1. Building Languages
for the JVM
Charles Oliver Nutter

2. Me
• Charles Oliver Nutter
• JRuby and JVM guy
• “headius” on most services
• headius@headius.com

3. Who Was I?
• Java EE architect
• Successfully!
• Never wrote a parser
• Never wrote a compiler
• But I wanted to learn how...

4. You
• Java?
• Ruby?
• Python?
• C#?
• Other?

5. Why Create Languages?
• Nothing is perfect
• New problems need new solutions
• Language design can be fun
• Fame and fortune?
• Not really.

6. Why Impl a Language?
• To learn it?
• Sort of...
• To learn the target platform
• Definitely!
• Fame and fortune?
• Well...getting there...

7. Challenges
• Community
• Platform
• Specifications
• Resources

8. Community
• Investment in status quo
• Afraid to stand out
• Known quantities
• Everything else sucks
• Gotta get paid!

9. Platform
• Matching language semantics
• JVM designed around Java
• JVM hides underlying platform
• Challenging to use
• Not bad...C/++ would be way worse
• Community may hate it ;-)

10. Specifications
• Incomplete
• Ruby had none for years
• ...and no complete test suites
• Difficult to implement
• Low level features
• Single-implementation quirks
• Hard or impossible to optimize

11. Resources
• You gotta eat
• Not much money in language work
• Some parts are hard
• OSS is a necessity

12. Why JVM?
• Because I am lazy
• Because VMs are *hard*
• Because I can’t be awesome at everything

13. Ok, Why Really?
• Cross-platform
• Libraries
• Languages
• Memory management
• Tools
• OSS

14. Cross-platform
• OpenJDK: Linux, Windows, Solaris, OS X,
xBSD
• J9: Linux, zLinux, AS/400, ...
• HP: OpenVMS, HP/UX, ...
• Dalvik (Android): Linux on ARM, x86

15. Libraries
• For any need, a dozen libraries
• And a couple of them are good!
• Cross-platform
• Leading edge

16. Selection of languages
• Java
• Scala
• Clojure
• JRuby
• Mirah
• Jython, Groovy, Fantom, Kotlin, Ceylon, ...

17. Memory management
• Best GCs in the world
• Fastest object allocation
• Safe escape hatches like NIO

18. Tools
• Debugging
• Profiling
• Monitoring
• JVM internals

19. Open source?
• FOSS reference impl (OpenJDK)
• Mostly OSS libraries
• Heavy OSS culture
• Strong OSS influence in OpenJDK core

20. Case Study: JRuby

21. Ruby on the JVM
• All of Ruby’s power and beauty
• Solid VM underneath
• “Just another JVM language”

22. JVM Language
• Full interop with Java
• Tricky to do...
• Very rewarding for 99% case
• VM concerns solved
• No need to write a GC
• No need to write a JIT
• ...oh, but wait...

23. More than a JVM
language
• Use native code where JDK fails us
• Paper over ugly bits like CLASSPATH
• Matching Ruby semantics exactly*
• Push JVM forward too!

24. Playing with JRuby
• Simple IRB demo
• JRuby on Rails - see Jano’s talk tomorrow
• JRuby performance
• PotC (???)

25. How did we do it?

26. JRuby Architecture
• Parser
• Abstract Syntax Tree (AST)
• Intermediate Representation (IR)
• Core classes
• Compiler

27. Parser
• Port of MRI’s Bison grammar
• “Jay” parser generator for Java
• Hand-written lexer
• Nearly as fast as the C version
• ...once it gets going

