Fast as C: How to Write Really Terrible Java

For years we’ve been told that the JVM’s amazing optimizers can take your running code and make it “fast” or “as fast as C++” or “as fast as C”…or sometimes “faster than C”. And yet we don’t often see this happen in practice, due in large part to (good and bad) development patterns that have taken hold in the Java world.
In this talk, we’ll explore the main reasons why Java code rarely runs as fast as C or C++ and how you can write really bad Java code that the JVM will do a better job of optimizing. We’ll take some popular microbenchmarks and burn them to the ground, monitoring JIT logs and assembly dumps along the way.

Transcript

1. Fast as C How to write really terrible Java
2. Me • Charles Oliver Nutter • Red Hat (yes, I have one; no, I don’t wear it) • JRuby and JVM languages • JVM hacking and spelunking • @headius
3. Benchmarks! • Lots of benchmarks out there, but… • Most of them already terrible Java • Usually very synthetic cases • Not particularly illustrative
4. for ( int i=idxMin; i<idxMax; ++i ) { // count flips if ( p0 != 0 ) { int pp0 = p0, pp1 = p1, pp2 = p2, pp3 = p3, pp4 = p4, pp5 = p5, pp6 = p6, pp7 = p7, pp8 = p8, pp9 = p9, pp10 = p10, pp11 = p11; int flips = 1; for ( ;; ++flips ) { int t = pp0; switch ( t ) { case 1: pp0 = pp1; pp1 = t; break; case 2: pp0 = pp2; pp2 = t; break; case 3: pp0 = pp3; pp3 = t; t = pp2; pp2 = pp1; pp1 = t; break; case 4: pp0 = pp4; pp4 = t; t = pp3; pp3 = pp1; pp1 = t; break; case 5: pp0 = pp5; pp5 = t; t = pp4; pp4 = pp1; pp1 = t; t = pp3; pp3 = pp2; pp2 = t; break;
5. What are we going to do today? • Look at some Java features and patterns • See how they’re compiled to bytecode • Watch what the JVM does with them • Examine the actual native code they become
6. WHY?!
7. Who Are You? • Java developers? • Performance engineers? • Debuggers? • All of the above?
8. Mechanical Sympathy • Features with hidden costs • Anonymous inner classes • Structural types in Scala • Serialization • Code design impacts performance • JVM can’t do everything for you
9. Sufficiently Smart Compiler “HighLevelLanguage H may be slower than the LowLevelLanguage L, but given a SufficientlySmartCompiler this would not be the case” http://c2.com/cgi/wiki?SufficientlySmartCompiler
10. Sufficiently Smart Compiler If you wait long enough*, the JVM will eventually optimize everything perfectly and even bad code will perform well. * for some definition of “long”
11. Pre-dive Prep • Profiling with various tools • YourKit, Flight Recorder, JMH • Algorithmic complexity • Allocation/GC overhead • Latency/blocking in IO and system calls
12. Part One: The Primer
13. Vocabulary • Source • The .java text that represents a program • Bytecode • The binary version of the program that all JVMs can load and execute
14. Vocabulary • Native code • Machine code specific to the current platform (OS, CPU) that represents the program in a form the CPU can execute directly • Heap • The JVM-controlled area of memory where Java objects live
15. Vocabulary • JIT • “Just In Time” (compilation) that turns one program form into a lower program form, e.g. bytecode into native code at runtime • AOT • Compilation that occurs before runtime
16. JVM 101 Java source JVM bytecode javac JVM bytecode Bytecode interpreter runs inside gather information JIT compiler triggers Native code produces executes backs off
17. Vocabulary • Inlining • Inserting the code of a called method into the caller, avoiding overhead of the call and optimizing the two together • Optimization • Doing the least amount of work needed to accomplish some goal
18. Inlining Instance Method Load target and arguments Target type is same as inlined? Method lookup Run target code directly Yes No Run target method as a call
