[dotnet-linker] Trim away the static constructor for protocol interfa…

…ces if we're registering protocols in the static registrar.

Pending:

* Update docs
* Tests
* Stats for size improvements

This might be the fix for #21002.

src/bgen/Generator.cs

@@ -5055,6 +5055,7 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
  foreach (var docId in docIds) {
  print ($"[DynamicDependencyAttribute (\"{docId}\")]");
  }
+ print ("[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]");
  print ($"static I{TypeName} ()");
  print ("{");
  print ("\tGC.KeepAlive (null);"); // need to do _something_ (doesn't seem to matter what), otherwise the static cctor (and the DynamicDependency attributes) are trimmed away.

tools/linker/CoreOptimizeGeneratedCode.cs

@@ -755,6 +755,9 @@ protected override void Process (MethodDefinition method)
  return; // nothing else to do here.
  }
 
+ if (ProcessProtocolInterfaceStaticConstructor (method))
+ return;
+
  var instructions = method.Body.Instructions;
  for (int i = 0; i < instructions.Count; i++) {
  var ins = instructions [i];
@@ -1191,6 +1194,19 @@ static Instruction GetPreviousSkippingNops (Instruction ins)
  return ins;
  }
 
+ static Instruction SkipNops (Instruction ins)
+ {
+ if (ins is null)
+ return null;
+
+ while (ins.OpCode == OpCodes.Nop) {
+ if (ins.Next is null)
+ return null;
+ ins = ins.Next;
+ }
+ return ins;
+ }
+
  int ProcessIsARM64CallingConvention (MethodDefinition caller, Instruction ins)
  {
  const string operation = "inline Runtime.IsARM64CallingConvention";
@@ -1345,5 +1361,66 @@ MethodReference GetBlockLiteralConstructor (MethodDefinition caller, Instruction
  }
  return caller.Module.ImportReference (block_ctor_def);
  }
+
+ bool ProcessProtocolInterfaceStaticConstructor (MethodDefinition method)
+ {
+ // The static cctor in protocol interfaces exists only to preserve the protocol's members, for inspection by the registrar(s).
+ // If we're registering protocols, then we don't need to preserve protocol members, because the registrar
+ // already knows everything about it => we can remove the static cctor.
+
+ if (!(method.DeclaringType.IsInterface && method.DeclaringType.IsInterface && method.IsStatic && method.IsConstructor && method.HasBody))
+ return false;
+
+ if (Optimizations.RegisterProtocols != true) {
+ Driver.Log (4, "Did not optimize static constructor in the protocol interface {0}: the 'register-protocols' optimization is disabled.", method.DeclaringType.FullName);
+ return false;
+ }
+
+ if (!method.DeclaringType.HasCustomAttributes || !method.DeclaringType.CustomAttributes.Any (v => v.AttributeType.Is ("Foundation", "ProtocolAttribute"))) {
+ Driver.Log (4, "Did not optimize static constructor in the protocol interface {0}: no Protocol attribute found.", method.DeclaringType.FullName);
+ return false;
+ }
+
+ var ins = SkipNops (method.Body.Instructions.First ());
+ if (ins is null) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_A /* Could not optimize the static constructor in the interface {0} because it did not have the expected instruction sequence (found end of method too soon). */, method.DeclaringType.FullName));
+ return false;
+ } else if (ins.OpCode != OpCodes.Ldnull) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_B /* Could not optimize the static constructor in the interface {0} because it had an unexpected instruction {1} at offset {2}. */, method.DeclaringType.FullName, ins.OpCode, ins.Offset));
+ return false;
+ }
+
+ ins = SkipNops (ins.Next);
+ var callGCKeepAlive = ins;
+ if (ins is null) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_A /* Could not optimize the static constructor in the interface {0} because it did not have the expected instruction sequence (found end of method too soon). */, method.DeclaringType.FullName));
+ return false;
+ } else if (callGCKeepAlive.OpCode != OpCodes.Call || !(callGCKeepAlive.Operand is MethodReference methodOperand) || methodOperand.Name != "KeepAlive" || !methodOperand.DeclaringType.Is ("System", "GC")) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_B /* Could not optimize the static constructor in the interface {0} because it had an unexpected instruction {1} at offset {2}. */, method.DeclaringType.FullName, ins.OpCode, ins.Offset));
+ return false;
+ }
+
+ ins = SkipNops (ins.Next);
+ if (ins is null) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_A /* Could not optimize the static constructor in the interface {0} because it did not have the expected instruction sequence (found end of method too soon). */, method.DeclaringType.FullName));
+ return false;
+ } else if (ins.OpCode != OpCodes.Ret) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_B /* Could not optimize the static constructor in the interface {0} because it had an unexpected instruction {1} at offset {2}. */, method.DeclaringType.FullName, ins.OpCode, ins.Offset));
+ return false;
+ }
+
+ ins = SkipNops (ins.Next);
+ if (ins is not null) {
+ ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.App, 2112, method, ins, Errors.MX2112_B /* Could not optimize the static constructor in the interface {0} because it had an unexpected instruction {1} at offset {2}. */, method.DeclaringType.FullName, ins.OpCode, ins.Offset));
+ return false;
+ }
+
+ // We can just remove the entire method
+ Driver.Log (4, "Optimized static constructor in the protocol interface {0} (static constructor was cleared and custom attributes removed)", method.DeclaringType.FullName);
+ method.Body.Instructions.Clear ();
+ method.Body.Instructions.Add (Instruction.Create (OpCodes.Ret));
+ method.CustomAttributes.Clear ();
+ return true;
+ }
  }
 }

tools/mtouch/Errors.resx

@@ -1327,6 +1327,13 @@
  </value>
  </data>
 
+ <data name="MX2112_A" xml:space="preserve">
+ <value>Could not optimize the static constructor in the interface {0} because it did not have the expected instruction sequence (found end of method too soon).</value>
+ </data>
+
+ <data name="MX2112_B" xml:space="preserve">
+ <value>Could not optimize the static constructor in the interface {0} because it had an unexpected instruction {1} at offset {2}.</value>
+ </data>
  <!-- 2200 -> 2299 is used by/reserved for ExceptionalSubStep subclasses in the linker -->
  <!-- 220x: PreserveSmartEnumConversionsSubStep -->
  <!-- 221x: RemoveBitcodeIncompatibleCodeStep -->