Comment by khrbtxyz

Large projects like the Linux kernel, which are mostly C, use the preprocessor in .S files quite heavily.

Linux has hardly any .s files.

  ~/linux$ find . -name '*.s'
  ./arch/x86/kernel/asm-offsets.s
  ./kernel/bounds.s
  ./scripts/mod/devicetable-offsets.s

But there are plenty of .S files with #includes

  ~/linux$ $ find . -name '*.S' | xargs grep -l '#include' | wc -l
  1217

Edit: simplify command as suggested