RFR: 8325494: C2: Broken graph after not skipping CastII node anymore for Assertion Predicates after JDK-8309902 [v2]

[Roland Westrelin]

> After range check elimination, a cast in the main loop becomes top
> because the type of its input (that depends on the iv phi) and the
> type recorded in the cast do not intersect. This is a case that's
> expected to be caught by assert predicates but, in this particular
> case, no assert predicate constant folds.
> 
> The stride for the loop is -2. The iv phi type is `min+1..0`
> 
> As a consequence, the init value for the main loop has type int.
> 
> The range check that causes the issue is for array access:
> 
> lArrFld[i11 + 1] = 6;
> 
> 
> The main loop is unrolled once. The second access in the loop is at
> `i11 - 1` which has type `min..-1`. The range check cast at that
> access becomes top. The assert predicates operates on an init value
> that has the shape:
> 
> 
> (CastII (AddI pre_loop_iv -2) int)
> 
> 
> and type int.
> 
> That `CastII` is inserted by `PhaseIdealLoop::cast_incr_before_loop()`.
> 
> The assert predicate for the first iteration in the main loop is for
> index:
> 
> 
> (AddI (CastII (AddI pre_loop_iv -2) int) 1)
> 
> 
> And for the second:
> 
> 
> (AddI (CastII (AddI pre_loop_iv -2) int) -1)
> 
> 
> Both have type int so the assert predicate can't constant fold.
> 
> I initially fixed this by changing the type of the cast from int to
> the type of the iv phi:
> 
> 
> (AddI (CastII (AddI pre_loop_iv -2) min+1..0) -1)
> 
> 
> That allows the assert predicate for the second iteration to constant
> fold. But I was then worried narrowing the type of the cast would
> causes issues going forward so instead, I propose proceeding as in
> 8282592 and have assert predicates skip over the CastII (that part of
> 8282592 was later undone):
> 
> 
> (AddI (AddI pre_loop_iv -2) 1)
> 
> 
> which allows the assert predicate for the first iteration in the main
> loop to constant fold.
> 
> The change from 8282592 caused issues because we used to narrow the
> type of a cast based on the condition that guards it. That was removed
> by 8319372.

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

  review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18724/files
  - new: https://git.openjdk.org/jdk/pull/18724/files/3531b4a2..3ce25154

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18724&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18724&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18724.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18724/head:pull/18724

PR: https://git.openjdk.org/jdk/pull/18724