[pizza234]

For busy waiting in general, AFAIK one should signal the intention to the compiler, via intrinsics (`_mm_pause` in C, `std::sync::atomic::spin_loop_hint` in Rust, and so on). The compiler and processor will attempt to make the best out of the pause, e.g. send the processor to a lower-consumption state.

It may be overkill for such cases, but if one keeps the strategy as reference, it's best to know the optimal one :-)

I even think it's not overkill for this case, as they're actually working around the compiler, and a clean approach is preferrable regardless.

[simias]

>e.g. send the processor to a lower-consumption state.

You have to be very careful in a bootloader, since you're initializing the clocks and other very low level subsystems you don't want to risk putting the CPU in a state you won't be able to get out of.

In general the standard method is to use a dummy busy delay like the "nop loop" shown above in the very early stages until you get to the point where you can enable interrupts and standby states and implement a proper sleep method that will effectively shut the CPU down waiting for an external event.

If you're talking about userland code (or even driver code) using busyloops you're right though.

[saagarjha]

Isn’t _mm_pause an Intel intrinsic?

[pizza234]

Based on what I see on [Stack Overflow](https://stackoverflow.com/a/51908785), it's recognized by all the major compilers

The [Microsoft Compiler help](https://docs.microsoft.com/en-us/cpp/intrinsics/x86-intrinsi...) includes it in the "x86 intrinsics list".

I've just compiled with success, for testing purposes, on an AMD processor.

C, relevant section:

    #include <immintrin.h>

    void test() {
        _mm_pause();
    }

Output ASM, relevant section:

    test:
    .LFB4006:
      .cfi_startproc
      endbr64
      pushq %rbp
      .cfi_def_cfa_offset 16
      .cfi_offset 6, -16
      movq  %rsp, %rbp
      .cfi_def_cfa_register 6
      rep nop
      nop
      nop
      popq  %rbp
      .cfi_def_cfa 7, 8
      ret
      .cfi_endproc
    .LFE4006:
      .size test, .-test
      .globl  main
      .type main, @function

It should be the `rep nop`.

Including `ammintrin.h` yields the same `rep nop`.

[saagarjha]

By “Intel” I really meant “x86” :P Is there an ARM compiler that recognizes it?

[secondcoming]

Unlikely, intrinsics are CPU, and sometimes CPU model, specific. Compiler builtins will instead use the intrinsic if it's available/suitable or generate equivalent code if it's not.

[pizza234]

Ah! Sorry for that; I have no idea.

It seems that at least some ARMs have [timers that can be used](https://electronics.stackexchange.com/q/450971), which is a different strategy, but still more appropriate (I suppose).