.NET IL: Into the Marianas Trench

Are you interested in writing compilers, targeting Web Assembly, finding security issues automatically, binary analysis, or understanding performance at a low level? While it’s always good to know how your language works, the benefits of understanding the intermediate language extend to metaprogramming and analysis across multiple source languages. Learning how to work with intermediate languages allows you to write programs which would seem unattainable otherwise. You will learn not only how IL works but how it compares with LLVM IR, Java Bytecode, and other intermediate representations. No mere “deep dive,” you’ll leave this talk really understanding how C# turns into microcode and how to use that information to do “impossible” things.

Transcript

1. C R A I G S T U N T Z ∈ I M P R O V I N G
   . N E T I L :
   I N T O T H E M A R I A N A S T R E N C H
   https://speakerdeck.com/craigstuntz
   

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2. N O A A O K E A N O S E X P L O R E R 2 0 1 6
   A L L P H O T O S F R O M
   http://oceanexplorer.noaa.gov/okeanos/explorations/ex1605/welcome.html
   

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3. P R E V I E W
   • What is an intermediate language?
   • Why work at this level?
   • Specifics of .NET IL
   • Cool tools and techniques
   • Impossible things!
   

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4. C O N T I N U O U S
   W H Y
   “Execution powered by a new IL interpreter”
   

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5. M O N O
   W H Y
   

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6. M O N O T O U C H
   W H Y
   

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7. U N I T Y
   W H Y
   

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8. J S I L
   W H Y ?
   

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9. C O M P I L E R S
   W H Y
   • Emit module binaries directly
   • Emit modules via Reflection.Emit
   • Emit IL text representation, compile that with ILASM
   

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10. E V E RY D E C O M P I L E R
    W H Y
    

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11. D O T F U S C AT O R
    W H Y
    “Dotfuscator uses ildasm and ilasm to process the input
    assemblies.”
    

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12. F I Z I L
    W H Y
    

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13. I N T E R M E D I AT E L A N G U A G E S
    A L L A B O U T
    

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14. C O M P I L E R S , G E N E R A L LY
    I N T E R P R E T E R
    I N P U T
    C O D E
    M A C H I N E
    C O D E
    C O M P I L E R
    C O D E
    M A C H I N E
    C O D E
    

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15. C O M P I L I N G C #
    C S C
    C O D E . N E T I L
    Compile (Developer Machine)
    R y u J I T
    I L
    M A C H I N E
    C O D E
    .NET Virtual Execution System (End User Machine)
    

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16. IL_0000: nop
    IL_0001: ldstr "Hello World!"
    IL_0006: call void
    [System.Console]System.Console::WriteLine(
    {
    Console.WriteLine("Hello World!");
    }
    00007FFBA9E60B43 mov rcx,208F7F93068h
    00007FFBA9E60B4D mov rcx,qword ptr [rcx]
    00007FFBA9E60B50 call 00007FFBA9E60708
    00007FFBA9E60B55 nop
    J I T C O M P I L AT I O N
    I L
    C# x 6 4
    

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17. I N T E R M E D I AT E L A N G U A G E S , E V E RY W H E R E …
    I L
    C# x 6 4
    B Y T E C O D E
    J AVA x 6 4
    WA S M
    C++ x 6 4
    

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18. B Y A N Y O T H E R N A M E …
    B Y T E C O D E
    I N T E R M E D I AT E
    R E P R E S E N TAT I O N
    I L
    WA S M
    S T G
    P - C O D E
    

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19. – D R . R I C H A R D H I P P
    “SQLite consists of…
    -Compiler to translate SQL into byte code
    -Virtual Machine to evaluate the byte code”
    Lecture presented at Ohio State University, 10 October 2017
    

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20. . N E T I L J AVA B Y T E C O D E
    S T R U C T S ?
    ✅ ❌
    U N S I G N E D I N T S ?
    ✅ ❌
    G E N E R I C S ?
    ✅ ❌
    o u t & re f A R G S ?
    ✅ ❌
    

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21. – J O H N B A C K U S
    “Underlying every programming language is a
    model of a computing system that its programs
    control.”
    

