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I am currently learning ASM by disassembling some of C codes. One thing interested me is that the gcc compiler generates code like this
movq %rax,%rax
which is obviously meaningless. So what is the purpose of doing that?
I am wondering if it is used for waste a few cycles of CPU in order to improve the pipeline?
Thank you for your hint!
It is basically a no-op, yes.
The compiler does this because branching to an address aligned on a 4-byte boundary is faster than branching to an unaligned address. So if you have a loop, the compiler will insert “padding” just before the start of it in order to get it into alignment.