There are a couple reasons you wouldn't know that (for an entire inline asm function): 1. x86_64 vs. x86_32. It's possible that you'd want to use more register

There are a couple reasons you wouldn't know that (for an entire inline asm function): 1. x86_64 vs. x86_32. It's possible that you'd want to use more registers than 32-bit has. In this case, it's usually optimal to use memory operands on x86_32 but registers on x86_64. 2. gcc _will not_ allow you to use ebx in inline asm with PIC code. Likewise with ebp with frame pointer. There's no technical reason gcc couldn't just save+restore those registers around the block, but it doesn't. 3. The case in the bugreport: you have various locations in memory you want to address, and you want gcc to figure out their addresses for you. Without analysis that eliminates identical registers, the naive implementation of using 1-2 new registers per operand will quickly fail.