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22. . N E T I L /
    J AVA B Y T E C O D E
    L LV M I R
    M A C H I N E
    I N D E P E N D E N T ?
    ✅ ❌
    Q U I C K T O J I T ?
    ✅ ❌
    U N D E F I N E D B E H AV I O R ? A T I N Y B I T
    E N O U G H T O
    I M P L E M E N T C
    R E G I S T E R S ?
    ❌ ✅
    

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23. I N T E R M E D I AT E
    L A N G U A G E S
    A S M
    O P T I M I Z E D ?
    ✅ ❌
    S T R O N G LY
    D E V I C E - S P E C I F I C ?
    ❌ ✅
    C A N B E C O M P I L E D ?
    ✅ ✅
    C A N B E I N T E R P R E T E D /
    J I T T E D ?
    ✅ ❌
    C A N B E R E - O P T I M I Z E D
    AT R U N T I M E ?
    ✅ ❌
    

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24. I N T E R M E D I AT E L A N G U A G E
    . N E T
    

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25. H E L L O W O R L D , R E V I S I T E D static void Main(string[] args)
    {
    Console.WriteLine("Hello World!");
    }
    .method private hidebysig static
    void Main (
    string[] args
    ) cil managed
    {
    $% Code Size: 13 (0xD) bytes
    .maxstack 8
    .entrypoint
    IL_0000: nop
    IL_0001: ldstr "Hello World!"
    IL_0006: call void [System.Console]System.Console::WriteLine(string)
    IL_000B: nop
    IL_000C: ret
    }
    

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26. R E P R E S E N TAT I O N S
    • Binary (Assembly/Module)
    • Text (ILAsm format)
    

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27. A R E A L LY C O N F U S I N G T H I N G
    • CIL: The instruction set understood by the VES
    • ILAsm: Short for “IL Assembly Language”
    • ILASM: A utility which compiles an extended form
    of ILAsm to binary assemblies.
    

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28. M E M O RY M O D E L S
    W i n / x 8 6
    R E G I S T E R S
    S TA C K
    H E A P
    I L
    A R G U M E N T TA B L E
    E VA L U AT I O N S TA C K
    L O C A L S TA B L E
    F I E L D S
    

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29. M E TA D ATA
    .NET IL Assembler, by Serge Lidin, p. 188
    

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30. P R I M I T I V E T Y P E S
    • Void
    • Bool
    • Certain numeric types
    • signed/unsigned int8, int16, int32, int64, native int
    • Single/double float
    • Chars
    • TypedRef
    • Pointers
    • Signed or unsigned, implementation defined size
    

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31. E VA L U AT I O N S TA C K T Y P E S
    • Certain numeric types
    • int32, int64, native int, F
    • Chars
    • Object references (types undistinguished)
    • Pointers
    • Managed
    • Unmanaged
    • User-defined value types
    

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32. FA K I N G O T H E R T Y P E S
    • Unsigned integers
    • Boolean
    • Chars
    • Arrays
    • Enums
    • Native (interop) types
    

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33. E N U M S
    .class public enum Color {
    .field public specialname int32 __value
    .field public static literal valuetype Color Red = int32 (1)
    .field public static literal valuetype Color Green = int32 (2)
    .field public static literal valuetype Color Blue = int32 (3)
    }
    

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34. C L A S S E S
    .namespace CodeMash.Food
    {
    .class public Pizza
    { $$. }
    }
    

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35. C L A S S E S
    .class public CodeMash.Food.Pizza
    { $$. }
    

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36. C L A S S E S
    .class public auto ansi abstract sealed beforefieldinit ImpossibLe.TailCall
    extends [mscorlib]System.Object
    {
    Auto field layout
    (default)
    Marshal strings to
    unmanaged code as ANSI
    This is complicated.
    

