RFR: 8374725: C2: assert(x_ctrl == get_late_ctrl_with_anti_dep(x->as_Load(), early_ctrl, x_ctrl)) failed: anti-dependences were already checked [v2]

[Roland Westrelin]

> `PhaseIdealLoop::dom_lca_for_get_late_ctrl_internal()` caches
> intermediate results in `_dom_lca_tags` when the late control is
> computed by `PhaseIdealLoop::get_late_ctrl()` for a node `n`: the code
> iterates over all uses of `n` potentially calling
> `PhaseIdealLoop::dom_lca_for_get_late_ctrl_internal()` multiple
> times. `_dom_lca_tags` is used to cache data that is specific to the
> lca computation for `n`. `_dom_lca_tags` is set to a tag that depends
> on `n` to mark the cached data as only valid during the lca
> computation for `n`.
> 
> `PhaseIdealLoop::try_sink_out_of_loop()` checks that all uses of a
> node are out of loop with
> `PhaseIdealLoop::ctrl_of_all_uses_out_of_loop()` which also needs to
> consider anti-dependences for `Load`s and also calls
> `PhaseIdealLoop::dom_lca_for_get_late_ctrl_internal()` through
> `PhaseIdealLoop::get_late_ctrl_with_anti_dep()`: this computes the
> late control for a node and one particular out of loop
> use. `_dom_lca_tags` values computed by an earlier
> `PhaseIdealLoop::get_late_ctrl()` should be ignored (because it
> computes the late control for a node and all its uses). To address
> that issue, the tag that's used by
> `PhaseIdealLoop::dom_lca_for_get_late_ctrl_internal()` is made
> different on each call from
> `PhaseIdealLoop::ctrl_of_all_uses_out_of_loop()` by incrementing
> `_dom_lca_tags_round`.
> 
> The issue here is that one `Load` node is input to a `Phi` twice. So
> the `Phi` is considered twice as a use of the node along 2 different
> paths. `PhaseIdealLoop::get_late_ctrl_with_anti_dep()` is called twice
> from `PhaseIdealLoop::ctrl_of_all_uses_out_of_loop()` but
> `_dom_lca_tags_round` is not incremented between the 2
> calls. `PhaseIdealLoop::dom_lca_for_get_late_ctrl_internal()` when
> called for the second `Phi` input uses incorrect cached data which, in
> turn, causes an incorrect computation.
> 
> The fix I propose is to make sure `_dom_lca_tags_round` is incremented
> for every call to `PhaseIdealLoop::get_late_ctrl_with_anti_dep()`.

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/29231/files
  - new: https://git.openjdk.org/jdk/pull/29231/files/7175ae52..16bc98a6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29231&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29231&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/29231.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29231/head:pull/29231

PR: https://git.openjdk.org/jdk/pull/29231