[14] RFR(S): 8231412: C2: InitializeNode::detect_init_independence() bails out on simple IR shapes

[Tobias Hartmann]

Hi Christian,

this looks good to me.

Best regards,
Tobias

On 11.10.19 12:34, Christian Hagedorn wrote:
> Hi
> 
> Please review the following enhancement:
> https://bugs.openjdk.java.net/browse/JDK-8231412
> http://cr.openjdk.java.net/~chagedorn/8231412/webrev.00/
> 
> This enhancement addresses the early bailout of capturing a field store to remove unnecessary
> zeroing [1] in simple methods containing only non-escaping objects. As a consequence, some
> allocations were marked not scalar replaceable by the escape analysis which prevented their
> elimination.
> 
> The patch translates the recursive algorithm of the InitializeNode::detect_init_independence()
> method into an iterative one to avoid processing the same nodes twice. Additionally, a dominator
> check is inserted to further reduce iterations: If a CFG node dominates the current allocation then
> there cannot be any dependency beyond that node. In order to successfully apply this check for the
> sample program from the original discussion where this enhancement came up [2] and the corresponding
> jtreg test in the patch, the constant [3] was increased. As a result the field store can be captured
> and the escape analysis can remove all allocations in those methods. The larger value of the
> constant [3] had no negative impact on performance (verified with some standard benchmarks).
> 
> Thank you!
> 
> Best regards,
> Christian
> 
> 
> [1] http://hg.openjdk.java.net/jdk/jdk/file/e98509cb3867/src/hotspot/share/opto/memnode.cpp#l3602
> [2] https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-September/035135.html
> [3] http://hg.openjdk.java.net/jdk/jdk/file/e98509cb3867/src/hotspot/share/opto/phaseX.cpp#l902