcesRoot)\System\OleAutBinder.cs" />
+ <SystemSources Condition="'$(FeatureSpanOfT)' == 'true'" Include="$(BclSourcesRoot)\System\ByReference.cs" />
  <SystemSources Condition="'$(FeatureSpanOfT)' == 'true'" Include="$(BclSourcesRoot)\System\Span.cs" />
  <SystemSources Condition="'$(FeatureSpanOfT)' == 'true'" Include="$(BclSourcesRoot)\System\ReadOnlySpan.cs" />
  </ItemGroup>

src/coreclr/src/mscorlib/src/System/ByReference.cs

@@ -0,0 +1,31 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Runtime.CompilerServices;
+
+namespace System
+{
+ // ByReference<T> is meant to be used to represent "ref T" fields. It is working
+ // around lack of first class support for byref fields in C# and IL. The JIT and 
+ // type loader has special handling for it that turns it into a thin wrapper around ref T.
+ internal struct ByReference<T>
+ {
+ private IntPtr _value;
+
+ public ByReference(ref T value)
+ {
+ // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
+ unsafe { _value = (IntPtr)Unsafe.AsPointer(ref value); }
+ }
+
+ public ref T Value
+ {
+ get
+ {
+ // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
+ unsafe { return ref Unsafe.As<IntPtr, T>(ref *(IntPtr*)_value); }
+ }
+ }
+ }
+}

src/coreclr/src/mscorlib/src/System/ReadOnlySpan.cs

@@ -13,10 +13,10 @@ namespace System
  /// characteristics on par with T[]. Unlike arrays, it can point to either managed
  /// or native memory, or to memory allocated on the stack. It is type- and memory-safe.
  /// </summary>
- public unsafe struct ReadOnlySpan<T>
+ public struct ReadOnlySpan<T>
  {
- /// <summary>A byref or a native ptr. Do not access directly</summary>
- private readonly IntPtr _rawPointer;
+ /// <summary>A byref or a native ptr.</summary>
+ private readonly ByReference<T> _pointer;
  /// <summary>The number of elements this ReadOnlySpan contains.</summary>
  private readonly int _length;
 
@@ -31,8 +31,7 @@ public ReadOnlySpan(T[] array)
  if (array == null)
  ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
 
- // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
- _rawPointer = (IntPtr)Unsafe.AsPointer(ref JitHelpers.GetArrayData(array));
+ _pointer = new ByReference<T>(ref JitHelpers.GetArrayData(array));
  _length = array.Length;
  }
 
@@ -54,8 +53,7 @@ public ReadOnlySpan(T[] array, int start)
  if ((uint)start > (uint)array.Length)
  ThrowHelper.ThrowArgumentOutOfRangeException();
 
- // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
- _rawPointer = (IntPtr)Unsafe.AsPointer(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
+ _pointer = new ByReference<T>(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
  _length = array.Length - start;
  }
 
@@ -78,8 +76,7 @@ public ReadOnlySpan(T[] array, int start, int length)
  if ((uint)start > (uint)array.Length || (uint)length > (uint)(array.Length - start))
  ThrowHelper.ThrowArgumentOutOfRangeException();
 
- // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
- _rawPointer = (IntPtr)Unsafe.AsPointer(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
+ _pointer = new ByReference<T>(ref Unsafe.Add(ref JitHelpers.GetArrayData(array), start));
  _length = length;
  }
 
@@ -105,7 +102,7 @@ public unsafe ReadOnlySpan(void* pointer, int length)
  if (length < 0)
  ThrowHelper.ThrowArgumentOutOfRangeException();
 
- _rawPointer = (IntPtr)pointer;
+ _pointer = new ByReference<T>(ref Unsafe.AsRef<T>(pointer));
  _length = length;
  }
 
@@ -114,26 +111,25 @@ public unsafe ReadOnlySpan(void* pointer, int length)
  /// </summary>
  internal ReadOnlySpan(ref T ptr, int length)
  {
- // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
- _rawPointer = (IntPtr)Unsafe.AsPointer(ref ptr);
+ _pointer = new ByReference<T>(ref ptr);
  _length = length;
  }
 
  /// <summary>
- /// An internal helper for accessing spans.
+ /// Returns a reference to the 0th element of the Span. If the Span is empty, returns a reference to the location where the 0th element
+ /// would have been stored. Such a reference can be used for pinning but must never be dereferenced.
  /// </summary>
- internal unsafe ref T GetRawPointer()
+ public ref T DangerousGetPinnableReference()
  {
- // TODO-SPAN: This has GC hole. It needs to be JIT intrinsic instead
- return ref Unsafe.As<IntPtr, T>(ref *(IntPtr *)_rawPointer);
+ return ref _pointer.Value;
  }
 