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37. F I E L D S
    .field private static valuetype [Mono.C ecil]Mono.C ecil.Cil.OpCode[] callInstructions
    

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38. M E T H O D S
    T I N Y H E A D E R
    C I L C O D E
    ( < 6 4 B Y T E S )
    FAT H E A D E R
    ( F L A G S , M A X S TA C K ,
    L O C A L S )
    C I L C O D E
    S E H TA B L E
    

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39. L O C A L S
    .locals init (
    [0] class ImpossibLe.TailCall/'<>c__DisplayClass0_0`2' 'CS$<>8__locals0',
    [1] string 'assembly',
    [2] string assemblyExt,
    [3] string tempFileName,
    [4] class [Mono.C ecil]Mono.C ecil.AssemblyDefinition assemblyDefinition,
    [5] class [Mono.C ecil]Mono.C ecil.TypeDefinition typeDefinition,
    [6] class [Mono.C ecil]Mono.C ecil.MethodDefinition definition,
    [7] class [Mono.C ecil]Mono.C ecil.MethodReference methodReference,
    [8] class [Mono.C ecil]Mono.C ecil.Cil.ILProcessor ilProcessor,
    [9] class ImpossibLe.TailCall/TailCallData tailCallData,
    [10] class [mscorlib]System.R eflection.Assembly rewrittenAssembly,
    [11] class [mscorlib]System.Type rewrittenType,
    [12] class [mscorlib]System.R eflection.MethodInfo rewrittenMethod,
    [13] class [System.Core]System.Linq.Expressions.Expression 'instance',
    [14] class [mscorlib]System.Func`3 result,
    [15] int32 index,
    [16] bool,
    [17] class [mscorlib]System.Collections.Generic.IEnumerator`1,
    [18] class ImpossibLe.TailCall/StlocBrReturnData otherExit,
    [19] bool,
    [20] class [mscorlib]System.Func`3
    )
    

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40. T H E E VA L U AT I O N S TA C K
    1 : i n t
    2 : i n t
    3 : i n t
    1 : i n t
    ldc.i4.1
    ldc.i4.2
    add
    

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41. a d d S TA C K T R A N S I T I O N A L B E H AV I O R
    The stack transitional behavior, in sequential order, is:
    1. value1 is pushed onto the stack.
    2. value2 is pushed onto the stack.
    3. value2 and value1 are popped from the stack; value1 is added
    to value2.
    4. The result is pushed onto the stack.
    https://msdn.microsoft.com/en-us/library/system.reflection.emit.opcodes.add(v=vs.110).aspx
    

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42. S TAT E O F T H E S TA C K
    • Empty on method entry and exit
    • Stack will be mapped to registers by
    the runtime when possible
    • “Stack underflow” errors?
    

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43. V E R I F I A B L E I L
    • Subset of “Correct” IL
    • Don’t confuse with C# “safe”
    • Restrictions on managed pointers
    and unmanaged code
    • Other instructions limited to
    verifiability conditions
    • Many other restrictions!
    

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44. V E R I F I A B L E I L
    PS> peverify C:\Users\craig\AppData\Local\Temp\tmpBC4F.tmp
    Microsoft (R) .NET Framework PE Verifier. Version 4.0.30319.0
    Copyright (c) Microsoft Corporation. All rights reserved.
    [IL]: Error: [C:\Users\craig\AppData\Local\Temp\tmpBC4F.tmp :
    ImpossibLe.Program::SumTailRecursive][offset 0x0000000D] Branch
    out of the method.
    

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45. I N S T R U C T I O N S
    ldc.i4 314
    ldc.i4 159
    bge SomeLabel
    

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46. S H O R T F O R M S , U N S I G N E D VA L U E S , C O N S I S T
    I N S T R U C T I O N PA R A M E T E R T Y P E
    E VA L U AT I O N S TA C K
    VA L U E T Y P E
    b g e L O N G I N T S I G N E D
    b g e . s S H O R T I N T S I G N E D
    b g e . u n L O N G I N T U N S I G N E D
    b g e . u n . s S H O R T I N T U N S I G N E D
    l d c . i 4 . 0 C O N S TA N T 0 N / A
    

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47. A R I T H M E T I C I N S T R U C T I O N S
    static int Add1(int number)
    {
    return number + 1;
    }
    ldarg.0
    ldc.i4.1
    add
    ret
    

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48. A R I T H M E T I C I N S T R U C T I O N S
    • nop
    • dup/pop
    • ldc.i4/ldc.r4/ldind.*/stind
    • add/sub/mul/div/rem/neg
    • and/or/xor/not
    • shl/shr
    • conv.*
    • ceq/cgt/clt/ckfinite
    • cpblk/initblk
    