19. Inlining Static or Special Method Load arguments Run target code directly
20. Our Tools • javac, obviously • javap to dump .class data • -XX:+PrintCompilation and
 -XX:+PrintInlining • -XX:+PrintAssembly • -XX:+LogCompilation and JITWatch
21. Hello, world! • We’ll start with something simple.
22. package com.headius.talks.geekout; public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!"); } }
23. Level 1: Bytecode • javap • Java class file disassembler • Dump structure, data, metadata, and code
24. $ javap -cp dist/GeekOut.jar \ com.headius.talks.geekout.HelloWorld Compiled from "HelloWorld.java" public class com.headius.talks.geekout.HelloWorld { public com.headius.talks.geekout.HelloWorld(); public static void main(java.lang.String[]); }
25. $ javap -cp dist/GeekOut.jar \ -c \ com.headius.talks.geekout.HelloWorld Compiled from "HelloWorld.java" public class com.headius.talks.geekout.HelloWorld { ... public static void main(java.lang.String[]); Code: 0: getstatic #2 // Field java/lang/ System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello, world! 5: invokevirtual #4 // Method java/io/ PrintStream.println:(Ljava/lang/String;)V 8: return }
26. Our First Bytecodes • getstatic/putstatic - static field access • ldc - load constant value on stack • invokevirtual - call a concrete instance method • return - return from a void method
27. $ javap -cp dist/GeekOut.jar \ -c \ com.headius.talks.geekout.HelloWorld Compiled from "HelloWorld.java" public class com.headius.talks.geekout.HelloWorld { ... public static void main(java.lang.String[]); Code: 0: getstatic #2 // Field java/lang/ System.out:Ljava/io/PrintStream; 3: ldc #3 // String Hello, world! 5: invokevirtual #4 // Method java/io/ PrintStream.println:(Ljava/lang/String;)V 8: return }
28. Level 2: Compiler Logs • -XX:+PrintCompilation • Display methods as they compile • -XX:+PrintInlining • Display inlined methods as nested
29. JVM JIT • Code is interpreted first • After some threshold, JIT fires • Classic JVM went straight to “client” or “server” • Tiered compiler goes to “client plus profiling” and later “server”
30. public class HelloWorld { public static void main(String[] args) { for (int i = 0; i < 100000; i++) { hello(); } } private static void hello() { System.err.println("Hello, world!"); } }
31. $ java -Xbatch -XX:-TieredCompilation \ -XX:+PrintCompilation \ -cp dist/GeekOut.jar \ com.headius.talks.geekout.HelloWorld \ 2> /dev/null
32. 83 1 java.lang.String::hashCode (55 bytes) 91 2 java.lang.String::indexOf (70 bytes) 121 3 sun.nio.cs.UTF_8$Encoder::encodeArrayLoop (489 bytes) 137 4 java.nio.Buffer::position (5 bytes) ... 283 47 java.lang.String::indexOf (7 bytes) 285 48 com.headius.talks.geekout.HelloWorld::hello (9 bytes) 285 49 ! java.io.PrintStream::println (24 bytes) 295 50 java.io.PrintStream::print (13 bytes) 296 51 ! java.io.PrintStream::write (83 bytes) 301 52 ! java.io.PrintStream::newLine (73 bytes) 302 53 java.io.BufferedWriter::newLine (9 bytes) 302 54 % com.headius.talks.geekout.HelloWorld::main @ 2 (18 bytes)
33. 83 1 java.lang.String::hashCode (55 bytes) 91 2 java.lang.String::indexOf (70 bytes) 121 3 sun.nio.cs.UTF_8$Encoder::encodeArrayLoop (489 bytes) 137 4 java.nio.Buffer::position (5 bytes) ... 283 47 java.lang.String::indexOf (7 bytes) 285 48 com.headius.talks.geekout.HelloWorld::hello (9 bytes) 285 49 ! java.io.PrintStream::println (24 bytes) 295 50 java.io.PrintStream::print (13 bytes) 296 51 ! java.io.PrintStream::write (83 bytes) 301 52 ! java.io.PrintStream::newLine (73 bytes) 302 53 java.io.BufferedWriter::newLine (9 bytes) 302 54 % com.headius.talks.geekout.HelloWorld::main @ 2 (18 bytes)
34. $ java -Xbatch \ -XX:-TieredCompilation \ -XX:+PrintCompilation \ -XX:+UnlockDiagnosticVMOptions \ -XX:+PrintInlining \ -cp dist/Geekout.jar \ com.headius.talks.geekout.HelloWorld 2> /dev/null
35. 82 1 b java.lang.String::hashCode (55 bytes) 94 2 b java.lang.String::indexOf (70 bytes) @ 66 java.lang.String::indexOfSupplementary (71 bytes) too big 132 3 b sun.nio.cs.UTF_8$Encoder::encodeArrayLoop (489 bytes) @ 1 java.nio.CharBuffer::array (35 bytes) inline (hot) @ 6 java.nio.CharBuffer::arrayOffset (35 bytes) inline (hot) ... 397 48 b com.headius.talks.geekout.HelloWorld::hello (9 bytes) !m @ 5 java.io.PrintStream::println (24 bytes) inline (hot) @ 6 java.io.PrintStream::print (13 bytes) inline (hot) ... 446 54 % b com.headius.talks.geekout.HelloWorld::main @ 2 (18 bytes) @ 8 com.headius.talks.geekout.HelloWorld::hello (9 bytes) already compiled into a big method
36. 82 1 b java.lang.String::hashCode (55 bytes) 94 2 b java.lang.String::indexOf (70 bytes) @ 66 java.lang.String::indexOfSupplementary (71 bytes) too big 132 3 b sun.nio.cs.UTF_8$Encoder::encodeArrayLoop (489 bytes) @ 1 java.nio.CharBuffer::array (35 bytes) inline (hot) @ 6 java.nio.CharBuffer::arrayOffset (35 bytes) inline (hot) ... 397 48 b com.headius.talks.geekout.HelloWorld::hello (9 bytes) !m @ 5 java.io.PrintStream::println (24 bytes) inline (hot) @ 6 java.io.PrintStream::print (13 bytes) inline (hot) ... 446 54 % b com.headius.talks.geekout.HelloWorld::main @ 2 (18 bytes) @ 8 com.headius.talks.geekout.HelloWorld::hello (9 bytes) already compiled into a big method
37. Level 3: Native Code • -XX:+PrintAssembly • Dumps “human readable” JITed code • Google for “hotspot printassembly” • Aren’t you excited?!
