[jdk17] RFR: 8269752: C2: assert(false) failed: Bad graph detected in build_loop_late [v3]

[Roland Westrelin]

> Some nodes are sunk out of the outer loop of vMeth(). A CastII node is
> added on the exit path of the loop to pin the nodes out of the
> loop. The dominating test of that CastII is:
> 
> if (i >= 132) {
> 
> so the CastII is updated with type int [132..]
> 
> The inner loop is eliminated, the outer loop is turned into a counted
> loop, pre/main/post loops are created. The CastII control is now a
> Region that merges paths from the pre/main/post loops. Its data input
> is a Phi that merges the iv Phis of the 3 loops.
> 
> The loop body includes an early exit:
> 
> if (i2 != 0) {
>     vMeth_check_sum += i3;
>     return;
> }
> 
> That test is guaranteed to be taken at the second iteration. As a
> consequence, the backedge of the main loop is removed. What's left of
> the main loop is still guarded by a an entry test that compares the iv
> value out of the pre loop with an Opaque1 node. The CastII is now only
> reachable through that test. The iv Phi of the pre loop constant folds
> to a value that's not >= 132. As a consequence the CastII becomes
> top. top propagates to other data node and as a consequence a Phi is
> replaced by its only non top input. That causes the assert failure.
> 
> The CastII should not be reachable anymore once the iv Phi of the pre
> loop has constant folded and the early exit in the main loop is always
> taken. But because the test that guards the main loop uses an Opaque1,
> it cannot constant fold. As a fix, I propose removing the Opaque1 once
> the backedge of the main loop is removed. This allows the entry test
> to constant fold. The Opaque1 has no purpose anymore given the loop
> doesn't exist at this point.

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

  review

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

Changes:
  - all: https://git.openjdk.java.net/jdk17/pull/247/files
  - new: https://git.openjdk.java.net/jdk17/pull/247/files/52028ab3..9664bbef

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk17&pr=247&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk17&pr=247&range=01-02

  Stats: 5 lines in 1 file changed: 3 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/247.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/247/head:pull/247

PR: https://git.openjdk.java.net/jdk17/pull/247