[13] 8221592: C2 compilation failed with assert(!q->is_MergeMem())

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8221592
http://cr.openjdk.java.net/~thartmann/8221592/webrev.00/

We hit an assert during parsing with incremental inlining when merging memory edges into a target
block because of a MergeMem that has another MergeMem as input. I've traced the MergeMem back to
this code:
http://hg.openjdk.java.net/jdk/jdk/file/47a8fdf84424/src/hotspot/share/opto/parse1.cpp#l1028

The memory input of the _exit map is a MergeMem with a useless Phi input that has another MergeMem
as input (see bug comments for details). After calling transform on this slice, the Phi is removed
and we end up with a MergeMem that is then set as input to the original MergeMem. This later
triggers the assert.

The proposed fix is to transform the original MergeMem after transforming the slices to get rid of
MergeMem inputs.

The problem only shows up with the fix for JDK-8059241 [1] which reduced the number of times
PhaseRemoveUseless is executed with incremental inlining. I don't think it's related though but just
triggers the bug.

Tested with multiple runs of the microbenchmark that triggered the assert and
hs-tier1,hs-tier2,hs-tier3,hs-precheckin-comp,jdk-tier1,jdk-tier2,jdk-tier3 (running).

Thanks,
Tobias

[1] https://bugs.openjdk.java.net/browse/JDK-8059241