RFR: 8264223: CodeHeap::verify fails extra_hops assertion in fastdebug test

[Hui Shi]

When test with -XX:+VerifyCodeCache, many tests fail due to extra_hops assertion in CodeHeap::verify. See full dump in JBS.

# Internal Error (/data/shihui/codes/shihui_jdk_master/src/hotspot/share/memory/heap.cpp:838), pid=1525697, tid=1525715
# assert((count == 0) || (extra_hops < (16 + 2*count))) failed: CodeHeap: many extra hops due to optimization. blocks: 234, extra hops: 484.
Discussion in https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2019-October/035508.html doesn't tell where assertion (extra_hops < (16 + 2*count) comes from.

In CodeHeap::mark_segmap_as_used wehn is_FreeBlock_join is true,  it creates extra hops in _segmap. _fragmentation_count in inced and trigger defrag_segmap when reach threshold. In my understanding, extra hop can not guarantee under (16 + 2*count). 

In following extreme case, before HeapBlock free, segmap is all 0 (each blob use 1 smallest segment), suppose free action is applied from right to left. This increase 9 unnecessary hop for 1 continous HeapBlock.  assertion (extra_hops < (16 + 2*count)  is not safe.
|0|0|0|0|0|0|0|0|0|0|
after free, it will be
|0|1|1|1|1|1|1|1|1|1|

Proposed fix is assert extra hops not exceed _fragmentation_count. And if it exceeds (16 + 2 * count), give warning on two many extra hops.

fastdebug tier1, tier2 with VerifyCodeCache passing, no extra assertion found.

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Commit messages:
 - 8264223: CodeHeap::verify fails extra_hops assertion in fastdebug test

Changes: https://git.openjdk.java.net/jdk/pull/3212/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3212&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264223
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3212.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3212/head:pull/3212

PR: https://git.openjdk.java.net/jdk/pull/3212