RFR: 8330158: C2: Loop strip mining uses ABS with min int [v2]

[Roland Westrelin]

> This fixes 3 calls to ABS with a min int argument. I think all of them
> are harmless:
> 
> - in `PhaseIdealLoop::exact_limit()`, I removed the call to ABS. The
>   check is for a stride of 1 or -1.
>   
> - in `OuterStripMinedLoopNode::adjust_strip_mined_loop()`, for the
>   computation of `scaled_iters_long`, the stride is passed to `ABS()`
>   and then implicitly casted to long. I now cast the stride to long
>   before `ABS()`. For a min int stride, `LoopStripMiningIter * stride`
>   overflows the int range for all values of `LoopStripMiningIter`
>   except 0 or 1. Those values are handled early on in that method. So
>   for a min in stride:
>   ```
>   (jlong)scaled_iters != scaled_iters_long
>   ```
>   is always true and the method returns early.
>   
> - in `OuterStripMinedLoopNode::adjust_strip_mined_loop()`, the
>   computation of `short_scaled_iters` also calls `ABS()` with the
>   stride as argument. But the result of that computation is only used
>   if the test for:
>   ```
>   (jlong)scaled_iters != scaled_iters_long
>   ```
>   doesn't cause an early return of the method. I reordered statements
>   so the `ABS()` calls happens after that test which will cause an early
>   return if the stride is min int.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  more

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18813/files
  - new: https://git.openjdk.org/jdk/pull/18813/files/ecd940f3..d876cbc9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18813&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18813&range=00-01

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18813.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18813/head:pull/18813

PR: https://git.openjdk.org/jdk/pull/18813