Case 133: TLS Model Flip (-ftls-model)

Field	Value
Verdict	🟡 COMPATIBLE_WITH_RISK
Category	Risk
Platforms	Linux
Flags	—
Detected ChangeKinds	tls_model_changed
Source files	examples/case133_tls_model_flip/

Category: Build mode | Verdict: 🟡 COMPATIBLE_WITH_RISK

Same source, same symbols; v1 built with -ftls-model=global-dynamic, v2 with -ftls-model=initial-exec. The generated CMake L3 build context reveals the change → tls_model_changed.

What this demonstrates

The thread-local storage model changes the TLS access sequence the compiler emits. A consumer built against the old model can use the wrong access pattern for an exported thread_local; initial-exec, in particular, cannot be used from a library dlopened after program start.

Why COMPATIBLE_WITH_RISK

A build-mode signal, not a proven binary break (ADR-028 D3); the artifact diff proves any concrete break, this localizes the risk.

How abicheck detects it

The CMake fixture builds v1 with -ftls-model=global-dynamic and v2 with -ftls-model=initial-exec; the generated build-dir compile_commands.json carries those flags. The L3 diff normalizes both to the canonical tls_model option and reports the switch.

Reproduce manually

cmake -S examples -B /tmp/abicheck-examples-build -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
cmake --build /tmp/abicheck-examples-build --target case133_tls_model_flip_v1 case133_tls_model_flip_v2
abicheck dump /tmp/abicheck-examples-build/case133_tls_model_flip/libv1.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v1.abi.json
abicheck dump /tmp/abicheck-examples-build/case133_tls_model_flip/libv2.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v2.abi.json
abicheck compare v1.abi.json v2.abi.json   # → tls_model_changed

How to fix

Choose a TLS model compatible with how the library is loaded (use global-dynamic for anything that may be dlopened) and keep it stable across releases.

---

Source files

• CMakeLists.txt
• app.cpp
• v1.cpp
• v2.cpp

See also: Examples overview · All COMPATIBLE_WITH_RISK cases · Category: Risk.