8374180: C2 crash in PhaseCCP::verify_type - fatal error: Not monotonic

[merykitty]

Hi,

The issue here is the inconsistency in computing the _widen field of the TypeInt. At the first step, the types of the operands are:

t1 = int:0
t2 = int:-2..3, widen = 3

Since the type of the first operand is a constant zero, AddNode::Value returns the type of the second operand directly, as x ^ 0 == x for all x. In the second step, t1 is widened to 0..2. This triggers the real computation of the result. The algorithm then splits t2 into t21 = int:-2..-1 and t22 = int:0..3. The Xor of these with t1 are r1 = int:-4..-1 and r2 = int:0..3. As both have _hi - _lo <= SMALL_TYPEINT_THRESHOLD == 3, their _widens are normalized to 0. As a result, their meet also has _widen == 0. This value is smaller than that from the previous step, which was 3, which leads to the failure.

The root cause here is that, the _widen value of a node should be computed and normalized on the whole range of the node, not on its subranges, which may normalize it to 0 in more cases than what is expected. As a result, my proposed solution is to ignore the _widen value of the subranges, and pass the expected _widen value when composing the final result.

Please take a look and leave your reviews, thanks a lot.

---

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• JDK-8374180: C2 crash in PhaseCCP::verify_type - fatal error: Not monotonic (Bug - P3)

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