  /// <summary>
  /// Defines an implicit conversion of a <see cref="Span{T}"/> to a <see cref="ReadOnlySpan{T}"/>
  /// </summary>
  public static implicit operator ReadOnlySpan<T>(Span<T> slice)
  {
- return new ReadOnlySpan<T>(ref slice.GetRawPointer(), slice.Length);
+ return new ReadOnlySpan<T>(ref slice.DangerousGetPinnableReference(), slice.Length);
  }
 
  /// <summary>
@@ -189,7 +185,7 @@ public T this[int index]
  if ((uint)index >= (uint)_length)
  ThrowHelper.ThrowIndexOutOfRangeException();
 
- return Unsafe.Add(ref GetRawPointer(), index);
+ return Unsafe.Add(ref DangerousGetPinnableReference(), index);
  }
  }
 
@@ -204,7 +200,7 @@ public T[] ToArray()
  return Array.Empty<T>();
 
  var destination = new T[_length];
- SpanHelper.CopyTo<T>(ref JitHelpers.GetArrayData(destination), ref GetRawPointer(), _length);
+ SpanHelper.CopyTo<T>(ref JitHelpers.GetArrayData(destination), ref DangerousGetPinnableReference(), _length);
  return destination;
  }
 
@@ -221,7 +217,7 @@ public ReadOnlySpan<T> Slice(int start)
  if ((uint)start > (uint)_length)
  ThrowHelper.ThrowArgumentOutOfRangeException();
 
- return new ReadOnlySpan<T>(ref Unsafe.Add(ref GetRawPointer(), start), _length - start);
+ return new ReadOnlySpan<T>(ref Unsafe.Add(ref DangerousGetPinnableReference(), start), _length - start);
  }
 
  /// <summary>
@@ -238,7 +234,7 @@ public ReadOnlySpan<T> Slice(int start, int length)
  if ((uint)start > (uint)_length || (uint)length > (uint)(_length - start))
  ThrowHelper.ThrowArgumentOutOfRangeException();
 
- return new ReadOnlySpan<T>(ref Unsafe.Add(ref GetRawPointer(), start), length);
+ return new ReadOnlySpan<T>(ref Unsafe.Add(ref DangerousGetPinnableReference(), start), length);
  }
 
  /// <summary>
@@ -248,7 +244,7 @@ public ReadOnlySpan<T> Slice(int start, int length)
  public bool Equals(ReadOnlySpan<T> other)
  {
  return (_length == other.Length) &&
- (_length == 0 || Unsafe.AreSame(ref GetRawPointer(), ref other.GetRawPointer()));
+ (_length == 0 || Unsafe.AreSame(ref DangerousGetPinnableReference(), ref other.DangerousGetPinnableReference()));
  }
 
  /// <summary>
@@ -261,7 +257,7 @@ public bool TryCopyTo(Span<T> destination)
  if ((uint)_length > (uint)destination.Length)
  return false;
 
- SpanHelper.CopyTo<T>(ref destination.GetRawPointer(), ref GetRawPointer(), _length);
+ SpanHelper.CopyTo<T>(ref destination.DangerousGetPinnableReference(), ref DangerousGetPinnableReference(), _length);
  return true;
  }
  }

src/coreclr/src/mscorlib/src/System/Runtime/CompilerServices/Unsafe.cs

@@ -23,7 +23,7 @@ internal static unsafe class Unsafe
  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public static void* AsPointer<T>(ref T value)
  {
- // The body of this function will be replaced by the EE with unsafe code that just returns sizeof !!T
+ // The body of this function will be replaced by the EE with unsafe code!!!
  // See getILIntrinsicImplementationForUnsafe for how this happens. 
  throw new InvalidOperationException();
  }
@@ -40,14 +40,26 @@ public static int SizeOf<T>()
  throw new InvalidOperationException();
  }
 
+ /// <summary>
+ /// Reinterprets the given location as a reference to a value of type<typeparamref name="T"/>.
+ /// </summary>
+ [NonVersionable]
+ [MethodImpl(MethodImplOptions.AggressiveInlining)]
+ public static ref T AsRef<T>(void * source)
+ {
+ // The body of this function will be replaced by the EE with unsafe code!!!
+ // See getILIntrinsicImplementationForUnsafe for how this happens. 
+ throw new InvalidOperationException();
+ }
+
  /// <summary>
  /// Reinterprets the given reference as a reference to a value of type <typeparamref name="TTo"/>.
  /// </summary>
  [NonVersionable]
  [MethodImpl(MethodImplOptions.AggressiveInlining)]
  public static ref TTo As<TFrom, TTo>(ref TFrom source)
  {
- // The body of this function will be replaced by the EE with unsafe code that just returns sizeof !!T
+ // The body of this function will be replaced by the EE with unsafe code!!!
  // See getILIntrinsicImplementationForUnsafe for how this happens. 
  throw new InvalidOperationException();
  }