38. $ java -Xbatch \ -XX:-TieredCompilation \ -XX:+UnlockDiagnosticVMOptions \ -XX:+PrintAssembly \ -cp dist/GeekOut.jar \ com.headius.talks.geekout.HelloWorld 2> /dev/null | less
39. Decoding compiled method 0x0000000110526110: Code: [Entry Point] [Verified Entry Point] [Constants] # {method} {0x00000001100a6420} 'hello' '()V' in 'com/headius/talks/geekout/HelloWorld' # [sp+0x70] (sp of caller) 0x0000000110526300: mov %eax,-0x14000(%rsp) 0x0000000110526307: push %rbp 0x0000000110526308: sub $0x60,%rsp ;*synchronization entry ; - com.headius.talks.geekout.HelloWorld::hello@-1 (line 13) 0x000000011052630c: movabs $0x7aaa80c78,%r10 ; {oop(a 'java/lang/Class' = 'java/lang/System')} 0x0000000110526316: mov 0x70(%r10),%r11d ;*getstatic err ; - com.headius.talks.geekout.HelloWorld::hello@0 (line 13) 0x000000011052631a: mov %r11d,0x10(%rsp) 0x000000011052631f: test %r11d,%r11d 0x0000000110526322: je 0x000000011052664e ;*invokevirtual println ; - com.headius.talks.geekout.HelloWorld::hello@5 (line 13)
40. Too big! • Server produces ~2700 bytes of ASM • Client produces ~594 bytes of ASM • Most of server output is from inlining • More profiling, more code, more perf • ...and slower startup
41. public class Tiny1 { public static void main(String[] args) { for (int i = 0; i < 100000; i++) { tiny(); } } public static int tiny() { return 1 + 1; } }
42. public static int tiny(); Code: 0: iconst_2 1: ireturn iconst_2: load integer 2 on stack ireturn: return int
43. 110 3 b com.headius.talks.geekout.Tiny1::tiny (2 bytes) 111 4 % b com.headius.talks.geekout.Tiny1::main @ 2 (19 bytes) @ 8 com.headius.talks.geekout.Tiny1::tiny (2 bytes) inline (hot)
44. {0x000000010994c3c0} 'tiny' '()I' in 'com/headius/talks/geekout/Tiny1' # [sp+0x40] (sp of caller) 0x0000000109e566a0: mov %eax,-0x14000(%rsp) 0x0000000109e566a7: push %rbp 0x0000000109e566a8: sub $0x30,%rsp ;*iconst_2 ; - com.headius.talks.geekout.Tiny1::tiny@0 (line 11) 0x0000000109e566ac: mov $0x2,%eax 0x0000000109e566b1: add $0x30,%rsp 0x0000000109e566b5: pop %rbp 0x0000000109e566b6: test %eax,-0x9a05bc(%rip) # 0x00000001094b6100 ; {poll_return} 0x0000000109e566bc: retq
45. {0x000000010994c3c0} 'tiny' '()I' in 'com/headius/talks/geekout/Tiny1' # [sp+0x40] (sp of caller) 0x0000000109e566a0: mov %eax,-0x14000(%rsp) 0x0000000109e566a7: push %rbp 0x0000000109e566a8: sub $0x30,%rsp ;*iconst_2 ; - com.headius.talks.geekout.Tiny1::tiny@0 (line 11) 0x0000000109e566ac: mov $0x2,%eax 0x0000000109e566b1: add $0x30,%rsp 0x0000000109e566b5: pop %rbp 0x0000000109e566b6: test %eax,-0x9a05bc(%rip) # 0x00000001094b6100 ; {poll_return} 0x0000000109e566bc: retq
46. {0x000000010e67d300} 'main' '([Ljava/lang/String;)V' in 'com/headius/talks/geekout/Tiny1' 0x000000010eb879a0: mov %eax,-0x14000(%rsp) 0x000000010eb879a7: push %rbp 0x000000010eb879a8: sub $0x40,%rsp ;*iconst_0 ; - com.headius.talks.geekout.Tiny1::main@0 (line 5) 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 ; - com.headius.talks.geekout.Tiny1::main@2 (line 5) 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} ;*goto ; - com.headius.talks.geekout.Tiny1::main@15 (line 5) 0x000000010eb879ba: test %eax,-0x9a08c0(%rip) # 0x000000010e1e7100 ;*goto ; - com.headius.talks.geekout.Tiny1::main@15 (line 5) ; {poll} 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge ; - com.headius.talks.geekout.Tiny1::main@5 (line 5) 0x000000010eb879c8: add $0x40,%rsp 0x000000010eb879cc: pop %rbp 0x000000010eb879cd: test %eax,-0x9a08d3(%rip) # 0x000000010e1e7100 ; {poll_return} 0x000000010eb879d3: retq ;*return ; - com.headius.talks.geekout.Tiny1::main@18 (line 8)
47. 0x000000010eb879a0: mov %eax,-0x14000(%rsp) 0x000000010eb879a7: push %rbp 0x000000010eb879a8: sub $0x40,%rsp ;*iconst_0 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879ba: test %eax,-0x9a08c0(%rip) # 0x000000010e1e7100 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879c8: add $0x40,%rsp 0x000000010eb879cc: pop %rbp 0x000000010eb879cd: test %eax,-0x9a08d3(%rip) # 0x000000010e1e7100 0x000000010eb879d3: retq ;*return
48. 0x000000010eb879a0: mov %eax,-0x14000(%rsp) 0x000000010eb879a7: push %rbp 0x000000010eb879a8: sub $0x40,%rsp ;*iconst_0 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879ba: test %eax,-0x9a08c0(%rip) # 0x000000010e1e7100 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879c8: add $0x40,%rsp 0x000000010eb879cc: pop %rbp 0x000000010eb879cd: test %eax,-0x9a08d3(%rip) # 0x000000010e1e7100 0x000000010eb879d3: retq ;*return