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49. L A B E L S A N D C O N T R O L F L O W
    static int Add1(int number)
    {
    return number + 1;
    }
    ldarg.0
    ldc.i4.1
    add
    ret
    

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50. L A B E L S A N D C O N T R O L F L O W
    static int Add1Log(int number)
    {
    try
    {
    return number + 1;
    }
    finally
    {
    Debug.Write("Done!");
    }
    }
    .try
    {
    ldarg.0
    ldc.i4.1
    add
    stloc.0
    leave.s IL_0016
    }
    finally
    {
    ldstr "Done!"
    call void [System.Diagnosti
    endfinally
    }
    ldloc.0
    ret
    

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51. L A B E L S A N D C O N T R O L F L O W
    • br
    • brfalse/brtrue
    • beq/bne
    • bge/bgt/ble/blt
    • switch
    • break
    • leave/endfilter/endfinally
    • ret
    

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52. A R G U M E N T S A N D L O C A L S
    static int Add1(int number)
    {
    return number + 1;
    }
    ldarg.0
    ldc.i4.1
    add
    ret
    

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53. A R G U M E N T S A N D L O C A L S
    • ldarg/ldarga
    • starg
    • arglist
    • ldloc/ldloca
    • stloc
    • localloc
    

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54. F I E L D S
    var x = Guid.Empty;
    ldsfld valuetype System.Guid::Empty
    stloc.0
    

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55. F I E L D S
    • ldfld/ldsfld/ldflda/ldsflda
    • stfld/stsfld
    • unaligned/volatile
    

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56. I N V O K I N G M E T H O D S
    static int Add1Log(int number)
    {
    try
    {
    return number + 1;
    }
    finally
    {
    Debug.Write("Done!");
    }
    }
    .try
    {
    ldarg.0
    ldc.i4.1
    add
    stloc.0
    leave.s IL_0016
    }
    finally
    {
    ldstr "Done!"
    call void [System.Diagnosti
    endfinally
    }
    ldloc.0
    ret
    

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57. I N V O K I N G M E T H O D S
    • jmp
    • call/callvirt
    • ldftn/ldvirtftn
    • calli
    • tail/constrained
    

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58. A D D R E S S I N G O B J E C T S
    var x = new Object();
    newobj instance void
    System.Object::.ctor()
    stloc.0
    

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59. A D D R E S S I N G O B J E C T S
    • ldnull
    • ldstr
    • ldobj/stobj/cpobj
    • newobj/initobj
    • castclass/isinst
    • box/unbox/unbox.any
    • mkrefany/refanytype/refanyval
    • ldtoken
    • sizeof
    • throw/rethrow
    

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60. A R R AY S
    var x = new int[1];
    x[0] = 1;
    ldc.i4.1
    newarr [System.Runtime]System.Int32
    stloc.0
    ldloc.0
    ldc.i4.0
    ldc.i4.1
    stelem.i4
    

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61. A R R AY S
    • newarr
    • ldlen
    • ldelema
    • ldelem.*/stelem.*
    • readonly
    

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62. D ATA S E C T I O N
    .class public value sealed MagicNumber
    {
    .field public static int32 MagicOne at D_00
    }
    .data D_00 = int32 (123)
    

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63. E X C E P T I O N S
    .try
    {
    IL_0002: call uint64 ImpossibLe.Program::WillNotCrash()
    IL_0007: stloc.0
    IL_0008: ldloca.s result
    IL_000A: call instance string [mscorlib]System.UInt64::ToString()
    IL_000F: call void [mscorlib]System.Console::WriteLine(string)
    IL_0016: leave.s IL_0034
    } $% end .try
    catch [mscorlib]System.Exception
    {
    IL_0018: stloc.1
    IL_001A: call class [mscorlib]System.IO.TextWriter [mscorlib]System.Console::get_Error()
    IL_001F: ldloc.1
    IL_0020: callvirt instance string [mscorlib]System.Object::ToString()
    IL_0025: callvirt instance void [mscorlib]System.IO.TextWriter::WriteLine(string)
    IL_002B: call string [mscorlib]System.Console::ReadLine()
    IL_0030: pop
    IL_0032: leave.s IL_0034
    } $% end handler
    IL_0034: ret
    

