suppress compiler warning about CYTHON_FALLTHROUGH
The `-Wno-unreachable-code-fallthrough` compiler flag suppresses warnings about fallthrough annotations in unreachable code.
In C switch statements, "fallthrough" occurs when execution continues from one case to the next without a break statement. This is often a source of bugs, so modern compilers warn about it. To indicate intentional fallthrough, developers use annotations like `__attribute__((fallthrough))`.
In Cython-generated C code, the `CYTHON_FALLTHROUGH` macro is defined to expand to the appropriate fallthrough annotation for the compiler being used. For example, in `compress.c`:
```c
#define CYTHON_FALLTHROUGH __attribute__((fallthrough))
```
The issue occurs because Cython generates code with conditional branches that may be unreachable on certain platforms or configurations. When these branches contain switch statements with fallthrough annotations, compilers like Clang issue warnings like:
```
warning: fallthrough annotation in unreachable code [-Wunreachable-code-fallthrough]
```
These warnings appear in the generated C code, not in the original Cython source. They're harmless but noisy, cluttering the build output with warnings about code we don't control.
By adding `-Wno-unreachable-code-fallthrough` to the compiler flags in `setup.py`, we specifically tell the compiler to ignore these particular warnings, resulting in a cleaner build output without affecting the actual functionality of the code.
This is a common practice when working with generated code - suppress specific warnings that are unavoidable due to the code generation process while keeping other useful warnings enabled.
Thomas Waldmann