Programs that Write Programs: How Compilers Work

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Programs that Write Programs Craig Stuntz https://speakerdeck.com/craigstuntz https://github.com/CraigStuntz/TinyLanguage

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What Do Developers Do?

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–Steve Yegge “You're actually surrounded by compilation problems. You run into them almost every day.” http://steve-yegge.blogspot.ca/2007/06/rich-programmer-food.html

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–Greenspun’s Tenth Rule “Any sufficiently complicated C or Fortran program contains an ad hoc, informally- specified, bug-ridden, slow implementation of half of Co!"on Lisp.” https://commons.wikimedia.org/wiki/File:Philip_Greenspun_and_Alex_the_dog.jpg

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The Hoover Dam

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Generalize the Problem

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–Eugene Wallingford “…compilers ultimately depend on a single big idea from the theory of computer science: that a certain kind of machine can simulate anything — including itself. As a result, this certain kind of machine, the Turing machine, is the very definition of computability.” http://www.cs.uni.edu/~wallingf/blog/archives/monthly/2015-09.html#e2015-09-03T15_26_47.htm

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Compiler

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Compiler Interpreter

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code exe

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code exe

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Useful Bits • Regular Expressions (lexing) • Deserializers (parsing) • Linters, static analysis (syntax, type checking) • Solvers, theorem provers (optimization) • Code migration tools (compilers!)

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A → B

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Source code → Program JPEG file → Image on screen Source code → Potential style error list JSON → Object graph Code with 2 digit years → Y2K compliant code VB6 → C# Object graph → User interface markup Algorithm → Faster, equivalent algorithm

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Designing with Formal Methods

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#define D define #D Y return #D R for #D e while #D I printf #D l int #D W if #D C y=v+111;H(x,v)*y++= *x #D H(a,b)R(a=b+11;au){R(w=i=0;i<4;i++)w+=(m=v[h[i]])==f?300:m==q?-300:(t=v[ih[i]])==f?-50: t==q?50:0;Y w;}H(z,0){W(E(v,z,f,100)){c++;w= -S(d+1,n,q,0,-b,-j);W(w>j){g=bz=z; j=w;W(w#$b%&w#$8003)Y w;}}}W(!c){g=0;W(_){H(x,v)c+= *x==f?1:*x==3-f?-1:0;Y c>0? 8000+c:c-8000;}C;j= -S(d+1,n,q,1,-b,-j);)bz=g;Y d#$u-1?j+(c'(3):j;}main(){R(;t< 1600;t+=100)R(m=0;m<100;m++)V[t+m]=m<11%&m>88%&(m+1)%10<2?3:0;I("Level:");V[44] =V[55]=1;V[45]=V[54]=2;s(u);e(lv>0){Z do{I("You:");s(m);}e(!E(V,m,2,0))*m+,99); W(m+,99)lv--;W(lv<15)*u<10)u+=2;U("Wait\n");I("Value:%d\n",S(0,V,1,0,-9000,9000 ));I("move: %d\n",(lv-=E(V,bz,1,0),bz));}}E(v,z,f,o)l*v;{l*j,q=3-f,g=0,i,w,*k=v +z;W(*k==0)R(i=7;i#$0;i--){j=k+(w=r[i]);e(*j==q)j+=w;W(*j==f)*j-w+,k){W(!g){g=1 ;C;}e(j+,k)*((j-=w)+o)=f;}}Y g;}

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Duff’s Device There Are No Edge Cases In Progra"#ing Languages send(to, from, count) register short *to, *from; register count; { register n = (count + 7) / 8; switch (count % 8) { case 0: do { *to = *from++; case 7: *to = *from++; case 6: *to = *from++; case 5: *to = *from++; case 4: *to = *from++; case 3: *to = *from++; case 2: *to = *from++; case 1: *to = *from++; } while (--n > 0); } }

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1 + 2 + 3 + … + 100 = 100 * 101 / 2 = 5050

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Lexer → Regular Expressions Parser → Context Free Gra!"ar Optimizer → Algebra Type Checker → Logical Inference Rules Code Generator → Denotational Semantics

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–Leslie Lamport “You don’t achieve simplicity by thinking in terms of complicated languages. Simplicity requires thinking abstractly before you start implementing.” http://www.heidelberg-laureate-forum.org/blog/video/lecture-monday-august-24-2015-leslie-lamport/ https://commons.wikimedia.org/wiki/File:Leslie_Lamport.jpg

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A Few Important Concepts

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Syntax x = x + 1; alert(x); Sequence Assign Invoke x add x 1 alert x

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Semantics name = "Nate" # +/ "Nate" String.upcase(name) # +/ "NATE" name # +/ "Nate" name = "Nate" # +/ "Nate" name.upcase! # +/ "NATE" name # +/ "NATE" http://www.natescottwest.com/elixir-for-rubyists-part-2/