28. system ~/projects/jruby $ jruby -y -e "1 + 1"
push state 0 value null
reduce tate 0 uncover 0
s rule (1) $$1 :
goto from state 0 to 2
push state 2 value null
lex tate 2 reading tIDENTIFIER value Token { Value=load,
s
Position=file:/Users/headius/projects/jruby/lib/jruby.jar!/
jruby/kernel.rb:6}
shift from state 2 to 33
push state 33 value Token { Value=load, Position=file:/Users/
headius/projects/jruby/lib/jruby.jar!/jruby/kernel.rb:6}
lex tate 33 reading tSTRING_BEG value Token { Value=',
s
Position=file:/Users/headius/projects/jruby/lib/jruby.jar!/
jruby/kernel.rb:6}
reduce tate 33 uncover 2
s rule (487) operation : tIDENTIFIER
goto from state 2 to 62
push state 62 value Token { Value=load, Position=file:/Users/
headius/projects/jruby/lib/jruby.jar!/jruby/kernel.rb:6}
reduce tate 62 uncover 62 rule (252) $$6 :
s

29. system ~/projects/jruby $ jruby -y -e "1 + 1"
push state 0 value null
reduce tate 0 uncover 0
s rule (1) $$1 :
goto from state 0 to 2
push state 2 value null
lex tate 2 reading tIDENTIFIER value Token { Value=load,
s
Position=file:/Users/headius/projects/jruby/lib/jruby.jar!/
jruby/kernel.rb:6}
shift from state 2 to 33
You will never need this.
push state 33 value Token { Value=load, Position=file:/Users/
headius/projects/jruby/lib/jruby.jar!/jruby/kernel.rb:6}
lex tate 33 reading tSTRING_BEG value Token { Value=',
s
Position=file:/Users/headius/projects/jruby/lib/jruby.jar!/
jruby/kernel.rb:6}
reduce tate 33 uncover 2
s rule (487) operation : tIDENTIFIER
goto from state 2 to 62
push state 62 value Token { Value=load, Position=file:/Users/
headius/projects/jruby/lib/jruby.jar!/jruby/kernel.rb:6}
reduce tate 62 uncover 62 rule (252) $$6 :
s

30. public class RubyYaccLexer {
public static final Encoding UTF8_ENCODING = UTF8Encoding.INSTANCE;
public static final Encoding USASCII_ENCODING = USASCIIEncoding.INSTANCE;
public static final Encoding ASCII8BIT_ENCODING = ASCIIEncoding.INSTANCE;
private static ByteList END_MARKER = new ByteList(new byte[] {'_', 'E', 'N', 'D', '_', '_'});
private static ByteList BEGIN_DOC_MARKER = new ByteList(new byte[] {'b', 'e', 'g', 'i', 'n'});
private static ByteList END_DOC_MARKER = new ByteList(new byte[] {'e', 'n', 'd'});
private static final HashMap<String, Keyword> map;
static {
map = new HashMap<String, Keyword>();
map.put("end", Keyword.END);
map.put("else", Keyword.ELSE);
map.put("case", Keyword.CASE);
map.put("ensure", Keyword.ENSURE);
map.put("module", Keyword.MODULE);
map.put("elsif", Keyword.ELSIF);
map.put("def", Keyword.DEF);
map.put("rescue", Keyword.RESCUE);
map.put("not", Keyword.NOT);
map.put("then", Keyword.THEN);
map.put("yield", Keyword.YIELD);
map.put("for", Keyword.FOR);
map.put("self", Keyword.SELF);
map.put("false", Keyword.FALSE);

