Adding a new Bytecode Instruction to the CLR

Now that the CoreCLR is open-source we can do fun things, for instance find out if it’s possible to add new IL (Intermediate Language) instruction to the runtime.

TL;DR it turns out that it’s easier than you might think!! Here are the steps you need to go through:

Update: turns out that I wasn’t the only person to have this idea, see Beachhead implements new opcode on CLR JIT for another implementation by Kouji Matsui.

Step 0

But first a bit of background information. Adding a new IL instruction to the CLR is a pretty rare event, that last time is was done for real was in .NET 2.0 when support for generics was added. This is part of the reason why .NET code had good backwards-compatibility, from Backward compatibility and the .NET Framework 4.5:

The .NET Framework 4.5 and its point releases (4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, and 4.7) are backward-compatible with apps that were built with earlier versions of the .NET Framework. In other words, apps and components built with previous versions will work without modification on the .NET Framework 4.5.

Side note: The .NET framework did break backwards compatibility when moving from 1.0 to 2.0, precisely so that support for generics could be added deep into the runtime, i.e. with support in the IL. Java took a different decision, I guess because it had been around longer, breaking backwards-comparability was a bigger issue. See the excellent blog post Comparing Java and C# Generics for more info.

Step 1

For this exercise I plan to add a new IL instruction (op-code) to the CoreCLR runtime and because I’m a raving narcissist (not really, see below) I’m going to name it after myself. So let me introduce the matt IL instruction, that you can use like so:

.method private hidebysig static int32 TestMattOpCodeMethod(int32 x, int32 y) 
        cil managed noinlining
    .maxstack 2
    matt  // yay, my name as an IL op-code!!!!

But because I’m actually a bit-British (i.e. I don’t like to ‘blow my own trumpet’), I’m going to make the matt op-code almost completely pointless, it’s going to do exactly the same thing as calling Math.Max(x, y), i.e. just return the largest of the 2 numbers.

The other reason for naming it matt is that I’d really like someone to make a version of the C# (Roslyn) compiler that allows you to write code like this:

Console.WriteLine("{0} m@ {1} = {2}", 1, 7, 1 m@ 7)); // prints '1 m@ 7 = 7'

I definitely want the m@ operator to be a thing (pronounced ‘matt’, not ‘m-at’), maybe the other ‘Matt Warren’ who works at Microsoft on the C# Language Design Team can help out!! Seriously though, if anyone reading this would like to write a similar blog post, showing how you’d add the m@ operator to the Roslyn compiler, please let me know I’d love to read it.

Update: Thanks to Marcin Juraszek (@mmjuraszek) you can now use the m@ in a C# program, see Adding Matt operator to Roslyn - Syntax, Lexer and Parser, Adding Matt operator to Roslyn - Binder and Adding Matt operator to Roslyn - Emitter for the full details.

Now we’ve defined the op-code, the first step is to ensure that the run-time and tooling can recognise it. In particular we need the IL Assembler (a.k.a ilasm) to be able to take the IL code above (TestMattOpCodeMethod(..)) and produce a .NET executable.

As the .NET runtime source code is nicely structured (+1 to the runtime devs), to make this possible we only need to makes changes in opcode.def:

--- a/src/inc/opcode.def
+++ b/src/inc/opcode.def
@@ -154,7 +154,7 @@ OPDEF(CEE_NEWOBJ,                     "newobj",           VarPop,             Pu
 OPDEF(CEE_CASTCLASS,                  "castclass",        PopRef,             PushRef,     InlineType,         IObjModel,   1,  0xFF,    0x74,    NEXT)
 OPDEF(CEE_ISINST,                     "isinst",           PopRef,             PushI,       InlineType,         IObjModel,   1,  0xFF,    0x75,    NEXT)
 OPDEF(CEE_CONV_R_UN,                  "conv.r.un",        Pop1,               PushR8,      InlineNone,         IPrimitive,  1,  0xFF,    0x76,    NEXT)
-OPDEF(CEE_UNUSED58,                   "unused",           Pop0,               Push0,       InlineNone,         IPrimitive,  1,  0xFF,    0x77,    NEXT)
+OPDEF(CEE_MATT,                       "matt",             Pop1+Pop1,          Push1,       InlineNone,         IPrimitive,  1,  0xFF,    0x77,    NEXT)
 OPDEF(CEE_UNUSED1,                    "unused",           Pop0,               Push0,       InlineNone,         IPrimitive,  1,  0xFF,    0x78,    NEXT)
 OPDEF(CEE_UNBOX,                      "unbox",            PopRef,             PushI,       InlineType,         IPrimitive,  1,  0xFF,    0x79,    NEXT)
 OPDEF(CEE_THROW,                      "throw",            PopRef,             Push0,       InlineNone,         IObjModel,   1,  0xFF,    0x7A,    THROW)