49. 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879ba: test %eax,-0x9a08c0(%rip) # 0x000000010e1e7100 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879cd: test %eax,-0x9a08d3(%rip) # 0x000000010e1e7100 0x000000010eb879d3: retq ;*return
50. 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879ba: test %eax,-0x9a08c0(%rip) # 0x000000010e1e7100 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879cd: test %eax,-0x9a08d3(%rip) # 0x000000010e1e7100 0x000000010eb879d3: retq ;*return
51. 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879d3: retq ;*return
52. 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b6: xchg %ax,%ax 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879d3: retq ;*return
53. 0x000000010eb879ac: mov $0x0,%esi 0x000000010eb879b1: jmpq 0x000000010eb879c0 ;*iload_1 0x000000010eb879b8: inc %esi ; OopMap{off=26} 0x000000010eb879c0: cmp $0x186a0,%esi 0x000000010eb879c6: jl 0x000000010eb879b8 ;*if_icmpge 0x000000010eb879d3: retq ;*return
54. 1: mov $0,%esi 2: jmpq 4: 3: inc %esi 4: cmp $1000000,%esi 5: jl 3: 6: retq
55. 1: retq
56. -XX:+LogCompilation • Combines PrintCompilation and PrintInlining in one horrendous XML file • OpenJDK tool “LogCompilation” for CLI • OpenJDK tool “JITWatch” for GUI
57. scopes_pcs_offset='1384' dependencies_offset='1576' handler_table_offset='1592' nul_chk_table_offset='1736' oops_offset='992' method='org/jruby/lexer/yacc/ByteArrayLexerSource$ByteArrayCursor read ()I' bytes='49' count='5296' backedge_count='1' iicount='10296' stamp='0.412'/> <writer thread='4425007104'/> <nmethod compile_id='21' compiler='C2' entry='4345862528' size='1152' address='4345862160' relocation_offset='288' insts_offset='368' stub_offset='688' scopes_data_offset='840' scopes_pcs_offset='904' dependencies_offset='1016' handler_table_offset='1032' oops_offset='784' method='org/jruby/lexer/yacc/ ByteArrayLexerSource forward (I)I' bytes='111' count='5296' backedge_count='1' iicount='10296' stamp='0.412'/> <writer thread='4300214272'/> <task_queued compile_id='22' method='org/jruby/lexer/yacc/ByteArrayLexerSource read ()I' bytes='10' count='5000' backedge_count='1' iicount='10000' stamp='0.433' comment='count' hot_count='10000'/> <writer thread='4426067968'/> <nmethod compile_id='22' compiler='C2' entry='4345885984' size='1888' address='4345885584' relocation_offset='288' insts_offset='400' stub_offset='912' scopes_data_offset='1104' scopes_pcs_offset='1496' dependencies_offset='1704' handler_table_offset='1720' nul_chk_table_offset='1864' oops_offset='1024' method='org/jruby/lexer/yacc/ByteArrayLexerSource read ()I' bytes='10' count='5044' backedge_count='1' iicount='10044' stamp='0.435'/> <writer thread='4300214272'/> <task_queued compile_id='23' method='java/util/HashMap hash (I)I' bytes='23' count='5000' backedge_count='1' iicount='10000' stamp='0.442' comment='count' hot_count='10000'/> <writer thread='4425007104'/> <nmethod compile_id='23' compiler='C2' entry='4345887808' size='440' address='4345887504' relocation_offset='288' insts_offset='304' stub_offset='368' scopes_data_offset='392' scopes_pcs_offset='400' dependencies_offset='432' method='java/util/HashMap hash (I)I' bytes='23' count='5039' backedge_count='1' iicount='10039' stamp='0.442'/> <writer thread='4300214272'/> <dependency_failed type='abstract_with_unique_concrete_subtype' ctxk='org/jruby/lexer/yacc/LexerSource' x='org/jruby/lexer/yacc/ByteArrayLexerSource' witness='org/jruby/lexer/yacc/InputStreamLexerSource' stamp='0.456'/> <dependency_failed type='abstract_with_unique_concrete_subtype' ctxk='org/jruby/lexer/yacc/LexerSource' x='org/jruby/lexer/yacc/ByteArrayLexerSource' witness='org/jruby/lexer/yacc/InputStreamLexerSource' stamp='0.456'/> <dependency_failed type='abstract_with_unique_concrete_subtype' ctxk='org/jruby/lexer/yacc/LexerSource' x='org/jruby/lexer/yacc/ByteArrayLexerSource' witness='org/jruby/lexer/yacc/InputStreamLexerSource' stamp='0.456'/> <dependency_failed type='abstract_with_unique_concrete_subtype' ctxk='org/jruby/lexer/yacc/LexerSource' x='org/jruby/lexer/yacc/ByteArrayLexerSource' witness='org/jruby/lexer/yacc/InputStreamLexerSource' stamp='0.456'/>
58. $ java -jar logc.jar hotspot.log 1 java.lang.String::hashCode (67 bytes) 2 Accumulator::addSqrt (7 bytes) 3 Accumulator::sqrt (6 bytes) logc with no flags = PrintCompilation
59. $ java -jar logc.jar -i hotspot.log 1 java.lang.String::hashCode (67 bytes) 2 Accumulator::addSqrt (7 bytes) @ 2 Accumulator::sqrt (6 bytes) (end time: 0.0660 nodes: 36) @ 2 java.lang.Math::sqrt (5 bytes) 3 Accumulator::sqrt (6 bytes) @ 2 java.lang.Math::sqrt (5 bytes) -i flag = PrintCompilation + PrintInlining
61. It’s not that hard
    once you know what to look at.
