Wrong code generated with -fextend-variable-liveness and coroutines

[avikivity]

In main (51db3c9) and clang 21.1.2 there is a regression with -fextend-variable-liveness and coroutines. I bisected it to 36af734, which makes -fextend-variable-liveness the default under -Og.

The bug manifests in a use-after-free running some coroutine.

Reproducer: (warning: very long)

podman run -it docker.io/avikivity/scylladb-fextend-variable-tracking-reproducer

This drops you into an environment with clang 21.1.2 with the reproducer code and all its dependencies. It is derived from Fedora 43 beta. You can substitute docker for podman if that's what you have.

./configure.py --mode debug
ninja build/debug/test/boost/combined_tests_g
./build/debug/test/boost/combined_tests_g -t restrictions_test -- --smp 1

To add flags to the compiler, append to the ./configure.py line:

./configure.py --mode debug --cflags=-fextend-variable-liveness=none

Which ought to make the crash go away (will confirm soon).

/cc @SLTozer

[avikivity]

./configure.py --mode debug --cflags=-fextend-variable-liveness=none

Which ought to make the crash go away (will confirm soon).

Confirmed.

[llvmbot]

@llvm/issue-subscribers-coroutines

Author: Avi Kivity (avikivity)

In main (51db3c9) and clang 21.1.2 there is a regression with -fextend-variable-liveness and coroutines. I bisected it to 36af734, which makes -fextend-variable-liveness the default under -Og.

The bug manifests in a use-after-free running some coroutine.

Reproducer: (warning: very long)

podman run -it docker.io/avikivity/scylladb-fextend-variable-tracking-reproducer

This drops you into an environment with clang 21.1.2 with the reproducer code and all its dependencies. It is derived from Fedora 43 beta. You can substitute docker for podman if that's what you have.

./configure.py --mode debug
ninja build/debug/test/boost/combined_tests_g
./build/debug/test/boost/combined_tests_g -t restrictions_test -- --smp 1

To add flags to the compiler, append to the ./configure.py line:

./configure.py --mode debug --cflags=-fextend-variable-liveness=none

Which ought to make the crash go away (will confirm soon).

/cc @SLTozer

[SLTozer]

Sorry for the delayed response, I've been away for a bit. Are you able to provide a reproducer without a dependency on docker/podman? Even if the reproducer is quite large, it would be easier for me to work with links to source repos and configure/build commands.

[avikivity]

The repo is https://github.com/scylladb/scylladb. However, it is only guaranteed to build on Fedora 42/43. Fedora 43 comes with clang 21 so a virtual machine with Fedora 43 would be the easiest non-container reproducer.

Instructions (note: one step requires sudo):

git clone --recurse-submodules https://github.com/scylladb/scylladb
cd scylladb
sudo ./install-dependencies.sh
./configure.py --mode debug
ninja build/debug/test/boost/combined_tests_g
./build/debug/test/boost/combined_tests_g -t restrictions_test -- --smp 1