I just picked the first available unused slot and added matt in there. It’s defined as Pop1+Pop1 because it takes 2 values from the stack as input and Push1 because after is has executed, a single result is pushed back onto the stack.

Note: all the changes I made are available in one-place on GitHub if you’d rather look at them like that.

Once this change was done ilasm will successfully assembly the test code file that contains TestMattOpCodeMethod(..) as shown above:

λ ilasm /EXE /OUTPUT=HelloWorld.exe -NOLOGO

Assembling ''  to EXE --> 'HelloWorld.exe'
Source file is ANSI

Assembled method HelloWorld::Main
Assembled method HelloWorld::TestMattOpCodeMethod

Creating PE file

Emitting classes:
Class 1:        HelloWorld

Emitting fields and methods:
Class 1 Methods: 2;
Resolving local member refs: 1 -> 1 defs, 0 refs, 0 unresolved

Emitting events and properties:
Class 1
Resolving local member refs: 0 -> 0 defs, 0 refs, 0 unresolved
Writing PE file
Operation completed successfully

Step 2

However at this point the matt op-code isn’t actually executed, at runtime the CoreCLR just throws an exception because it doesn’t know what to do with it. As a first (simpler) step, I just wanted to make the .NET Interpreter work, so I made the following changes to wire it up:

--- a/src/vm/interpreter.cpp
+++ b/src/vm/interpreter.cpp
@@ -2726,6 +2726,9 @@ void Interpreter::ExecuteMethod(ARG_SLOT* retVal, __out bool* pDoJmpCall, __out
         case CEE_REM_UN:
+        case CEE_MATT:
+            BinaryArithOp<BA_Matt>();
+            break;
         case CEE_AND:

--- a/src/vm/interpreter.hpp
+++ b/src/vm/interpreter.hpp
@@ -298,10 +298,14 @@ void Interpreter::BinaryArithOpWork(T val1, T val2)
             res = val1 / val2;
-        else 
+        else if (op == BA_Rem)
             res = RemFunc(val1, val2);
+        else if (op == BA_Matt)
+        {
+            res = MattFunc(val1, val2);
+        }

and then I added the methods that would actually implement the interpreted code:

--- a/src/vm/interpreter.cpp
+++ b/src/vm/interpreter.cpp
@@ -10801,6 +10804,26 @@ double Interpreter::RemFunc(double v1, double v2)
     return fmod(v1, v2);
+INT32 Interpreter::MattFunc(INT32 v1, INT32 v2)
+	return v1 > v2 ? v1 : v2;
+INT64 Interpreter::MattFunc(INT64 v1, INT64 v2)
+	return v1 > v2 ? v1 : v2;
+float Interpreter::MattFunc(float v1, float v2)
+	return v1 > v2 ? v1 : v2;
+double Interpreter::MattFunc(double v1, double v2)
+	return v1 > v2 ? v1 : v2;

So fairly straight-forward and the bonus is that at this point the matt operator is fully operational, you can actually write IL using it and it will run (interpreted only).

Step 3

However not everyone wants to re-compile the CoreCLR just to enable the Interpreter, so I want to also make it work for real via the Just-in-Time (JIT) compiler.

