gas vs. nasm: which assembler produces the best code?

Question

Both tools translate assembly instructions directly into machine code, but is it possible to determine which one produces the fastest and cleanest code?

Answer 1

I don't know about these two specific tools, but there are some instructions that can be encoded differently:

• ADD AX,1 is either 05 01 or 81 c0 01 or fe c0
• INT 3 is either cc or cd 03
• New AVX instructions that extend two-byte SSE instructions will either have a 2-byte or 3-byte prefix. All 2-byte prefixes can be encoded as 3-byte prefixes as well.

These are just a few examples off the top of my head of how assemblers can encode the same instruction differently, so the question does in fact make sense.