assembly - What does `rep ret` mean?

Question

I was testing some code on Visual Studio 2008 and noticed security_cookie. I can understand the point of it, but I don't understand what the purpose of this instruction is.

    rep ret /* REP to avoid AMD branch prediction penalty */

Of course I can understand the comment :) but what is this prefix exaclty doing in context with the ret and what happens if ecx is != 0? Apparently the loop count from ecx is ignored when I debug it, which is to be expected.

The code where I found this was here (injected by the compiler for security):

void __declspec(naked) __fastcall __security_check_cookie(UINT_PTR cookie)
{
    /* x86 version written in asm to preserve all regs */
    __asm {
        cmp ecx, __security_cookie
        jne failure
        rep ret /* REP to avoid AMD branch prediction penalty */
failure:
        jmp __report_gsfailure
    }
}

Answer 1

There's a whole blog named after this instruction. And the first post describes the reason behind it: http://repzret.org/p/repzret/

Basically, there was an issue in the AMD's branch predictor when a single-byte ret immediately followed a conditional jump as in the code you quoted (and a few other situations), and the workaround was to add the rep prefix, which is ignored by CPU but fixes the predictor penalty.