This is the original source of the document: https://gist.github.com/axic/5bf506728005864461454dd4d37a7f37

And it refers to “Appendix C” of https://software.intel.com/sites/default/files/managed/9e/bc/64-ia-32-architectures-optimization-manual.pdf of which @cetrio copy pasted.

Anyway, I wrote it over 3 years so don’t remember where the 80 came from.