I remember some rules from a time ago (pre-32bit Intel processors), when was quite frequent (at least for me) having to analyze the assembly output generated by C/C++ compilers (in my case, Borland/Turbo at that time) to find performance bottlenecks, and to safely mix assembly routines with C/C++ code. Things like using the SI register for the I remember some rules from a time ago (pre-32bi