Redundant Assembly Instructions?

Question

Recently I've been trying to learn reverse-engineering. As such, I've been delving into a lot of assembly code. I'm mystified by the following:

movq    %rax,0xf8(%rbp)
movq    0xf8(%rbp),%rax

I've seen this several times. Is it not redundant? Why does the compiler do this? The binary I'm looking at was compiled with gcc.

Answer 1

You probably compiled without optimization (-O). What you're seeing is a direct, naive translation of the intermediate representation. Snippets like this are usually due to the value being stored in a local variable, in this case 0xf8(%rbp). The value is then used immediately afterwards, so it loads it again into a register, %rax. The optimizer will spot that storing from %rax only to restore back to the very same register is redundant and remove the sequence altogether. If all optimization stages fail, at the very least a peephole will spot these two instructions being consecutive.

If you really do have optimization turned on, then this is indeed odd, but might be explained if you post a larger (but not excessively large) sequence. There's still plenty of cases where something blatantly sub-optimal will be generated, but nothing as blatant as that.