GNU Compiler Collection (GCC) Internals: Troubleshooting

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### 23.7 Troubleshooting the garbage collector

With the current garbage collector implementation, most issues should show up as GCC compilation errors. Some of the most commonly encountered issues are described below.

- Gengtype does not produce allocators for a `GTY`-marked type. Gengtype checks if there is at least one possible path from GC roots to at least one instance of each type before outputting allocators. If there is no such path, the `GTY` markers will be ignored and no allocators will be output. Solve this by making sure that there exists at least one such path. If creating it is unfeasible or raises a “code smell”, consider if you really must use GC for allocating such type.
- Link-time errors about undefined `gt_ggc_r_foo_bar` and similarly-named symbols. Check if your `foo_bar` source file has `#include "gt-foo_bar.h"` as its very last line.