The full changes to make this work were spread across multiple files, but were mostly housekeeping so I won’t include them all here, check-out the full diff if you’re interested. But the significant parts are below:

--- a/src/jit/importer.cpp
+++ b/src/jit/importer.cpp
@@ -11112,6 +11112,10 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                 oper = GT_UMOD;
                 goto MATH_MAYBE_CALL_NO_OVF;
+            case CEE_MATT:
+                oper = GT_MATT;
+                goto MATH_MAYBE_CALL_NO_OVF;
                 ovfl = false;

--- a/src/vm/jithelpers.cpp
+++ b/src/vm/jithelpers.cpp
@@ -341,6 +341,14 @@ HCIMPL2(UINT32, JIT_UMod, UINT32 dividend, UINT32 divisor)
+HCIMPL2(INT32, JIT_Matt, INT32 x, INT32 y)
+    return x > y ? x : y;
 HCIMPL2_VV(INT64, JIT_LDiv, INT64 dividend, INT64 divisor)

In summary, these changes mean that during the JIT’s ‘Morph phase’ the IL containing the matt op code is converted from:

fgMorphTree BB01, stmt 1 (before)
       [000004] ------------             ▌  return    int   
       [000002] ------------             │  ┌──▌  lclVar    int    V01 arg1        
       [000003] ------------             └──▌  m@        int   
       [000001] ------------                └──▌  lclVar    int    V00 arg0               

into this:

fgMorphTree BB01, stmt 1 (after)
       [000004] --C--+------             ▌  return    int   
       [000003] --C--+------             └──▌  call help int    HELPER.CORINFO_HELP_MATT
       [000001] -----+------ arg0 in rcx    ├──▌  lclVar    int    V00 arg0         
       [000002] -----+------ arg1 in rdx    └──▌  lclVar    int    V01 arg1                 


When this is finally compiled into assembly code it ends up looking like so:

// Assembly listing for method HelloWorld:TestMattOpCodeMethod(int,int):int             
// Emitting BLENDED_CODE for X64 CPU with AVX                                           
// optimized code                                                                       
// rsp based frame                                                                      
// partially interruptible                                                              
// Final local variable assignments                                                     
//  V00 arg0         [V00,T00] (  3,  3   )     int  ->  rcx                            
//  V01 arg1         [V01,T01] (  3,  3   )     int  ->  rdx                            
//  V02 OutArgs      [V02    ] (  1,  1   )  lclBlk (32) [rsp+0x00]                     
// Lcl frame size = 40                                    
       4883EC28             sub      rsp, 40                                           
       E8976FEB5E           call     CORINFO_HELP_MATT                                 
       90                   nop                                                        
       4883C428             add      rsp, 40                                           
       C3                   ret                                                        

I’m not entirely sure why there is a nop instruction in there? But it works, which is the main thing!!

Step 4

In the CLR you can also dynamically emit code at runtime using the methods that sit under the ‘System.Reflection.Emit’ namespace, so the last task is to add the OpCodes.Matt field and have it emit the correct values for the matt op-code.

--- a/src/mscorlib/src/System/Reflection/Emit/OpCodes.cs
+++ b/src/mscorlib/src/System/Reflection/Emit/OpCodes.cs
@@ -139,6 +139,7 @@ internal enum OpCodeValues
         Castclass = 0x74,
         Isinst = 0x75,
         Conv_R_Un = 0x76,
+        Matt = 0x77,
         Unbox = 0x79,
         Throw = 0x7a,
         Ldfld = 0x7b,
@@ -1450,6 +1451,16 @@ private OpCodes()
             (0 << OpCode.StackChangeShift)
+        public static readonly OpCode Matt = new OpCode(OpCodeValues.Matt,
+            ((int)OperandType.InlineNone) |
+            ((int)FlowControl.Next << OpCode.FlowControlShift) |
+            ((int)OpCodeType.Primitive << OpCode.OpCodeTypeShift) |
+            ((int)StackBehaviour.Pop1_pop1 << OpCode.StackBehaviourPopShift) |
+            ((int)StackBehaviour.Push1 << OpCode.StackBehaviourPushShift) |
+            (1 << OpCode.SizeShift) |
+            (-1 << OpCode.StackChangeShift)
+        );
         public static readonly OpCode Unbox = new OpCode(OpCodeValues.Unbox,
             ((int)OperandType.InlineType) |
             ((int)FlowControl.Next << OpCode.FlowControlShift) |

This lets us write the code shown below, which emits, compiles and then executes the matt op-code:

DynamicMethod method = new DynamicMethod(
		returnType: typeof(int),
		parameterTypes: new [] { typeof(int), typeof(int) }, 
		m: typeof(TestClass).Module);

// Emit the IL
var generator = method.GetILGenerator();
generator.Emit(OpCodes.Matt); // Use the new 'matt' IL OpCode