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64. F I N A L LY
    .method public static
    int32 main (
    string[] argv
    ) cil managed
    {
    /* $$. */
    IL_0000: call void [Fizil.Instrumentation]Fizil.Instrumentation.Instrument::Open()
    .try
    {
    /* Lots. Of. Code.... */
    IL_00FF: leave.s IL_0107
    } $% end .try
    finally
    {
    IL_0101: call void [Fizil.Instrumentation]Fizil.Instrumentation.Instrument::Close()
    IL_0106: endfinally
    } $% end handler
    L_0107: ldloc.s 12
    IL_0109: ret
    } $% end of method Program::main
    

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65. I L U N U S E D I N C #
    • Global (*to a module) variables
    • Classless methods
    • Tail calls
    • Memberless value types as static data
    

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66. D I S A S S E M B LY
    

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67. I L D A S M
    >ildasm /TEXT Program.exe
    $% Microsoft (R) .NET Framework IL Disassembler. Version 4.6.1055.0
    $% Copyright (c) Microsoft Corporation. All rights reserved.
    $% Metadata version: v4.0.30319
    .assembly extern mscorlib
    {
    .publickeytoken = (B7 7A 5C 56 19 34 E0 89 ) $% .z\V.4..
    .ver 4:0:0:0
    }
    .assembly extern Mono.C ecil
    {
    .publickeytoken = (07 38 EB 9F 13 2E D7 56 ) $% .8.....V
    .ver 0:9:6:0
    }
    .assembly extern System.Core
    {
    .publickeytoken = (B7 7A 5C 56 19 34 E0 89 ) $% .z\V.4..
    .ver 4:0:0:0
    }
    

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68. L I N Q PA D
    

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69. d n S p y
    

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70. A S S E M B LY
    

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71. I L A S M
    

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72. R E F L E C T I O N . E M I T
    let private emit (ilg : Emit.ILGenerator) inst =
    match inst with
    | Add $→ ilg.Emit(OpCodes.Add)
    | Call mi $→ ilg.Emit(OpCodes.Call, mi)
    | Callvirt mi $→ ilg.Emit(OpCodes.Callvirt, mi)
    | DeclareLocal t $→ ignore(ilg.D eclareLocal(t))
    | Div $→ ilg.Emit(OpCodes.Div)
    | LdArg_0 $→ ilg.Emit(OpCodes.Ldarg_0)
    | Ldc_I4 n $→ ilg.Emit(OpCodes.Ldc_I4, n)
    | Ldc_I4_0 $→ ilg.Emit(OpCodes.Ldc_I4_0)
    https://github.com/CraigStuntz/TinyLanguage/blob/ea4b7dd5a2bd454a5366247de65e18b2aa5de9da/TinyLanguage/Il.fs#L61
    

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73. M O N O . C E C I L / D N L I B
    let private removeStrongName (assemblyDefinition : AssemblyDefinition) =
    let name = assemblyDefinition.Name;
    name.HasPublicKey 0← false;
    name.P ublicKey 0← Array.empty;
    assemblyDefinition.Modules
    |> Seq.iter (
    fun moduleDefinition $→
    moduleDefinition.Attributes 0← moduleDefinition.Attributes &&& ˜˜˜ModuleAttributes.StrongNameSigned)
    let aptca = assemblyDefinition.CustomAttributes.FirstOrDefault(
    fun attr $→ attr.AttributeType.FullName =
    typeof.FullName)
    assemblyDefinition.CustomAttributes.Remove aptca
    |> ignore
    

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74. I L I N T E R P R E T E R
    http://mattwarren.org/2017/03/30/The-.NET-IL-Interpreter/
    

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75. I M P O S S I B L E T H I N G S
    

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76. M O D I F Y I N G B I N A R I E S
    let private insertTraceInstruction(ilProcessor, before, state) =
    let compileTimeRandom = state.Random.Next(0, UInt16.MaxValue |> Convert.ToInt32)
    let ldArg = ilProcessor.Create(OpCodes.Ldc_I4, compileTimeRandom)
    let callTrace = ilProcessor.Create(OpCodes.Call, state.Trace)
    ilProcessor.InsertBefore(before, ldArg)
    ilProcessor.InsertAfter (ldArg, callTrace)
    https://github.com/CraigStuntz/Fizil/blob/master/Fizil.Fuzzer/CilInstrument.fs
    

