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Subject: Re: [std-proposals] Remove infinite loop UB
From: Tony V E (tvaneerd_at_[hidden])
Date: 2020-05-08 21:22:36


I don't understand the situation. How is your loop different than:

for(;;) ;

And if it's not different, how does it ever get out of that, or does it not, and some other part of the system deals with it?

What's your infinite loop doing?

Sent from my BlackBerry portable Babbage Device
From: connor horman via Std-Proposals
Sent: Friday, May 8, 2020 9:03 PM
To: std-proposals@lists.isocpp.org
Reply To: std-proposals@lists.isocpp.org
Cc: connor horman
Subject: [std-proposals] Remove infinite loop UB

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


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