Why is the loop instruction slow? Couldn't Intel have implemented it efficiently?

Question

LOOP (Intel ref manual entry) decrements ecx / rcx, and then jumps if non-zero. It\'s slow, but couldn\'t Intel have cheaply made it fast? dec/jnz already mac

Answer 1

In 1988, IBM fellow Glenn Henry had just come on board at Dell, which had a few hundred employees at the time, and in his first month he gave a tech talk about 386 internals. A bunch of us BIOS programmers had been wondering why LOOP was slower than DEC/JNZ so during the question/answer section somebody posed the question.

His answer made sense. It had to do with paging.

LOOP consists of two parts: decrementing CX, then jumping if CX is not zero. The first part cannot cause a processor exception, whereas the jump part can. For one, you could jump (or fall through) to an address outside segment boundaries, causing a SEGFAULT. For two, you could jump to a page that is swapped out.

A SEGFAULT usually spells the end for a process, but page faults are different. When a page fault occurs, the processor throws an exception, and the OS does the housekeeping to swap in the page from disk into RAM. After that, it restarts the instruction that caused the fault.

Restarting means restoring the state of the process to what it was just before the offending instruction. In the case of the LOOP instruction in particular, it meant restoring the value of the CX register. One might think you could just add 1 to CX, since we know CX got decremented, but apparently, it's not that simple. For example, check out this erratum from Intel:

The protection violations involved usually indicate a probable software bug and restart is not desired if one of these violations occurs. In a Protected Mode 80286 system with wait states during any bus cycles, when certain protection violations are detected by the 80286 component, and the component transfers control to the exception handling routine, the contents of the CX register may be unreliable. (Whether CX contents are changed is a function of bus activity at the time internal microcode detects the protection violation.)

To be safe, they needed to save the value of CX on every iteration of a LOOP instruction, in order to reliably restore it if needed.

It's this extra burden of saving CX that made LOOP so slow.

Intel, like everyone else at the time, was getting more and more RISC. The old CISC instructions (LOOP, ENTER, LEAVE, BOUND) were being phased out. We still used them in hand-coded assembly, but compilers ignored them completely.