box and val (c2 crash)

[Remi Forax]

I believe the crash comes from the method "next()" (or indexedElements() because it the same kind of code),

  Cursor.box next() {
    if (index + 1 == array.length) {
      return null;
    }
    return new Cursor(array, index + 1);
  }

There is a merge of either null or a Cursor (which is a value type) into a register typed Cursor.box.

Trying to insert a local variable in between result in a crash in a different methods
  Cursor.box next() {
    Cursor.box box;
    if (index + 1 == array.length) {
      box = null;
    } else {
      box = new Cursor(array, index + 1);
    }
    return box;
  }

Current CompileTask:
C2:    308  117             fr.umlv.valuetype.perf.TupleLoopBenchMark$Cursor::next (38 bytes)

Stack: [0x00007fb3b253c000,0x00007fb3b263d000],  sp=0x00007fb3b2639d00,  free space=1015k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0xde246f]  ValueTypeNode::make_from_oop(GraphKit*, Node*, ciValueKlass*)+0x9f
V  [libjvm.so+0xc06567]  Parse::return_current(Node*)+0x8d7
V  [libjvm.so+0xc14dc4]  Parse::do_one_bytecode()+0x2714
V  [libjvm.so+0xc03077]  Parse::do_one_block()+0x197
V  [libjvm.so+0xc034e2]  Parse::do_all_blocks()+0x102
V  [libjvm.so+0xc05bd0]  Parse::Parse(JVMState*, ciMethod*, float)+0xa70
V  [libjvm.so+0x53da48]  ParseGenerator::generate(JVMState*)+0x88
V  [libjvm.so+0x61bab8]  Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int, bool, bool, bool, DirectiveSet*)+0xe98
V  [libjvm.so+0x53cf46]  C2Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0xe6
V  [libjvm.so+0x624182]  CompileBroker::invoke_compiler_on_method(CompileTask*)+0x3a2
V  [libjvm.so+0x6255c8]  CompileBroker::compiler_thread_loop()+0x608
V  [libjvm.so+0xda42f8]  JavaThread::thread_main_inner()+0xf8
V  [libjvm.so+0xda0fc1]  Thread::call_run()+0x171
V  [libjvm.so+0xbd5598]  thread_native_entry(Thread*)+0xf8

Rémi

----- Mail original -----
> De: "Remi Forax" <forax at univ-mlv.fr>
> À: "valhalla-dev" <valhalla-dev at openjdk.java.net>
> Envoyé: Jeudi 3 Janvier 2019 16:14:22
> Objet: box and val (c2 crash)

> Now, i've tried to play with the .box/.val trying to implement the clojure
> Cursor protocol,
> which iterate like it's an abstract linked list, so at each step either you
> return a new Cursor (a value type) or null at the end (which seems to be a good
> test for a nullable value type).
> 
> The following code compiles !
> I had to had the fix method because the compier refuse to find any methods on
> Cursor.box and
>  Cursor.box boxed = ...
>  ((Cursor)boxed).next
> doesn't compile too.
> 
> If you try to execute the code, it crashes the VM (in the C2 compiler thread it
> seems)
> 
> Current CompileTask:
> C2:    338  119             fr.umlv.valuetype.perf.TupleLoopBenchMark::sum (46
> bytes)
> 
> Stack: [0x00007ff4c0a37000,0x00007ff4c0b38000],  sp=0x00007ff4c0b34700,  free
> space=1013k
> Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted,
> Vv=VM code, C=native code)
> V  [libjvm.so+0xe19ed6]  VMError::report_and_die(int, char const*, char const*,
> __va_list_tag*, Thread*, unsigned char*, void*, void*, char const*, int,
> unsigned long)+0x186
> V  [libjvm.so+0xe1ac87]  VMError::report_and_die(Thread*, void*, char const*,
> int, char const*, char const*, __va_list_tag*)+0x47
> V  [libjvm.so+0x67a68e]  report_vm_error(char const*, int, char const*, char
> const*, ...) [clone .constprop.51]+0xee
> V  [libjvm.so+0xdbc39e]  TypePtr::xmeet_helper(Type const*) const+0x5e
> V  [libjvm.so+0xdb80e0]  TypePtr::xmeet(Type const*) const+0x10
> V  [libjvm.so+0x5564c1]  PhiNode::Value(PhaseGVN*) const+0x131
> V  [libjvm.so+0xc38986]  PhaseGVN::transform_no_reclaim(Node*)+0x86
> V  [libjvm.so+0xc00c2d]  Parse::do_exits()+0x4bd
> V  [libjvm.so+0xc05c16]  Parse::Parse(JVMState*, ciMethod*, float)+0xab6
> V  [libjvm.so+0x53da48]  ParseGenerator::generate(JVMState*)+0x88
> V  [libjvm.so+0x70f87d]  Parse::do_call()+0x2dd
> V  [libjvm.so+0xc15998]  Parse::do_one_bytecode()+0x32e8
> V  [libjvm.so+0xc03077]  Parse::do_one_block()+0x197
> V  [libjvm.so+0xc034e2]  Parse::do_all_blocks()+0x102
> V  [libjvm.so+0xc05bd0]  Parse::Parse(JVMState*, ciMethod*, float)+0xa70
> V  [libjvm.so+0x53da48]  ParseGenerator::generate(JVMState*)+0x88
> V  [libjvm.so+0x61bab8]  Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int,
> bool, bool, bool, DirectiveSet*)+0xe98
> V  [libjvm.so+0x53cf46]  C2Compiler::compile_method(ciEnv*, ciMethod*, int,
> DirectiveSet*)+0xe6
> V  [libjvm.so+0x624182]
> CompileBroker::invoke_compiler_on_method(CompileTask*)+0x3a2
> V  [libjvm.so+0x6255c8]  CompileBroker::compiler_thread_loop()+0x608
> V  [libjvm.so+0xda42f8]  JavaThread::thread_main_inner()+0xf8
> V  [libjvm.so+0xda0fc1]  Thread::call_run()+0x171
> V  [libjvm.so+0xbd5598]  thread_native_entry(Thread*)+0xf8
> 
> 
> Rémi
> 
> ---
>  static value class Tuple {
>    private final int index;
>    private final int element;
>    
>    
>    private Tuple(int index, int element) {
>      this.index = index;
>      this.element = element;
>    }
>  }
>  
>  static value class Cursor {
>    private final int[] array;
>    private final int index;
>    
>    private Cursor(int[] array, int index) {
>      this.array = array;
>      this.index = index;
>    }
>    
>    Tuple current() {
>      return new Tuple(index, array[index]);
>    }
>    
>    Cursor.box next() {
>      if (index + 1 == array.length) {
>        return null;
>      }
>      return new Cursor(array, index + 1);
>    }
>  }
>  
>  private static Cursor.box indexedElements(int[] array) {
>    if (array.length == 0) {
>      return null;
>    }
>    return new Cursor(array, 0);
>  }
>  
>  private static Cursor fix(Cursor.box cursor) {
>    return cursor;
>  }
>  
>  @Benchmark
>  public int sum() {
>    int sum = 0;
>    for(Cursor.box cursor = indexedElements(ARRAY); cursor != null; cursor =
>    fix(cursor).next()) {
>      Tuple tuple = fix(cursor).current();
>      sum += tuple.index + tuple.element;
>    }
>    return sum;
>   }