RFR: 8331575: C2: crash when ConvL2I is split thru phi at LongCountedLoop [v2]

[Roland Westrelin]

> In the test case:
> 
> 
> long i;
> for (; i > 0; i--) {
>     res += 42 / ((int) i);
> 
> 
> The long counted loop phi has type `[1..100]`. As a consequence, the
> `ConvL2I` also has type `[1..100]`. The `DivI` node that follows can't
> fault: it is not guarded by a zero check and has no control set.
> 
> The `ConvL2I` is split through phi and so is the `DiVI` node:
> `PhaseIdealLoop::cannot_split_division()` returns true because the
> value coming from the backedge into the `DivI` (when it is about to be
> split thru phi) is the result of the `ConvL2I` which has type
> `[1..100`] so is not zero as far as the compiler can tell.
> 
> On the last iteration of the loop, i is 1. Because the DivI was split
> thru Phi, it computes the value for the following iteration, so for i
> = 0. This causes a crash when the compiled code runs.
> 
> The same problem can't happen with an int counted loop because logic
> in `PhaseIdealLoop::split_thru_phi()` prevents a `ConvI2L` from being
> split thru phi. I propose to fix this the same way: in the test case,
> it's not true that once the `ConvL2I` is split thru phi it keeps type
> `[1..100]`. The fix is fairly conservative because it's base on the
> existing logic for `ConvI2L`: we would want to not split a `ConvL2I`
> only a counted loopd but. I suppose the same is true for the `ConvI2L`
> and I thought it would be best to revisit both together.

Roland Westrelin has updated the pull request incrementally with two additional commits since the last revision:

 - test case tweaks
 - fuzzer test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19086/files
  - new: https://git.openjdk.org/jdk/pull/19086/files/d48443c3..3c417dc2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19086&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19086&range=00-01

  Stats: 63 lines in 2 files changed: 61 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19086.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19086/head:pull/19086

PR: https://git.openjdk.org/jdk/pull/19086