On Thu, 2 Jul 2026 13:34:28 GMT, Christian Hagedorn <[email protected]> wrote:
> In the test case, we are trying to merge a dead path in a live target block > which is unexpected and we fail with an assertion failure. > > We call `Parse::do_acmp()` with `BoolTest::ne`. This means that the target > block is reached over `ne_region`, which collects all the branching paths, > and the fall-through block is `eq_region`, which collects all the > fall-through paths. > > We emit several checks to see if we need to branch off and capture those in > `ne_region`. In the test case, we either fold such a branch directly with the > available information or we emit an uncommon trap in which case we also will > not branch off to the target block. At the end of `do_acmp`, we only > collected dead paths in the `ne_region` and thus are left with only `top` > inputs. This means that we will not branch off to the target block and will > fall through. We transform the `ne_region` with `top` inputs only, get `top` > back and set it as current control: > https://github.com/openjdk/valhalla/blob/6adeca933a1361c6f44f51c9c266bd1a20a42380/src/hotspot/share/opto/parse2.cpp#L2436 > Nothing unusual, this is perfectly fine to signal that this path is dead. So > far, so good. > > Since `do_acmp` handles both the `BoolTest::ne` and `BoolTest::eq` case, we > actually need to swap the fall-through control to be the `eq_region` instead: > https://github.com/openjdk/valhalla/blob/6adeca933a1361c6f44f51c9c266bd1a20a42380/src/hotspot/share/opto/parse2.cpp#L2447-L2450 > > Before doing that, we merge the `ne_region` into the target block: > https://github.com/openjdk/valhalla/blob/6adeca933a1361c6f44f51c9c266bd1a20a42380/src/hotspot/share/opto/parse2.cpp#L2441-L2445 > > However, if the target block is live by already having merged another block > as in the test case, we will merge a dead path with a live path. This is not > anticipated by `merge()` and we fail with an assertion failure: The caller > needs to make sure to only call `merge()` with live paths. > > The solution is straight forward: Only `merge()` if we have not `stopped()`. > Otherwise, mark the block as handled and do nothing. > > Thanks, > Christian > > --------- > - [x] I confirm that I make this contribution in accordance with the [OpenJDK > Interim AI Policy](https://openjdk.org/legal/ai). This pull request has now been integrated. Changeset: e16732cc Author: Christian Hagedorn <[email protected]> URL: https://git.openjdk.org/valhalla/commit/e16732cc12cff297859b3a58f48237201c6be771 Stats: 23 lines in 2 files changed: 21 ins; 1 del; 1 mod 8386602: [lworld] C2: assert(n != top() || r->in(pnum) == top()) failed: live value must not be garbage Reviewed-by: thartmann ------------- PR: https://git.openjdk.org/valhalla/pull/2618