31. public enum Keyword {
END ("end", Tokens.kEND, Tokens.kEND, LexState.EXPR_END),
ELSE ("else", Tokens.kELSE, Tokens.kELSE, LexState.EXPR_BEG),
CASE ("case", Tokens.kCASE, Tokens.kCASE, LexState.EXPR_BEG),
ENSURE ("ensure", Tokens.kENSURE, Tokens.kENSURE, LexState.EXPR_BEG),
MODULE ("module", Tokens.kMODULE, Tokens.kMODULE, LexState.EXPR_BEG),
ELSIF ("elsif", Tokens.kELSIF, Tokens.kELSIF, LexState.EXPR_BEG),
DEF ("def", Tokens.kDEF, Tokens.kDEF, LexState.EXPR_FNAME),
RESCUE ("rescue", Tokens.kRESCUE, Tokens.kRESCUE_MOD, LexState.EXPR_MID),
NOT ("not", Tokens.kNOT, Tokens.kNOT, LexState.EXPR_BEG),
THEN ("then", Tokens.kTHEN, Tokens.kTHEN, LexState.EXPR_BEG),
YIELD ("yield", Tokens.kYIELD, Tokens.kYIELD, LexState.EXPR_ARG),
FOR ("for", Tokens.kFOR, Tokens.kFOR, LexState.EXPR_BEG),
SELF ("self", Tokens.kSELF, Tokens.kSELF, LexState.EXPR_END),
FALSE ("false", Tokens.kFALSE, Tokens.kFALSE, LexState.EXPR_END),
RETRY ("retry", Tokens.kRETRY, Tokens.kRETRY, LexState.EXPR_END),
RETURN ("return", Tokens.kRETURN, Tokens.kRETURN, LexState.EXPR_MID),
TRUE ("true", Tokens.kTRUE, Tokens.kTRUE, LexState.EXPR_END),
IF ("if", Tokens.kIF, Tokens.kIF_MOD, LexState.EXPR_BEG),
DEFINED_P ("defined?", Tokens.kDEFINED, Tokens.kDEFINED, LexState.EXPR_ARG),

32. private int yylex() throws IOException {
int c;
boolean spaceSeen = false;
boolean commandState;
if (lex_strterm != null) {
int tok = lex_strterm.parseString(this, src);
if (tok == Tokens.tSTRING_END || tok == Tokens.tREGEXP_END) {
lex_strterm = null;
setState(LexState.EXPR_END);
}
return tok;
}
commandState = commandStart;
commandStart = false;
loop: for(;;) {
c = src.read();
switch(c) {

33. case '<':
return lessThan(spaceSeen);
case '>':
return greaterThan();
case '"':
return doubleQuote();
case '`':
return backtick(commandState);
case ''':
return singleQuote();
case '?':
return questionMark();
case '&':
return ampersand(spaceSeen);
case '|':
return pipe();
case '+':
return plus(spaceSeen);

34. private int lessThan(boolean spaceSeen) throws IOException {
int c = src.read();
if (c == '<' && lex_state != LexState.EXPR_DOT && lex_state !=
LexState.EXPR_CLASS &&
!isEND() && (!isARG() || spaceSeen)) {
int tok = hereDocumentIdentifier();
if (tok != 0) return tok;
}
determineExpressionState();
switch (c) {
case '=':
if ((c = src.read()) == '>') {
yaccValue = new Token("<=>", getPosition());
return Tokens.tCMP;

35. %%
program : {
lexer.setState(LexState.EXPR_BEG);
support.initTopLocalVariables();
} top_compstmt {
// ENEBO: Removed !compile_for_eval which probably is to reduce
warnings
if ($2 != null) {
/* last expression should not be void */
if ($2 instanceof BlockNode) {
support.checkUselessStatement($<BlockNod
e>2.getLast());
} else {
support.checkUselessStatement($2);
}
}
support.getResult().setAST(support.addRootNode($
2, support.getPosition($2)));
}

36. stmt : kALIAS fitem {
lexer.setState(LexState.EXPR_FNAME);
} fitem {
$$ = support.newAlias($1.getPosition(), $2, $4);
}
| kALIAS tGVAR tGVAR {
$$ = new VAliasNode($1.getPosition(), (String)
$2.getValue(), (String) $3.getValue());
}
| kALIAS tGVAR tBACK_REF {
$$ = new VAliasNode($1.getPosition(), (String)
$2.getValue(), "$" + $<BackRefNode>3.getType());
}
| kALIAS tGVAR tNTH_REF {
support.yyerror("can't make alias for the number
variables");
}
| kUNDEF undef_list {
$$ = $2;
}
| stmt kIF_MOD expr_value {
$$ = new IfNode(support.getPosition($1),
support.getConditionNode($3), $1, null);
}