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Semantics Imports System Namespace Hello Class HelloWorld Overloads Shared Sub Main(ByVal args() As String) Dim name As String = "VB.NET" 'See if argument passed If args.Length = 1 Then name = args(0) Console.WriteLine("Hello, " & name & "!") End Sub End Class End Namespace using System; namespace Hello { public class HelloWorld { public static void Main(string[] args) { string name = "C#"; !" See if argument passed if (args.Length == 1) name = args[0]; Console.WriteLine("Hello, " + name + "!"); } } } http://www.harding.edu/fmccown/vbnet_csharp_comparison.html

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Front End: Understand Language Back End: Emit Code

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Lexer IL Generator Parser Type Checker Optimizer Optimizer Object Code Generator Binder

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OK, so let’s compile something already! module Compiler let compile = Lexer.lex 01 Parser.parse 01 Binder.bind 01 Optimize Binding.optimize 01 IlGenerator.codegen 01 Railway.map OptimizeIl.optimize 01 Railway.map Il.toAssemblyBuilder

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(inc -1)

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(inc -1) Ldc.i4 -1 Ldc.i4 1 Add

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(inc -1) Ldc.i4 -1 Ldc.i4 1 Add Ldc.i4.0

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(inc -1) Lex LeftParen, Identifier(inc), Number(-1), RightParen Parse Apply “inc” to -1 Type check “inc” exists and takes an int argument, and -1 is an int. Great! Optimize -1 + 1 = 0, so just emit int 0! IL generate Ldc.i4 0 Optimize Ldc.i4 0 → Ldc.i4.0 Object code Produce assembly with entry point which contains the IL generated

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(defun add-1 (int x) (inc x)) (defun main () (print (add-1 2)))

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Lexer What Problem Are We Solving? String → Sequence of tokens Non-Compiler Example Text search

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Lexer Search “am” I am. You are.

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Lexer

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Lexer (inc -1)

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Lexer (inc -1) “(“ “inc” “-1” “)” LeftParenthesis Identifier(“inc”) LiteralInt(-1) RightParenthesis

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Lexer ( -1)

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Lexer let rec private lexChars (source: char list) : Lexeme list = match source with | '(' :: rest !→ LeftParenthesis :: lexChars rest | ')' :: rest !→ RightParenthesis :: lexChars rest | '"' :: rest !→ lexString(rest, "") | c :: rest when isIdentifierStart c !→ lexName (source, "") | d :: rest when System.Char.IsDigit d !→ lexNumber(source, "") | [] !→ [] | w :: rest when System.Char.IsWhiteSpace w !→ lexChars rest | c :: rest !→ Unrecognized c :: lexChars rest

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Lexer http://stackoverﬂow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags

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Lexer http://www.regular-expressions.info/email.html “So even when following official standards, there are still trade-offs to be made. Don't blindly copy regular expressions from online libraries or discussion forums.” -Jan Goyvaerts, regular-expressions.info

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Parser What Problem Are We Solving? Sequence of tokens → Syntax tree Non-Compiler Example Deserialization

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PEMDAS 1 + 2 * 3 1 + (2 * 3)

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Gra"#ar := | := | := “(defun” identifier “)” := number | string | := “(” identifier “)”

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Parser LeftParenthesis Identifier(“inc”) LiteralInt(-1) RightParenthesis

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Parser LeftParenthesis Identifier(“inc”) LiteralInt(-1) RightParenthesis Invoke “inc” -1

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Parser LeftParenthesis Identifier(“inc”) LiteralInt(-1) LiteralInt(-1)

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Parser LeftParenthesis Identifier(“inc”) LiteralInt(-1) LiteralInt(-1) “Expected ‘)’”

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Parser let rec private parseExpression (state : ParseState): ParseState = match state.Remaining with | LeftParenthesis :: Identifier "defun" :: Identifier name :: rest !→ let defun = parseDefun (name, { state with Remaining = rest }) match defun.Expressions, defun.Remaining with | [ ErrorExpr _ ], _ !→ defun | _, RightParenthesis :: remaining !→ { defun with Remaining = remaining } | _, [] !→ error ("Expected ')'.") | _, wrong :: _ !→ error (sprintf "Expected ')'; found %A." wrong) | LeftParenthesis :: Identifier name :: argumentsAndBody !→ let invoke = parseInvoke (name, { state with Remaining = argumentsAndBody }) match invoke.Remaining with | RightParenthesis :: remaining !→ { invoke with Remaining = remaining } | [] !→ error ("Expected ')'.") | wrong :: _ !→ error (sprintf "Expected ')'; found %A." wrong) | LeftParenthesis :: wrong !→ error (sprintf "%A cannot follow '('." wrong)

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Parser

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–Guy Steele “If it's worth telling another progra!"er, it's worth telling the compiler, I think.” https://joshvarty.wordpress.com/2015/08/03/learn-roslyn-now-part-11-introduction-to-code-ﬁxes/

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Parser https://joshvarty.wordpress.com/2015/08/03/learn-roslyn-now-part-11-introduction-to-code-ﬁxes/

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Scope What Problem Are We Solving? What does “x” mean right now? Non-Compiler Example Bounded Context in Domain Driven Design

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Scope https://msujaws.wordpress.com/2011/05/03/static-vs-dynamic-scoping/

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Binding InvokeExpr “inc” -1

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InvokeBinding { FunctionName = "inc" Function = Inc Argument = IntBinding -1} Binding InvokeExpr “inc” -1

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InvokeExpr { Name = "not-a-function" Argument = StringExpr "" } Binding

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InvokeExpr { Name = "not-a-function" Argument = StringExpr "" } Binding “Undefined function ‘not-a-function’.”

