RFR: 8278518: String(byte[], int, int, Charset) constructor and String.translateEscapes() miss bounds check elimination

[Roland Westrelin]

In the micro benchmark for this bug, the bytecode has a single loop
head with 2 backedges. One branch is taken a lot more frequently than
the other (for instance, profiling reports 3820938 for the most
common, 134068 for the least common). When ciTypeFlow builds its loop
tree, the least common loop ends up as the inner loop (ciTypeFlow uses
the tail block number to order loops with a shared header). Then
ciTypeFlow clones the inner loop head.

The results I get then is:

StringBenchmark.safeDecoding    avgt    5  141.965 ± 53.511  ns/op
StringBenchmark.unsafeDecoding  avgt    5  123.051 ± 77.855  ns/op

The unsafe version uses unsafe instead of standard array accesses. The
reason for the unsafe version is to demonstrate that a missed range
check elimination in the common path hurts performance (the range
check is not performed on the loop iv but another phi so can't be
eliminated).

The patch I propose takes back branch count (from profile data) into
account when building the ciTypeFlow tree. As a consequence, the inner
loop (the one for which the loop head is cloned), is the most most
frequent one. The range check in the common path is also hoisted in
that case (it's now performed on the iv). Running the micro benchmark
again, I get:

StringBenchmark.safeDecoding    avgt    5  53.368 ± 2.232  ns/op
StringBenchmark.unsafeDecoding  avgt    5  55.565 ± 5.124  ns/op

Both are improved (2-3x) and the unsafe version doesn't perform better.

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Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/7034/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7034&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278518
  Stats: 82 lines in 2 files changed: 73 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7034.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7034/head:pull/7034

PR: https://git.openjdk.java.net/jdk/pull/7034