62. Part 2: The Fun Stuff
63. Java Features • final fields • synchronized and volatile • string switch • lambda • single-implementer interfaces • transient objects
64. #1: Final Fields • Final fields can’t be modified • The pipeline can take advantage • ...but it doesn’t always
65. public class Fields { private static final String MY_STRING = "This is a static string"; private static final String MY_PROPERTY = System.getProperty("java.home"); public static void main(String[] args) { System.out.println(MY_STRING); System.out.println(MY_PROPERTY); } }
66. public static void main(java.lang.String[]); Code: 0: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #9 // String This is a static string 5: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream; 11: getstatic #11 // Field MY_PROPERTY:Ljava/lang/String; 14: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V private static final String MY_STRING = "This is a static string"; private static final String MY_PROPERTY = System.getProperty("java.home");
67. private static int addHashes() { return MY_STRING.hashCode() + MY_PROPERTY.hashCode(); }
68. movabs $0x7aab6c4f8,%r10 ; {oop("This is a static string")} mov %eax,0x10(%r10) ;*iload_1 ; - String::hashCode@53 (line 1467) ; - Fields::addHashes@2 (line 36) movabs $0x7aaa97a98,%rcx ; {oop(".../jdk1.8.0.jdk/Contents/Home/jre")} mov 0x10(%rcx),%r10d ;*getfield hash ; - String::hashCode@1 (line 1458) ; - Fields::addHashes@8 (line 36)
69. private final String myString = "This is an instance string"; private final String myProperty = System.getProperty("java.home"); public int addHashes2() { return myString.hashCode() + myProperty.hashCode(); }
70. private int addHashes2(); Code: 0: ldc #2 // String This is an instance string 2: invokevirtual #18 // Method java/lang/String.hashCode:()I 5: aload_0 6: getfield #6 // Field myProperty:Ljava/lang/String; 9: invokevirtual #18 // Method java/lang/String.hashCode:()I 12: iadd 13: ireturn
71. movabs $0x7aab6d318,%rcx ; {oop("This is an instance string")} mov 0x10(%rcx),%r10d ;*getfield hash ; - String::hashCode@1 (line 1458) ; - Fields::addHashes2@2 (line 40)
72. mov 0x10(%rsi),%ecx ;*getfield myProperty ; - Fields::addHashes2@6 (line 40) mov 0x10(%r12,%rcx,8),%eax ;*getfield hash ; - String::hashCode@1 (line 1458) ; - Fields::addHashes2@9 (line 40)
73. ACHIEVEMENT UNLOCKED:
    Find something Hotspot could do better
74. #2: Concurrency Stuff • What does “synchronized” do? • What does “volatile” do?
75. public class Concurrency { public static void main(String[] args) { System.out.println(getTime()); System.out.println(getTimeSynchronized()); } public static long getTime() { return System.currentTimeMillis(); } public static synchronized long getTimeSynchronized() { return System.currentTimeMillis(); } }
76. public static void main(java.lang.String[]); Code: 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: invokestatic #3 // Method getTime:()J 6: invokevirtual #4 // Method java/io/PrintStream.println:(J)V 9: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 12: invokestatic #5 // Method getTimeSynchronized:()J 15: invokevirtual #4 // Method java/io/PrintStream.println:(J)V
77. public static long getTime(); Code: 0: invokestatic #7 // Method java/lang/System.currentTimeMillis:()J 3: lreturn public static synchronized long getTimeSynchronized(); Code: 0: invokestatic #7 // Method java/lang/System.currentTimeMillis:()J 3: lreturn
78. 'getTime' '()J' in 'com/headius/talks/geekout/Concurrency' movabs $0x1015dbd3e,%r10 callq *%r10 ;*invokestatic currentTimeMillis ; - Concurrency::getTime@0 (line 22) retq
79. movabs $0x7aab6bee8,%r10 ; {oop(a 'java/lang/Class' = '.../Concurrency')} mov (%r10),%rax mov %rax,%r10 and $0x7,%r10 cmp $0x5,%r10 jne 0x000000010ef0665f mov $0xdf3803fe,%r11d ; {metadata('java/lang/Class')} mov 0xa8(%r12,%r11,8),%r10 mov %r10,%r11 or %r15,%r11 mov %r11,%r8 xor %rax,%r8 $0xffffffffffffff87,%r8 jne 0x000000010ef068e4 mov %r14d,(%rsp) ;*synchronization entry ; - Concurrency::getTimeSynchronized@-1 (line 26) ; - Concurrency::main@58 (line 16) movabs $0x10de5ad3e,%r10 callq *%r10 ;*invokestatic currentTimeMillis ; - Concurrency::getTimeSynchronized@0 (line 26) ; - Concurrency::main@58 (line 16)