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https://msdn.microsoft.com/en-us/library/ms228296.aspx?f=255&MSPPError=-2147217396

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About Those Errors [] member this.``should return error for unbound invocation``() = let source = "(bad-method 2)" let expected = ErrorBinding ( "Undefined function 'bad-method'.", EmptyBinding) let actual = bind source actual |> should equal expected

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About Those Errors http://www.drdobbs.com/architecture-and-design/so-you-want-to-write-your-own-language/240165488?pgno=2

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About Those Errors • Die in a fire http://www.drdobbs.com/architecture-and-design/so-you-want-to-write-your-own-language/240165488?pgno=2

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About Those Errors • Die in a fire • Guess what I meant, not what I said http://www.drdobbs.com/architecture-and-design/so-you-want-to-write-your-own-language/240165488?pgno=2

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About Those Errors • Die in a fire • Guess what I meant, not what I said • Poisoning http://www.drdobbs.com/architecture-and-design/so-you-want-to-write-your-own-language/240165488?pgno=2

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Type Checking What Problem Are We Solving? AST → Boolean “Is it valid?” Non-Compiler Example Linter

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Type Checking ldstr "Hi" ldstr "Hi" div This is bad. Don’t do this.

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Type Inference Rules Γ ⊢ A Γ ⊢ B Γ ⊢ A×B Γ ⊢ v1 :Int Γ ⊢ v2 :Int Γ ⊢ v1 +v2 :Int

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Type Checking • Statically typed • Unityped (“dynamic language”) • Untyped

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Type Checking let rec private toBinding (environment: Map) match expression with | IntExpr n !→ IntBinding n | StringExpr str !→ String Binding str

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Type Checking | InvokeExpr (name, argument) !→ match environment.TryFind name with | Some (Function Binding func) !→ let argumentBinding = toInvokedArgumentBinding environment argument match argumentTypeError argumentBinding func with | None !→ InvokeBinding { FunctionName = name Function = func Argument = argumentBinding } | Some argumentTypeErrorMessage !→ ErrorBinding (argumentTypeErrorMessage, EmptyBinding) | Some bindingType !→ ErrorBinding (sprintf "Expected function; found %A" bindingType, EmptyBinding) | None !→ ErrorBinding (sprintf "Undefined function '%s'." name, EmptyBinding)

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InvokeExpr { Name = "inc" Argument = StringExpr “Oops!" } Type Checking

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InvokeExpr { Name = "inc" Argument = StringExpr “Oops!" } Type Checking “Expected integer; found ‘Oops!’.”

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Optimizers What Problem Are We Solving? Program → Faster, but equivalent program Non-Compiler Example Theorem prover

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Optimization (I) InvokeBinding “inc” -1

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Optimization (I) InvokeBinding “inc” -1 IntBinding 0

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Optimization (I) Invoke “some-method” -1

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Optimization (I) Invoke “some-method” -1 Invoke “some-method” -1

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IL Generation IntBinding 0

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IL Generation IntBinding 0 Ldc.i4 0

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IL Generation IntBinding 0 Ldc.i4 0 Ldc.i4.0

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IL Generation let private writeLineMethod = typeof.GetMethod( "WriteLine", [| typeof |] let private codegenOper = function | IncInt !→ [ Instruction.Ldc_I4_1 Instruction.Add ] | WriteLine !→ [ Instruction.Call writeLineMethod ]

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Optimization (II) Ldc.i4 0

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Optimization (II) Ldc.i4 0 Ldc.i4.0

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Special Tools!

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Compare!

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https://www.ece.cmu.edu/~ganger/712.fall02/papers/p761-thompson.pdf

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Trusting Trust Compiler Executable Compiler Source Code Compiler Executable

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Trusting Trust Compiler Executable Compiler Source Code Trojaned Compiler Executable Trojan Code

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Trusting Trust Trojaned Compiler Executable Benign App Source Code Trojaned App Executable

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Trusting Trust Trojaned Compiler Executable (Benign!) Compiler Source Code Trojaned Compiler Executable

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Conclusion

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Further Reading • Progra!"ing Language Concepts, by Peter Sestoft • Modern Compiler Implementation in ML, by Andrew W. Appel • miniml (608 line implementation of ML subset), by Andrej Bauer • Coursera Compilers Course, by Alex Aiken

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Craig Stuntz @craigstuntz [email protected] https://www.craigstuntz.com https://www.meetup.com/Papers-We-Love-Columbus/ https://speakerdeck.com/craigstuntz https://github.com/CraigStuntz/TinyLanguage