I learned from Agner's guides, the Intel and AMD optimization manuals and experimentation.

It didn't exist in its current form whenn I started, but the x86 tag wiki is a great source for x86 specific stuff:

https://stackoverflow.com/tags/x86/info

I focus on x86 but I would say 75% of the knowledge is transferable to other architectures. 25% is weird x86 stuff that you aren't likely to find anywhere else.