80. movabs $0x7aab6bee8,%r10 ; {oop(a 'java/lang/Class' = '.../Concurrency')} mov (%r10),%rax mov %rax,%r10 and $0x7,%r10 cmp $0x5,%r10 jne 0x000000010ef0665f mov $0xdf3803fe,%r11d ; {metadata('java/lang/Class')} mov 0xa8(%r12,%r11,8),%r10 mov %r10,%r11 or %r15,%r11 mov %r11,%r8 xor %rax,%r8 $0xffffffffffffff87,%r8 jne 0x000000010ef068e4 mov %r14d,(%rsp) ;*synchronization entry ; - Concurrency::getTimeSynchronized@-1 (line 26) ; - Concurrency::main@58 (line 16) movabs $0x10de5ad3e,%r10 callq *%r10 ;*invokestatic currentTimeMillis ; - Concurrency::getTimeSynchronized@0 (line 26) ; - Concurrency::main@58 (line 16)
81. movabs $0x7aab6bee8,%r10 ; {oop(a 'java/lang/Class' = '.../Concurrency')} mov (%r10),%rax mov %rax,%r10 and $0x7,%r10 cmp $0x5,%r10 jne 0x000000010ef0665f mov $0xdf3803fe,%r11d ; {metadata('java/lang/Class')} mov 0xa8(%r12,%r11,8),%r10 mov %r10,%r11 or %r15,%r11 mov %r11,%r8 xor %rax,%r8 $0xffffffffffffff87,%r8 jne 0x000000010ef068e4 mov %r14d,(%rsp) ;*synchronization entry ; - Concurrency::getTimeSynchronized@-1 (line 26) ; - Concurrency::main@58 (line 16) movabs $0x10de5ad3e,%r10 callq *%r10 ;*invokestatic currentTimeMillis ; - Concurrency::getTimeSynchronized@0 (line 26) ; - Concurrency::main@58 (line 16)
82. 0x000000010ef0665f: movabs $0x7aab6bee8,%r11 ; {oop(a 'java/lang/Class' = '.../Concurrency')} 0x000000010ef06669: lea 0x10(%rsp),%rbx 0x000000010ef0666e: mov (%r11),%rax 0x000000010ef06671: test $0x2,%eax 0x000000010ef06676: jne 0x000000010ef0669f 0x000000010ef0667c: or $0x1,%eax 0x000000010ef0667f: mov %rax,(%rbx) 0x000000010ef06682: lock cmpxchg %rbx,(%r11) 0x000000010ef06687: je 0x000000010ef066bc
83. Volatile • Forces memory visibility, access ordering • Prevents some optimizations • Similar impact to unnecessary locking • ...but it can’t ever be removed
84. 11345d823: mov 0x70(%r8),%r9d ;*getstatic NULL_OBJECT_ARRAY ; - org.jruby.RubyBasicObject::<init>@5 (line 76) ; - org.jruby.RubyObject::<init>@2 (line 118) ; - org.jruby.RubyNumeric::<init>@2 (line 111) ; - org.jruby.RubyInteger::<init>@2 (line 95) ; - org.jruby.RubyFixnum::<init>@5 (line 112) ; - org.jruby.RubyFixnum::newFixnum@25 (line 173) 11345d827: mov %r9d,0x14(%rax) 11345d82b: lock addl $0x0,(%rsp) ;*putfield varTable ; - org.jruby.RubyBasicObject::<init>@8 (line 76) ; - org.jruby.RubyObject::<init>@2 (line 118) ; - org.jruby.RubyNumeric::<init>@2 (line 111) ; - org.jruby.RubyInteger::<init>@2 (line 95) ; - org.jruby.RubyFixnum::<init>@5 (line 112) ; - org.jruby.RubyFixnum::newFixnum@25 (line 173) LOCK Code from a RubyBasicObject’s default constructor. Why are we doing a volatile write in the constructor?
85. public class RubyBasicObject ... { private static final boolean DEBUG = false; private static final Object[] NULL_OBJECT_ARRAY = new Object[0]; // The class of this object protected transient RubyClass metaClass; // zeroed by jvm protected int flags; // variable table, lazily allocated as needed (if needed) private volatile Object[] varTable = NULL_OBJECT_ARRAY; LOCK Maybe it’s not such a good idea to pre-init a volatile?
86. public static Object getVariable(RubyBasicObject object, int index) { Object[] ivarTable; if (index < 0 || (ivarTable = object.varTable) == null) return null; if (ivarTable.length > index) return ivarTable[index]; return null; } Yuck!
87. #3: String Switch • Added in Java 7 • ...and there was much rejoicing • But how does it really work?
88. A Normal Switch • Variable switch parameter • Constant case values • Branch based on a table (fast) for narrow range of cases • Branch based on a lookup (less fast) for broad range of cases
89. public class StringSwitch { public static void main(String[] args) { String count = "unknown"; switch (args.length) { case 0: count = "zero"; break; case 1: count = "one"; break; case 2: count = "two"; break; } ...
