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Subject: Re: [ub] launder and aliasing
From: Robert Haberlach (rh633_at_[hidden])
Date: 2016-02-26 06:58:15


On 02/26/2016 12:45 PM, Jens Maurer wrote:
> On 02/26/2016 01:12 PM, Robert Haberlach wrote:
>> I apologize in advance if this was discussed before; I didn't check the entire archive. Consider
>>
>> static_assert(alignof(float) >= alignof(int) && sizeof(float) >= sizeof(int));
>> int foo(float f) {
>> return *std::launder(reinterpret_cast<int*>(&f)); }
>>
>> As it stands, invocation of foo is undefined, as the argument to launder is not pointing to an object of type int (or similar) within its lifetime,
>> violating launder's requirement.
>>
>> launder is designed to inhibit address propagation analysis, which is the only concerning optimization. Moreover, as long as all usual conditions are
>> met (alignment, size & trap values), this should be fine on any implementation. If so, can we make the above formally well-defined?
>> Being able to use launder in such scenarios would render circumlocutions via memcpy superfluous.
>
> That the suggested approach not enough to make this work; see 3.10p10.

Sorry if I was unclear; I didn't (yet) mean to suggest any specific strategy to make this defined. Clearly, more than just the specification of
launder needs to be adjusted. My question is rather whether allowing the shown code itself is sensible.


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