It’s admittedly pretty specialized, but there’s very little accessible information out there about ARM assembly in general (documentation from ARM always feels  sadly neglected and half-hearted to me), but I came across this wonderfully precise and accessible summary of inline ARM assembly with GCC that really surprised me.  It’s not something you’ll use all the time even if you do a fair amount of development on ARM, but it’s a treasure chest of information if you’re trying to figure out how to optimize some particularly picky function: ARM GCC Inline Assembler Cookbook.