From: "cosgroveb (Brian Cosgrove) via ruby-core" <ruby-core@...>
Date: 2026-08-04T22:21:11+00:00
Subject: [ruby-core:126252] [Ruby Bug#22225] YJIT regenerates a branch while a duplicate target still points to the invalidated block

Issue #22225 has been updated by cosgroveb (Brian Cosgrove).


luke-gru (Luke Gruber) wrote in #note-3:
> I think this is one of those instances I *should* have used an LLM, sorry! It has me thinking that this is not a bug after all. The JIT team will still take a look at it, but this may not be the repro to your bug that you were hoping for.

Thanks for taking an early look, nevertheless, if that ends up being so. This is a hairy thing. We'll see.

----------------------------------------
Bug #22225: YJIT regenerates a branch while a duplicate target still points to the invalidated block
https://bugs.ruby-lang.org/issues/22225#change-118337

* Author: cosgroveb (Brian Cosgrove)
* Status: Open
* Assignee: jit
* ruby -v: ruby 4.1.0dev (2026-08-03T03:11:21Z master f622e12503) +YJIT dev +PRISM [aarch64-linux]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
YJIT adds one incoming-list entry for each branch target that points to a block. When both targets point to the same block, that block's incoming list contains the same `BranchRef` twice.

`invalidate_block_version` processes those entries separately. For the first entry, it redirects one target and immediately calls `regenerate_branch`. The other target still references
the invalidated block.

The following assertion fails immediately before that call:

```patch
diff --git a/yjit/src/core.rs b/yjit/src/core.rs
index d08cd1fb26..4555081ae2 100644
--- a/yjit/src/core.rs
+++ b/yjit/src/core.rs
@@ -4282,6 +4282,18 @@ pub fn invalidate_block_version(blockref: &BlockRef) {
             }
         }

+        assert!(
+            branch.targets.iter().all(|target| unsafe {
+                // SAFETY: no mutation.
+                target
+                    .ref_unchecked()
+                    .as_ref()
+                    .and_then(|target| target.get_block())
+                    != Some(*blockref)
+            }),
+            "regeneration started while a branch target still referenced the invalidated block"
+        );
+
         // Rewrite the branch with the new jump target address
         let old_branch_size = branch.code_size();
         regenerate_branch(cb, branch);
```

This Ruby script triggers the assertion:

```ruby
# duplicate_target_invalidation.rb
module DuplicateTargetInvalidation
  TARGET = 1

  def self.call(condition)
    if condition
      1
    else
      2
    end

    TARGET
  end
end

3.times do |index|
  DuplicateTargetInvalidation.call(index.even?)
end

DuplicateTargetInvalidation.send(:remove_const, :TARGET)

puts "completed"
```

Save the patch as `assert-no-invalidated-target-before-regeneration.patch` and the script as `duplicate_target_invalidation.rb`. From CRuby `ruby 4.1.0dev`:

```sh
git apply assert-no-invalidated-target-before-regeneration.patch
./autogen.sh
mkdir build
cd build
../configure --enable-yjit=dev
make -j"$(nproc)"
./ruby --yjit-call-threshold=1 ../duplicate_target_invalidation.rb
```

The process aborts before printing `completed`:

```text
[BUG] YJIT: panicked at yjit/src/core.rs:4285:9:
regeneration started while a branch target still referenced the invalidated block
```


regenerate_branch runs with one target redirected to a stub and the other still pointing at the invalidated block. That can leave generated code jumping back into code YJIT just invalidated.





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