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Author here, I did look at some of the assembly output from C code, especially for frequently called functions like get_block/set_block, but other than that I used no reference but the original C code to write the assembly.

You can also read it as, write C code, test. Repeat until you have functionality you want. Pick block of C code, replace with assembly equivalent. Repeat.

He probably asked the compiler to emit the assembly it generates (e.g. gcc's -S option); disassembling or decompiling would be overkill.

He COMPILED the C code into its assembly stage with a gcc option. He probably just looked at the .S file from gcc..