“ret”会阻止管道吗？

Question

由于ret指令是间接调用，x86上的ret指令是否会使管道停止，或者它是否以某种方式进行了优化以表现得像更直接的调用？

Answer 1

From the Intel Optimization Reference Manual, the branch prediction unit contains a Return Stack Buffer precisely to predict ret instructions (section 2.2.2.1) more accurately. The instruction queueing and decode unit also tracks changes in the stack pointer to improve decoding bandwidth (section 2.2.2.5).

In more detail, section 3.4.1.4 describes some "rules", mostly directed to compiler writers, to benefit from inlining, calls & returns - the most relevant is probably that a near/far call must be paired with a near/far return, which means pushing the return address on the stack and jumping to the callee is not recommended. Also, the call depth is recommended to not exceed 16 nested calls (the size of the RSB).

If those rules are followed, you can effectively treat them like indirect branches during branch selection (section 3.4.1.6), with everything that implies. You will most likely never encounter a stall on a ret , except on pathological cases or self-modifying code.