8339526: C2: store incorrectly removed for clone() transformed to series of loads/stores

[rwestrel]

In the test1() method of the test case:

inlined2() calls clone() for an object loaded from field field
that has inexact type A at parse time. The intrinsic for clone()
inserts an Allocate and an ArrayCopy nodes. When igvn runs, the
load of field is optimized out because it reads back a newly
allocated B written to field in the same method. ArrayCopy can
now be optimized because the type of its src input is known. The
type of its dest input is the CheckCastPP from the allocation of
the cloned object created at parse time. That one has type A. A
series of Loads/Stores are created to copy the fields of class B
from src (of type B) to dest of (type A).

Writting to dest with offsets for fields that don't exist in A,
causes this code in Compile::flatten_alias_type():

    } else if (offset < 0 || offset >= ik->layout_helper_size_in_bytes()) {
      // Static fields are in the space above the normal instance
      // fields in the java.lang.Class instance.
      if (ik != ciEnv::current()->Class_klass()) {
        to = nullptr;
        tj = TypeOopPtr::BOTTOM;
        offset = tj->offset();
      }

to assign it some slice that doesn't match the one that's used at the
same offset in B.

That causes an assert in ArrayCopyNode::try_clone_instance() to
fire. With a release build, execution proceeds. test1() also has a
non escaping allocation. That one causes EA to run and
ConnectionGraph::split_unique_types() to move the store to the non
escaping allocation to a new slice. In the process, when it iterates
over MergeMem nodes, it notices the stores added by
ArrayCopyNode::try_clone_instance(), finds that some are not on the
right slice, tries to move them to the correct slice (expecting they
are from a non escaping EA). That causes some of the Stores to be
disconnected. When the resulting code runs, execution fails as some
fields are not copied.

The fix I propose is to skip ArrayCopyNode::try_clone_instance()
when src and dest classes don't match as this seems like a rare
enough corner case.

---

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Issue

• JDK-8339526: C2: store incorrectly removed for clone() transformed to series of loads/stores (Bug - P3)

Reviewers

• Christian Hagedorn (@chhagedorn - Reviewer)

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8339526: C2: store incorrectly removed for clone() transformed to series of loads/stores

Reviewed-by: chagedorn

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[mlbridge]

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[chhagedorn]

That looks reasonable to me.

[chhagedorn]

Can you move the field up to the other field?

[chhagedorn]

Suggested change

	// At parse time, exact type of the object to clone was not known. That inexact type was captured by the CheckCastPP
	// of the newly allocated cloned object (in dest). Exact type is now known (in src), but type for the cloned object
	// (dest) was not updated. When copying fields below, Store nodes may write to offsets for fields that don't exist in
	// the inexact class. The stores would then be assigned an incorrect slice.
	// At parse time, the exact type of the object to clone was not known. That inexact type was captured by the CheckCastPP
	// of the newly allocated cloned object (in dest). The exact type is now known (in src), but the type for the cloned object
	// (dest) was not updated. When copying the fields below, Store nodes may write to offsets for fields that don't exist in
	// the inexact class. The stores would then be assigned an incorrect slice.