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Subject: Re: [std-proposals] Remove infinite loop UB
From: Brian Bi (bbi5291_at_[hidden])
Date: 2020-05-08 20:14:01


On Fri, May 8, 2020 at 9:03 PM connor horman via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> Currently in the C++ language, it is undefined behaviour to have an
> infinite loop without observable side effects.
>

Is that true? I thought the rule was that the compiler is allowed to assume
that the loop will eventually terminate.

> While this is nice on paper, it has a few issues. Any time you need to
> loop forever, you basically need to insert a (potentially costly) side
> effect. This applies any time you need are, say, running a game on a old
> console, and just need to spin while waiting for interrupts to do stuff
> (same with an os kernel). This results in requiring assembly to so this, or
> maybe insert an instruction to wait for an interrupt in a loop (both llvm
> and gcc allow the volatile specifier in an assembly declaration, which is
> treated as observable, so x86_64 code could use hlt (in CPL=0), 65816 could
> WAI). This, however, seems to violate one of C++'s core principles, that
> there is no lower level language.
> A further case would be an init process on linux, which cannot terminate
> (doing so results in a kernel panic), nor can just call hlt in a loop, as
> that's one way to get a #GP which probably translates to SIGILL. Once an
> init process is done setting up, it could want to just spin forever, and do
> so using as little system resources as possible. The logical idea would be
> just for(;;) std::this_thread::yield();, but that would be UB as yield() is
> not observable (correct me if I'm wrong).
>
> Having the ability to spin forever, without wasting time actually doing
> stuff, seems like a reasonable thing to have in low-level code, so its very
> curious (and in many cases, annoying) that you cannot actually do this in
> real code that might have to.
>

Couldn't you just do: `volatile int x; x = 0;`? This is considered a side
effect.

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-- 
*Brian Bi*


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