// Compile the IL into a delegate (uses the JITter under-the-hood)
var mattOpCodeInvoker = 
    (Func<int, int, int>)method.CreateDelegate(typeof(Func<int, int, int>));

// prints "1 m@ 7 = 7"
Console.WriteLine("{0} m@ {1} = {2} (via IL Emit)", 1, 7, mattOpCodeInvoker(1, 7));
// prints "12 m@ 9 = 12"
Console.WriteLine("{0} m@ {1} = {2} (via IL Emit)", 12, 9, mattOpCodeInvoker(12, 9)); 

Step 5

Finally, you may have noticed that I cheated a little bit in Step 3 when I made changes to the JIT. Even though what I did works, it is not the most efficient way due to the extra method call to CORINFO_HELP_MATT. Also the JIT generally doesn’t use helper functions in this way, instead prefering to emit assembly code directly.

As a future exercise for anyone who has read this far (any takers?), it would be nice if the JIT emitted more efficient code. For instance if you write C# code like this (which does the same thing as the matt op-code):

private static int MaxMethod(int x, int y)
    return x > y ? x : y;

It’s turned into the following IL by the C# compiler

IL to import:
IL_0000  02                ldarg.0     
IL_0001  03                ldarg.1     
IL_0002  30 02             bgt.s        2 (IL_0006)
IL_0004  03                ldarg.1     
IL_0005  2a                ret         
IL_0006  02                ldarg.0     
IL_0007  2a                ret         

Then when the JIT runs it’s processed as 3 basic-blocks (BB01, BB02 and BB03):

Importing BB01 (PC=000) of 'TestNamespace.TestClass:MaxMethod(int,int):int'
    [ 0]   0 (0x000) ldarg.0
    [ 1]   1 (0x001) ldarg.1
    [ 2]   2 (0x002) bgt.s
           [000005] ------------             ▌  stmtExpr  void  (IL 0x000...  ???)
           [000004] ------------             └──▌  jmpTrue   void  
           [000002] ------------                │  ┌──▌  lclVar    int    V01 arg1         
           [000003] ------------                └──▌  >         int   
           [000001] ------------                   └──▌  lclVar    int    V00 arg0         

Importing BB03 (PC=006) of 'TestNamespace.TestClass:MaxMethod(int,int):int'
    [ 0]   6 (0x006) ldarg.0
    [ 1]   7 (0x007) ret
           [000009] ------------             ▌  stmtExpr  void  (IL 0x006...  ???)
           [000008] ------------             └──▌  return    int   
           [000007] ------------                └──▌  lclVar    int    V00 arg0         

Importing BB02 (PC=004) of 'TestNamespace.TestClass:MaxMethod(int,int):int'
    [ 0]   4 (0x004) ldarg.1
    [ 1]   5 (0x005) ret
           [000013] ------------             ▌  stmtExpr  void  (IL 0x004...  ???)
           [000012] ------------             └──▌  return    int   
           [000011] ------------                └──▌  lclVar    int    V01 arg1         

Before finally being turned into the following assembly code, which is way more efficient. It contains just a cmp, a jg and a couple of mov instructions, but crucially it’s all done in-line, it doesn’t need call out to another method.

// Assembly listing for method TestNamespace.TestClass:MaxMethod(int,int):int
// Emitting BLENDED_CODE for X64 CPU with AVX
// optimized code
// rsp based frame
// partially interruptible
// Final local variable assignments
//   V00 arg0         [V00,T00] (  4,  3.50)     int  ->  rcx
//   V01 arg1         [V01,T01] (  4,  3.50)     int  ->  rdx
// # V02 OutArgs      [V02    ] (  1,  1   )  lclBlk ( 0) [rsp+0x00]
// Lcl frame size = 0


       3BCA                 cmp      ecx, edx
       7F03                 jg       SHORT G_M32709_IG04
       8BC2                 mov      eax, edx

       C3                   ret

       8BC1                 mov      eax, ecx

       C3                   ret


I got the idea for doing this from the Appendix of the excellent book Shared Source CLI Essentials - Amazon, you can also download a copy of the 2nd edition if you don’t want to purchase the print one.

In Appendix B the authors of the book reproduced the work that Peter Drayton did to add an Exponentiation op-code to the SSCLI, which inspired this entire post, so thanks for that!!

Appendix B - Add a new CIL opcode

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