90. public static void main(java.lang.String[]); Code: 0: ldc #2 // String unknown 2: astore_1 3: aload_0 4: arraylength 5: tableswitch { // 0 to 2 0: 32 1: 38 2: 44 default: 47 } 32: ldc #3 // String zero 34: astore_1 35: goto 47 38: ldc #4 // String one 40: astore_1 41: goto 47 44: ldc #5 // String two 46: astore_1 Direct branch
91. switch (args.length) { case 2000000: count = "two million"; break; case 1000000: count = "one million"; break; case 3000000: count = "three million"; break; }
92. 49: lookupswitch { // 3 1000000: 90 2000000: 84 3000000: 96 default: 99 } Binary search
93. Comparison • tableswitch is O(1) • Indexed lookup of target • lookupswitch is O(log n) • Binary search for target
94. String Switch • What kind of switch do we use for String? • Table doesn’t work for hashcodes • Lookup might collide • Answer: both, plus .equals()
95. static String chooseGreeting(String language) { switch (language) { case "Java": return "I love to hate you!"; case "Scala": return "I love you, I think!"; case "Clojure": return "(love I you)"; case "Groovy": return "I love ?: you"; case "Ruby": return "I.love? you # => true"; default: return "Who are you?"; } }
96. static java.lang.String chooseGreeting(java.lang.String); Code: 0: aload_0 1: astore_1 2: iconst_m1 3: istore_2 4: aload_1 5: invokevirtual #16 // Method java/lang/String.hashCode:()I 8: lookupswitch { // 5 -1764029756: 88 2301506: 60 2558458: 116 79698214: 74 2141368366: 102 default: 127 } Hidden int variable... Hash and jump target for “Scala”
97. 74: aload_1 75: ldc #14 // String Scala 77: invokevirtual #17 // Method String.equals:(Ljava/lang/Object;)Z 80: ifeq 127 83: iconst_1 84: istore_2 Same hidden int variable now = 1
98. 127: iload_2 128: tableswitch { // 0 to 4 0: 164 1: 167 2: 170 3: 173 4: 176 default: 179 } 164: ldc #20 // String I love to hate you! 166: areturn 167: ldc #21 // String I love you, I think! 169: areturn 170: ldc #22 // String (love I you) 172: areturn 173: ldc #23 // String I love ?: you 175: areturn 176: ldc #24 // String I.love? you # => true 178: areturn 179: ldc #25 // String Who are you? 181: areturn A-ha! There it is! Scala’s index and target
99. static String chooseGreeting2(String language) { int hash = language.hashCode(); int target = -1; switch (hash) { case 2301506: if (language.equals("Java")) target = 0; break; case 79698214: if (language.equals("Scala"))target = 1; break; case -1764029756: if (language.equals(“Clojure")) target = 2; break; case 2141368366: if (language.equals(“Groovy")) target = 3; break; case 2558458: if (language.equals(“Ruby")) target = 3; break; } switch (target) { case 0: return "I love to hate you!"; case 1: return "I love you, I think!"; case 2: return "(love I you)"; case 3: return "I love ?: you"; case 4: return "I.love? you # => true"; default: return "Who are you?"; } }
100. It’s just a hash table!
101. #4: Lambda Expressions • New for Java 8 • ...and there was much rejoicing • Key goals • Lighter-weight than inner classes • No class-per-lambda • Optimizable by JVM
102. public class LambdaStuff { public static void main(String[] args) { List<String> list = Arrays.asList( "Clojure", "Java", "Ruby", "Groovy", "Scala" ); for (int i = 0; i < 100000; i++) { doSort(list); getRest(list); getAllCaps(list); getInitials(list); getInitialsManually(list); }
103. public static void doSort(List<String> input) { Collections.sort(input, (a,b)->Integer.compare(a.length(), b.length())); }
104. public static void doSort(java.util.List<java.lang.String>); Code: 0: aload_0 1: invokedynamic #36, 0 // InvokeDynamic #4:compare:()Ljava/util/Comparator; 6: invokestatic #37 // Method java/util/Collections.sort ... 9: return
105. public static void doSort(java.util.List<java.lang.String>); Code: 0: aload_0 1: invokedynamic #36, 0 // InvokeDynamic #4:compare:()Ljava/util/Comparator; 6: invokestatic #37 // Method java/util/Collections.sort ... 9: return InvokeDynamic is used to create the initial lambda object and then cache it forever. Compare to anonymous inner classes, where an instance is created every time.
106. $ javap -cp dist/GeekOut.jar \ -verbose \ -c \ com.headius.talks.geekout.LambdaStuff
107. BootstrapMethods: ... 4: #142 invokestatic java/lang/invoke/LambdaMetafactory.metafactory... ...bunch of types here Method arguments: #167 (Ljava/lang/Object;Ljava/lang/Object;)I #168 invokestatic LambdaStuff.lambda$2:(Ljava/lang/String;Ljava/lang/String;)I #169 (Ljava/lang/String;Ljava/lang/String;)I LambdaMetaFactory generates an implementation of our interface (Comparator here) using Method Handles (from JSR292)
108. private static int lambda$2(java.lang.String, java.lang.String); Code: 0: aload_0 1: invokevirtual #53 // Method java/lang/String.length:()I 4: aload_1 5: invokevirtual #53 // Method java/lang/String.length:()I 8: invokestatic #54 // Method java/lang/Integer.compare:(II)I 11: ireturn Lambda body is just a static method; all state is passed to it. Because the wrapper is generated and the body is just a static method, we have no extra class files and potentially no allocation.