37. public Object yyparse (RubyYaccLexer yyLex) throws java.io.IOException {
if (yyMax <= 0) yyMax = 256;"" " // initial size
int yyState = 0, yyStates[] = new int[yyMax];" // state stack
Object yyVal = null, yyVals[] = new Object[yyMax];" // value stack
int yyToken = -1;" " " " " // current input
int yyErrorFlag = 0;" " " " // #tokens to shift
yyLoop: for (int yyTop = 0;; ++ yyTop) {
if (yyTop >= yyStates.length) {" " " // dynamically increase
int[] i = new int[yyStates.length+yyMax];
System.arraycopy(yyStates, 0, i, 0, yyStates.length);
yyStates = i;
Object[] o = new Object[yyVals.length+yyMax];
System.arraycopy(yyVals, 0, o, 0, yyVals.length);
yyVals = o;
}
yyStates[yyTop] = yyState;
yyVals[yyTop] = yyVal;
if (yydebug != null) yydebug.push(yyState, yyVal);

38. if (state == null) {
yyVal = yyDefault(yyV > yyTop ? null : yyVals[yyV]);
} else {
yyVal = state.execute(support, lexer, yyVal, yyVals, yyTop);
}

39. states[23] = new ParserState() {
public Object execute(ParserSupport support, RubyYaccLexer lexer, Object
yyVal, Object[] yyVals, int yyTop) {
yyVal = new
IfNode(support.getPosition(((Node)yyVals[-2+yyTop])),
support.getConditionNode(((Node)yyVals[0+yyTop])), ((Node)yyVals[-2+yyTop]),
null);
return yyVal;
}
};
Never look at this.
states[24] = new ParserState() {
public Object execute(ParserSupport support, RubyYaccLexer lexer, Object
yyVal, Object[] yyVals, int yyTop) {
yyVal = new
IfNode(support.getPosition(((Node)yyVals[-2+yyTop])),
support.getConditionNode(((Node)yyVals[0+yyTop])), null,
((Node)yyVals[-2+yyTop]));
return yyVal;
}
};

40. AST
• Interpreted directly
• Specialized in places
• Large and rich

41. $ ast -e "a = true; if a; 2; else; 3; end"
AST:
RootNode 0
BlockNode 0
NewlineNode 0
LocalAsgnNode:a 0
TrueNode:true 0
NewlineNode 0
IfNode 0
LocalVarNode:a 0
NewlineNode 0
FixnumNode 0
NewlineNode 0
FixnumNode 0

42. public class IfNode extends Node {
private final Node condition;
private final Node thenBody;
private final Node elseBody;
public IfNode(ISourcePosition position, Node condition, Node thenBody,
Node elseBody) {
super(position);
assert condition != null : "condition is not null";
// assert thenBody != null : "thenBody is not null";
// assert elseBody != null : "elseBody is not null";
this.condition = condition;
this.thenBody = thenBody;
this.elseBody = elseBody;
}

43. @Override
public IRubyObject interpret(Ruby runtime, ThreadContext context,
IRubyObject self, Block aBlock) {
ISourcePosition position = getPosition();
context.setFile(position.getFile());
context.setLine(position.getStartLine());
IRubyObject result =
condition.interpret(runtime, context, self, aBlock);
if (result.isTrue()) {
return thenBody == null ?
runtime.getNil() :
thenBody.interpret(runtime, context, self, aBlock);
} else {
return elseBody == null ?
runtime.getNil() :
elseBody.interpret(runtime, context, self, aBlock);
}
}

44. IR (future work)
• Control flow graph
• Ruby-specific instruction set
• Optimizing compiler
• Ruby-level optimizations

45. jruby -e “1 + 1”

46. 2011-11-04T05:23:09.375-03:00: IR_Printer:
instrs:
0 %self = recv_self
1 %block(0:0) = recv_closure
2 file_name(-e)
3 line_num(0)
4 %v_0 = call(+, 1:fixnum, [1:fixnum])
5 return(%v_0)
2011-11-04T05:23:09.375-03:00: IR_Printer:
live variables:
%v_0: 4-5

