The last time I used assembly language was a one-line inline asm opcode to call the RDTSC instruction on X86.

The last significant amount of asm I wrote was a function to convert UTF-16 to UTF-8. I was inspired by a comment in the reference implementation saying "I wish there was a better way to do this" where there is a bunch of if/else chains to test how many leading 1 bits are in a byte. Well, the CPU has an instruction for that. Then, it proved easier just to write it all in asm than to interface a helper function. After all, its work is to push bits around, something asm is good at.

So, instructions that are not modeled in your high-level language is a good reason to dig to asm.

—John