109. Will It Blend?
110. public static String getInitials(List<String> input) { return input.stream() .map(x->x.substring(0,1)) .collect(Collectors.joining()); } public static String getInitialsManually(List<String> input) { StringBuilder builder = new StringBuilder(); UnaryOperator<String> initial = (String x)->x.substring(0,1); for (String s : input) { builder.append(initial.apply(s)); } return builder.toString(); }
111. public static void time(Object name, int iterations, Runnable body) { long start = System.currentTimeMillis(); for (int i = 0; i < iterations; i++) { body.run(); } System.out.println(name.toString() + ": " + (System.currentTimeMillis() - start)); }
112. Function<List<String>, String> getInitials = LambdaStuff::getInitials; Function<List<String>, String> getInitialsManually = LambdaStuff::getInitialsManually; for (int i = 0; i < 10; i++) { time("getInitials", 1000000, ()->getInitials.apply(list)); time("getInitialsManually", 1000000, ()->getInitialsManually.apply(list)); }
113. Drum roll, please...
114. public static String getInitials(List<String> input) { return input.stream() .map(x->x.substring(0,1)) .collect(Collectors.joining()); } mov %r10d,0x24(%r9) ;*putfield nextStage ; - java.util.stream.AbstractPipeline::<init>@28 (line 200) ; - java.util.stream.ReferencePipeline::<init>@3 (line 94) ; - java.util.stream.ReferencePipeline$StatelessOp::<init>@3 (line 627) ; - java.util.stream.ReferencePipeline$3::<init>@16 (line 188) ; - java.util.stream.ReferencePipeline::map@22 (line 187) ; - com.headius.talks.geekout.LambdaStuff::getInitials@11 (line 57) Methods like map() and collect() inline...
115. public static String getInitials(List<String> input) { return input.stream() .map(x->x.substring(0,1)) .collect(Collectors.joining()); } callq 0x0000000105973f20 ; OopMap{rbp=Oop [0]=NarrowOop off=2776} ;*invokeinterface apply ; - java.util.stream.ReferencePipeline::collect@118 (line 512) ; {runtime_call} But they can’t inline all those lambdas.
116. The Problem • In order to inline code, we need: • A consistent target method • A unique path through the code • Collections.sort’s lambda callback • Will see many different methods • Will be called via many different paths
117. Caller 1 Caller 2 Caller 3 Caller 4 sort Lambda 1 Lambda 2 Lambda 3 Lambda 4 Too many paths! JVM can’t cope!
118. mov 0x60(%r15),%rcx mov %rcx,%r10 add $0x18,%r10 cmp 0x70(%r15),%r10 jae 0x0000000104548d78 mov %r10,0x60(%r15) prefetchnta 0xc0(%r10) mov $0xdf3802e6,%r10d ; {metadata('java/lang/String')} mov 0xa8(%r12,%r10,8),%r10 mov %r10,(%rcx) movl $0xdf3802e6,0x8(%rcx) ; {metadata('java/lang/String')} mov %r12d,0xc(%rcx) mov %r12,0x10(%rcx) ;*new ; - String::substring@65 (line 1961) ; - LambdaStuff::lambda$6@3 (line 75) ; - LambdaStuff$$Lambda$9::apply@4 ; - LambdaStuff::getInitialsManually@45 (line 77) public static String getInitialsManually(List<String> input) { StringBuilder builder = new StringBuilder(); UnaryOperator<String> initial = (String x)->x.substring(0,1); for (String s : input) { builder.append(initial.apply(s)); } return builder.toString(); } Yuck!
119. mov 0x60(%r15),%rcx mov %rcx,%r10 add $0x18,%r10 cmp 0x70(%r15),%r10 jae 0x0000000104548d78 mov %r10,0x60(%r15) prefetchnta 0xc0(%r10) mov $0xdf3802e6,%r10d ; {metadata('java/lang/String')} mov 0xa8(%r12,%r10,8),%r10 mov %r10,(%rcx) movl $0xdf3802e6,0x8(%rcx) ; {metadata('java/lang/String')} mov %r12d,0xc(%rcx) mov %r12,0x10(%rcx) ;*new ; - String::substring@65 (line 1961) ; - LambdaStuff::lambda$6@3 (line 75) ; - LambdaStuff$$Lambda$9::apply@4 ; - LambdaStuff::getInitialsManually@45 (line 77) public static String getInitialsManually(List<String> input) { StringBuilder builder = new StringBuilder(); UnaryOperator<String> initial = (String x)->x.substring(0,1); for (String s : input) { builder.append(initial.apply(s)); } return builder.toString(); } Yuck! Yay!
120. #5 Single-impl Interface • Interfaces are everywhere • Frequently using a common base class • Frequently single implementor of a method
121. IRubyObject getMetaClass() RubyBasicObject final getMetaClass() RubyObject RubyArray RubyString RubyHash
122. @Override public final RubyClass getMetaClass() { return metaClass; }
123. public static boolean testType(RubyClass original, IRubyObject self) {
 return self.getMetaClass() == original;
 }
124. 450 Bootstrap::testType (16 bytes) @ 1 IRubyObject::getMetaClass (0 bytes) (end time: 0.0000) type profile IRubyObject -> RubyArray (41%) JVM sees only target type, even though there’s one impl of method
125. Single-implementer interfaces look like many implementers!
126. public static boolean testType(RubyClass original, IRubyObject self) { return ((RubyBasicObject)self).getMetaClass() == original; } Yuck!
127. Lessons
128. The JVM is not perfect.
129. Every feature has a cost.
130. You’ll be a better developer if you remember those facts...
131. ...and you aren’t afraid
     to look under the covers.
132. Thank You! • Charles Oliver Nutter • @headius • headius@headius.com • http://blog.headius.com