(This probably deserves to be a new topic.)
The ideal thing would be a way to embed snippets of assembly language....
esp8266 uses gcc, so it has the same sort of inline assembly language capabilities as all the other gcc variants.
The core CPU is a Tensilica Xtensa lx106 MCU, which you can find documentation for.
Note that the ESP8266 is a multi-tasking system, with the arduino-like loop() being one of the tasks. I would not be surprised if there were multiple stacks...