47. a = 1; while a < 10; puts a; a += 1; end

48. 2011-11-04T05:25:23.517-03:00: IR_Printer:
instrs:
0 %self = recv_self
1 %block(0:0) = recv_closure
2 file_name(-e)
3 line_num(0)
4 a(0:1) = 1:fixnum
5 _LOOP_BEGIN_0:
6 %v_1 = call(<, a(0:1), [10:fixnum])
7 beq(%v_1, true, _ITER_BEGIN_0)
8 %v_0 = nil
9 jump _LOOP_END_0
10 _ITER_BEGIN_0:
11 %v_2 = call(puts, %self, [a(0:1)])
12 %v_3 = call(+, a(0:1), [1:fixnum])
13 a(0:1) = copy(%v_3)
14 %v_0 = copy(%v_3)
15 thread_poll
16 _ITER_END_0:
17 jump _LOOP_BEGIN_0
18 _LOOP_END_0:
19 return(%v_0)
2011-11-04T05:25:23.517-03:00: IR_Printer:
live variables:
%v_0: 8-19
%v_1: 6-7
%v_3: 12-14

49. 2011-11-04T05:25:23.518-03:00: IRScope: ################## After
CFG Linearize##################
2011-11-04T05:25:23.518-03:00: IR_Printer:
----------------------------------------
2011-11-04T05:25:23.518-03:00: IR_Printer: Method [root]:
[script]:-e
2011-11-04T05:25:23.518-03:00: IR_Printer:
Graph:
BB [4:LBL_3]:>[7], <[3]
BB [1:LBL_1]:>[8,2]
BB [2:LBL_2]:>[3], <[1]
BB [7:_LOOP_END_0]:>[8], <[4]
BB [8:LBL_4]:<[1,7]
BB [3:_LOOP_BEGIN_0]:>[5,4], <[6,2]
BB [6:_ITER_END_0]:>[3], <[5]
BB [5:_ITER_BEGIN_0]:>[6], <[3]

50. 2011-11-04T05:25:23.517-03:00: IR_Printer:
instrs:
0 %self = recv_self
1 %block(0:0) = recv_closure
2 file_name(-e)
3 line_num(0)
4 a(0:1) = 1:fixnum
5 _LOOP_BEGIN_0:
6 %v_1 = call(<, a(0:1), [10:fixnum])
7 beq(%v_1, true, _ITER_BEGIN_0)
8 %v_0 = nil
9 jump _LOOP_END_0
10 _ITER_BEGIN_0:
11 %v_2 = call(puts, %self, [a(0:1)])
12 %v_3 = call(+, a(0:1), [1:fixnum])
13 a(0:1) = copy(%v_3)
14 %v_0 = copy(%v_3)
15 thread_poll
16 _ITER_END_0:
17 jump _LOOP_BEGIN_0
18 _LOOP_END_0:
19 return(%v_0)
2011-11-04T05:25:23.517-03:00: IR_Printer:
live variables:
%v_0: 8-19
%v_1: 6-7
%v_3: 12-14

51. 2011-11-04T05:25:23.518-03:00: IR_Printer:
Instructions:
BB [4:LBL_3]
%v_0 = nil
BB [1:LBL_1]
BB [2:LBL_2]
%self = recv_self
%block(0:0) = recv_closure
file_name(-e)
line_num(0)
a(0:1) = 1:fixnum
BB [7:_LOOP_END_0]
return(%v_0)
BB [8:LBL_4]
return(nil)

52. Core classes
• Mostly Java-based
• Leverage JDK where possible
• Work around JDK where necessary
• Use Ruby when possible

53. @JRubyClass(name="Fixnum", parent="Integer", include="Precision")
public class RubyFixnum extends RubyInteger {
public static RubyClass createFixnumClass(Ruby runtime) {
RubyClass fixnum = runtime.defineClass("Fixnum", runtime.getInteger(),
ObjectAllocator.NOT_ALLOCATABLE_ALLOCATOR);

54. @JRubyMethod(name = "+")
public IRubyObject op_plus(ThreadContext context, IRubyObject other) {
if (other instanceof RubyFixnum) {
return addFixnum(context, (RubyFixnum)other);
}
return addOther(context, other);
}

