[9] RFR(S): 8066763: fatal error "assert(false) failed: unexpected yanked node" in postaloc.cpp:139

[Tobias Hartmann]

Hi,

please review the following patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8066763
Webrev: http://cr.openjdk.java.net/~thartmann/8066763/webrev.00/

Problem:
A null check for an oop loaded by a LoadNNode (300) consists of a CmpNNode
(388), a BoolNode (356) and a IfNode (357) [1].
After matching, the CmpNNode is replaced by a testN_reg node (56) and a jmpCon
node (55) [2]. With implicit null checks enabled (default), the compiler notices
that the oop is accessed by a nearby loadINode (131) and optimizes the code by
using an implicit null check: Instead of checking the oop explicitly, the
loadINode is moved upwards and executed without a check. Code is added to handle
the SIGSEGV and throw the corresponding NullPointerException [3].

The problem is that after the optimization the testN_reg node is dead (see [3])
and triggers the assert in the register allocator. With implicit null checks
disabled (-XX:-ImplicitNullChecks) the bug does not appear.

Solution:
The proposed fix checks for dead input nodes after removing the old compare
node. This removes the dead testN_reg node (see [4]).

Testing:
- Failing test
- JPRT

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/secure/attachment/24158/1_before_matching.png
[2] https://bugs.openjdk.java.net/secure/attachment/24187/2_global_code_motion.png
[3] https://bugs.openjdk.java.net/secure/attachment/24159/1_global_code_motion.png
[4] https://bugs.openjdk.java.net/secure/attachment/24161/3_global_code_motion.png