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shakna an hour ago

Only if there would be no side-effects. Which there are.

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saagarjha an hour ago | parent | next [-]

No this is irrelevant for making this decision

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shakna 36 minutes ago | parent [-]

I've mentioned elsewhere the standards, and compilers as well, disagreeing with you here.

But feel free to run against the various compilers through godbolt. [0] They won't optimise the branch away. Access to a volatile, must be preserved, in the order that they exist. No optimisation, UB or otherwise, is allowed to impede that. Because an access is a side-effect.

[0] https://godbolt.org/z/85cGhq3Ta

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saagarjha 20 minutes ago | parent [-]

That they won’t is as most a courtesy to you but they are not required to do this.

	▲	shakna 11 minutes ago \| parent [-]
		> Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, except as modified by the unknown factors mentioned previously. I quoted the C standard, first. Not compiler behaviour. I showed where it requires the compiler not to optimise this. How about, instead of one-line throwaway disagreements, you point out where they are permitted to do this, instead?

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nilamo 34 minutes ago | parent | prev [-]

This looks like a long back and fourth, that can easily be solved by a minute or two on godbolt...

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aw1621107 22 minutes ago | parent | next [-]

> that can easily be solved by a minute or two on godbolt...

Unfortunately it's not that simple when it comes to UB. If the snippet in question does in fact exhibit UB then there's no guarantee whatever Godbolt shows will generalize to other programs/versions/compilers/environments/etc.

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nilamo 14 minutes ago | parent [-]

That's very funny to me.

A) x is always removed.

B) no, it's never removed if volatile.

But neither person can prove what a compiler will actually do, despite claiming they'll always act a certain way given 5 lines of code.

	▲	tialaramex 4 minutes ago \| parent \| next [-]
		Also, at behavioural edges what you'll see on Godbolt is compiler bugs. So you learn nothing about what should happen. All popular modern C++ compilers have known bugs and while I'm sure there are C compilers with no known bugs that will be because nobody tested very hard.
	▲	saagarjha 8 minutes ago \| parent \| prev [-]
		I made the weaker claim that x can be removed. This is something I could prove with compiler output but I would have to find a compiler willing to make this optimization which is not something I can guarantee.

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saagarjha 19 minutes ago | parent | prev [-]

No, compilers will often choose to not optimize on UB.