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77. W E I R D
    I N S T R U C T I O N S
    • tail
    • Global functions
    

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78. TA I L C A L L S
    1 + 2 + 3 + 4 + 5 = 15
    

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79. TA I L C A L L S
    static UInt64 SumTailRecursive(UInt64 n, UInt64 accum)
    {
    if (n < 1)
    {
    return accum;
    }
    else
    {
    return SumTailRecursive(n - 1, n + accum);
    }
    }
    return SumTailRecursive(30000, 0);
    

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80. TA I L C A L L S
    

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81. TA I L C A L L S
    var tailCallVersion = TailCall.Rewrite(null, SumTailRecursive);
    return tailCallVersion(50000, 0);
    

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82. TA I L C A L L S
    

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83. IL_0000: nop
    IL_0001: ldarg.0
    IL_0002: ldc.i4.1
    IL_0003: conv.i8
    IL_0004: clt.un
    IL_0006: stloc.0
    IL_0007: ldloc.0
    IL_0008: brfalse.s IL_000F
    IL_000A: nop
    IL_000B: ldarg.1
    IL_000C: stloc.1
    IL_000D: br.s IL_001F
    IL_000F: nop
    IL_0010: ldarg.0
    IL_0011: ldc.i4.1
    IL_0012: conv.i8
    IL_0013: sub
    IL_0014: ldarg.0
    IL_0015: ldarg.1
    IL_0016: add
    IL_0017: call uint64 ImpossibLe.Program::SumTailRecursive(uint64,
    uint64)
    IL_001C: stloc.1
    IL_001D: br.s IL_001F
    IL_001F: ldloc.1
    IL_0020: ret
    IL_0000: nop
    IL_0001: ldarg.0
    IL_0002: ldc.i4.1
    IL_0003: conv.i8
    IL_0004: clt.un
    IL_0006: stloc.0
    IL_0007: ldloc.0
    IL_0008: brfalse.s IL_000D
    IL_000A: nop
    IL_000B: ldarg.1
    IL_000C: ret
    IL_000D: nop
    IL_000E: ldarg.0
    IL_000F: ldc.i4.1
    IL_0010: conv.i8
    IL_0011: sub
    IL_0012: ldarg.0
    IL_0013: ldarg.1
    IL_0014: add
    IL_0015: tail.
    IL_0017: call uint64
    RewrittenNamespace.RewrittenType::SumTailRecursive(uint64, uint64)
    IL_001C: ret
    ❌ Step 1
    Step 2
    Step 3
    TA I L C A L L S
    Base case
    exit
    Recursive
    case exit
    

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84. TA I L C A L L S
    TailCallData tailCallData = TryFindTailCall(definition.Body.Instructions, definition);
    while (tailCallData 02 null)
    {
    $% rewrite as tail call
    $% Step 1: Remove stloc/ldloc/br.s instructions before ret instruction
    for (var index = tailCallData.RetIndex -1; index > tailCallData.CallIndex; index--)
    {
    ilProcessor.Remove(definition.Body.Instructions[index]);
    }
    $% Step 2: Rewrite any stloc/br "returns" as proper rets
    foreach (var otherExit in tailCallData.OtherExits)
    {
    ilProcessor.R eplace(otherExit.BrInstruction, Instruction.Create(OpCodes.Ret));
    ilProcessor.Remove(otherExit.StlocInstruction);
    }
    $% Step 3: Insert "tail" IL instruction before call
    ilProcessor.InsertBefore(tailCallData.CallInstruction, ilProcessor.Create(OpCodes.Tail));
    $% any more?
    tailCallData = TryFindTailCall(definition.Body.Instructions, definition);
    }
    https://github.com/CraigStuntz/ImpossibLe
    

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85. F U R T H E R R E A D I N G
    • .NET IL Assembler, by Serge Lidin
    • ECMA-335 Standard, Common Language Infrastructure
    • LLVM Language Reference Manual
    

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86. View Slide

87. C O N TA C T
    craig.stuntz@improving.com
    @craigstuntz
    http://paperswelove.org/chapter/columbus/
    https://speakerdeck.com/craigstuntz
    https://github.com/CraigStuntz/
    

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