RFR: 8336729: C2: Div/Mod nodes without zero check could be split through iv phi of outer loop of long counted loop nest resulting in SIGFPE [v2]

[Christian Hagedorn]

> The previous fix for preventing `Div/Mod` nodes to be split through iv phis when their divisor could become zero (see [JDK-8299259](https://bugs.openjdk.org/browse/JDK-8299259)) is not complete. Originally, we thought that it is enough to prevent a split through the phi if it belongs to a `BaseCountedLoop`:
> https://github.com/openjdk/jdk/blob/da7311bbe37c2b9632b117d52a77c659047820b7/src/hotspot/share/opto/loopopts.cpp#L302-L304
> 
> The reasoning behind this was that the iv phi type of `BaseCountedLoops` can be improved in such a way that it does not contain zero but the backedge input can include zero:
> https://github.com/openjdk/jdk/blob/da7311bbe37c2b9632b117d52a77c659047820b7/test/hotspot/jtreg/compiler/splitif/TestSplitDivisionThroughPhi.java#L69-L78
> 
> This optimization is not possible for `LoopNodes` where we know nothing about the number of iterations. 
> 
> However, there was an oversight: A `LongCountedLoop` is later split into an inner and an outer `LoopNode` where the inner loop is transformed into a `CountedLoop` while the outer loop stays a `LoopNode`. Both loops share the same optimized iv phi type as the original `LongCountedLoop`! Therefore, we can have the same situation as fixed in JDK-8299259 but with a `LoopNode` instead of a `CountedLoopNode` (see https://github.com/openjdk/jdk/pull/11900 for more details).
> 
> The simplest way to fix this is to extend the bailout fix of JDK-8299259 to not only apply to `BaseCountedLoopNodes` but to `LoopNodes` in general which is what I propose with this patch.
> 
> Thanks to @eme64 for extracting the original simpler reproducer to reproduce this problem easier. I've added an even simpler non-jasm reproducer in this patch in favor of the original jasm reproducer.
> 
> Thanks,
> Christian

Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:

  add bug number

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20594/files
  - new: https://git.openjdk.org/jdk/pull/20594/files/8ba74170..c11cee54

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20594&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20594&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20594/head:pull/20594

PR: https://git.openjdk.org/jdk/pull/20594