55. private IRubyObject addFixnum(ThreadContext context, RubyFixnum other) {
long otherValue = other.value;
long result = value + otherValue;
if (additionOverflowed(value, otherValue, result)) {
return addAsBignum(context, other);
}
return newFixnum(context.getRuntime(), result);
}

56. Compiler
• AST-walking
• ASM bytecode library
• Minimal optimizations
• Invokedynamic really helps
• IR offers new opportunities

57. jruby --bytecode -e “1 + 1”

58. ALOAD 0
INVOKEVIRTUAL ruby/__dash_e__.getCallSite0
ALOAD 1
ALOAD 2
ALOAD 1
GETFIELD org/jruby/runtime/ThreadContext.runtime
INVOKESTATIC org/jruby/RubyFixnum.one
LDC 1
INVOKEVIRTUAL org/jruby/runtime/CallSite.call
ARETURN

59. public class ASTCompiler {
private boolean isAtRoot = true;
public void compileBody(Node node, BodyCompiler context, boolean expr) {
Node oldBodyNode = currentBodyNode;
currentBodyNode = node;
compile(node, context, expr);
currentBodyNode = oldBodyNode;
}
public void compile(Node node, BodyCompiler context, boolean expr) {
if (node == null) {
if (expr) context.loadNil();
return;
}
switch (node.getNodeType()) {
case ALIASNODE:
compileAlias((AliasNode) node, context, expr);
break;
case ANDNODE:
compileAnd(node, context, expr);
break;

60. public void compileIf(Node node, BodyCompiler context, final boolean expr) {
final IfNode ifNode = (IfNode) node;
// optimizations if we know ahead of time it will always be true or false
Node actualCondition = ifNode.getCondition();
while (actualCondition instanceof NewlineNode) {
actualCondition = ((NewlineNode)actualCondition).getNextNode();
}
if (actualCondition.getNodeType().alwaysTrue()) {
// compile condition as non-expr and just compile "then" body
compile(actualCondition, context, false);
compile(ifNode.getThenBody(), context, expr);
} else if (actualCondition.getNodeType().alwaysFalse()) {
// always false or nil
compile(ifNode.getElseBody(), context, expr);
} else {

61. BranchCallback trueCallback = new BranchCallback() {
public void branch(BodyCompiler context) {
if (ifNode.getThenBody() != null) {
compile(ifNode.getThenBody(), context, expr);
} else {
if (expr) context.loadNil();
}
}
};
BranchCallback falseCallback = new BranchCallback() {
public void branch(BodyCompiler context) {
if (ifNode.getElseBody() != null) {
compile(ifNode.getElseBody(), context, expr);
} else {
if (expr) context.loadNil();
}
}
};
// normal
compile(actualCondition, context, true);
context.performBooleanBranch(trueCallback, falseCallback);
}

62. public abstract class BaseBodyCompiler implements BodyCompiler {
protected SkinnyMethodAdapter method;
protected VariableCompiler variableCompiler;
protected InvocationCompiler invocationCompiler;
protected int argParamCount;
protected Label[] currentLoopLabels;
protected Label scopeStart = new Label();
protected Label scopeEnd = new Label();
protected Label redoJump;
protected boolean inNestedMethod = false;
private int lastLine = -1;
private int lastPositionLine = -1;
protected StaticScope scope;
protected ASTInspector inspector;
protected String methodName;
protected String rubyName;
protected StandardASMCompiler script;

63. public void performBooleanBranch(BranchCallback trueBranch,
BranchCallback falseBranch) {
Label afterJmp = new Label();
Label falseJmp = new Label();
// call isTrue on the result
isTrue();
method.ifeq(falseJmp); // EQ == 0 (i.e. false)
trueBranch.branch(this);
method.go_to(afterJmp);
// FIXME: optimize for cases where we have no false branch
method.label(falseJmp);
falseBranch.branch(this);
method.label(afterJmp);
}

64. More Demos!
• JRuby + JVM flags
• JRuby concurrency
• Invokedynamic (Java 7)
• Redcar Editor
• Ruboto (Android)
• VisualVM

65. Gracias!
• Charles Oliver Nutter
• “headius” on most services
• headius@headius.com