Den lör 25 maj 2019 kl 10:13 skrev René J.V. Bertin <[email protected]>:
>
> Hi,
>
> I can't seem to wrap my head around what one can do with Q_ASSUME, i.e. which
> will be the code for which the compiler won't emit code (and how the compiler
> could know not to emit code as a function of a runtime condition?!)
I hadn't heard about this compiler hint, but isn't it simply that
putting in Q_ASSUME(cond), then the compiler is free in its code
generation to not generate any code to handle cases when cond is
false?
E.g.
Q_ASSUME(!atWar);
if (atWar) {
fireNukes(); <-- Oops, nukes may not be fired, even if at war,
because compiler may have taken the hint and assumed we're not at war
}
Microsoft seems to have a nice article about their __assume (which
Q_ASSUME expands to):
https://docs.microsoft.com/en-us/cpp/intrinsics/assume?view=vs-2019
>
> Squinting at the macros it seems evident that you cannot do something like
> this, which to me the documentation suggests (and I think) you SHOULD be able
> to do:
>
> Q_ASSUME(conditionsMet, {
> // do something that should be done only when conditions are met
> });
I don't understand. That just looks like an if statement?
Elvis
>
>
> R
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