Suspected miscompilation (as observed in a BLE Mesh function)

[ok32]

Hello. I've stumbled upon a peculiar error which was quite hard to trace.

The minimal reproducible example app: https://github.com/ok32/esp-rs-ble-miscomp. I build it in the default devcontainer.

Build target: xtensa-esp32s3-espidf.
esp-idf versions tried: 5.0.5, 5.1.1, 5.1.2.

I tried to initialize BLE Mesh (with the Nimble stack) by replicating the sequence of C calls from an example from the esp-idf repo. I ended up getting an internal assertion violation during one of the calls (esp_ble_mesh_node_prov_enable()). Furhter investigation showed that the problem was not in that particular function. Instead, it was here:
https://github.com/espressif/esp-idf/blob/482a8fb2d78e3b58eb21b26da8a5bedf90623213/components/bt/esp_ble_mesh/mesh_core/nimble_host/mesh_bearer_adapt.c#L1293-L1312

If I use it as it is, it somehow manages to skip the else branch of the if statement completely, resulting in some garbage in the uuid variable. But once I add anything there, it starts to work. It works with this patch:

diff --git a/components/bt/esp_ble_mesh/mesh_core/nimble_host/mesh_bearer_adapt.c b/components/bt/esp_ble_mesh/mesh_core/nimble_host/mesh_bearer_adapt.c
index a651b54284..5e5af02f91 100644
--- a/components/bt/esp_ble_mesh/mesh_core/nimble_host/mesh_bearer_adapt.c
+++ b/components/bt/esp_ble_mesh/mesh_core/nimble_host/mesh_bearer_adapt.c
@@ -1299,6 +1299,8 @@ int bt_mesh_gatts_service_start(struct bt_mesh_gatt_service *svc)
         uuid = BLE_UUID16_DECLARE(BT_UUID_MESH_PROXY_VAL);
     } else {
         uuid = BLE_UUID16_DECLARE(BT_UUID_MESH_PROV_VAL);
+        // just add this line and it works:
+        BT_ERR("(bt_mesh_gatts_service_start:else) abcabcabca");
     }

     rc = ble_gatts_find_svc(uuid, &handle);

What's in the log when it fails: https://github.com/ok32/esp-rs-ble-miscomp/blob/master/debug_stuff/log_fail.txt
What's in the log when it's ok (with the patch)': https://github.com/ok32/esp-rs-ble-miscomp/blob/master/debug_stuff/log_ok.txt

Anybody has any ideas what can be causing this and how to debug it further?

[ivmarkov]

@ok32 If you suspect miscompilation, we better ping @MabezDev as he is having the best grasp of the xtensa backlend.