[foreign-memaccess] RFR: 8227424: C2 assertion failures with debug build

[Nick Gasson]

Hi,

I tried the foreign-memaccess branch on AArch64. It works fine but I ran
into a crash and an assertion failure when I tested with a fastdebug
Hotspot build. I've got a patch to fix these two issues here:

Bug: https://bugs.openjdk.java.net/browse/JDK-8227424 (crash log attached)
Webrev: http://cr.openjdk.java.net/~ngasson/foreign/8227424/webrev.0/

The first failure is a segfault in C2 LoadNode::make. The
foreign-memaccess branch has a change to
LibraryCallKit::generate_current_thread to bypass GraphKit::make_load
and call LoadNode::make directly to load the current Thread
pointer. LoadNode::make takes a TypePtr* adr_type argument that is only
used in an assertion:

  assert(!(adr_type->isa_oopptr() &&
           adr_type->offset() == oopDesc::klass_offset_in_bytes()),
         "use LoadKlassNode instead");

generate_current_thread passes NULL for this argument so the assertion
crashes. Replaced NULL with p->bottom_type()->is_ptr() which is what
GraphKit::make_load would have done.

The second failure is an assertion error while running the
TestByteBuffer jtreg test:

  #  Internal Error (src/hotspot/share/opto/parse3.cpp:85), pid=22340, tid=22379
  #  assert(_gvn.type(obj)->higher_equal(tjp)) failed: cast_up is no longer needed

This happens when C2 is parsing the bytecodes for
VarHandleMemoryAddressAsInts1::get:

  static int get(java.lang.invoke.VarHandleMemoryAddressBase, java.lang.Object, long);
    Code:
       0: ldc           #51                 // String CONSTANT_PLACEHOLDER_4
       2: checkcast     #53                 // class java/lang/invoke/MethodHandle
       5: aload_0
       6: aload_1
       7: aload_0
       8: getfield      #56                 // Field java/lang/invoke/VarHandleMemoryAddressBase.offset:J
      11: aload_0
      12: getfield      #16                 // Field dim0:J
      15: lload_2
      16: lmul
      17: ladd
      18: invokevirtual #59                 // Method java/lang/invoke/MethodHandle.invokeExact:(Ljava/lang/invoke/VarHandleMemoryAddressBase;Ljava/lang/Object;J)I
      21: ireturn

The error occurs at the getfield instruction at BCI 12 above. In this
assertion `obj' is the object reference pushed by the aload_0 at BCI 11
which has type VarHandleMemoryAddressBase. The getfield is referencing
VarHandleMemoryAddressAsInts1::dim0 and `tjp' is the type of the field
holder, VarHandleMemoryAddressAsInts1. The assert then expects the type
of obj to be equal to tjp. As far as I can tell this isn't required by
the JVMLS though.

I worked around this by adding a checkcast before the getfield, but I
wonder if the assertion in C2 is wrong / redundant? It's been there
since the initial import in 2007 so I don't know what the history is.

Tested foreign-memaccess jdk_foreign on x86 and AArch64.

Thanks,
Nick