P0528R3 The curious case of padding bits, featuring atomic compare-and-exchange #2154
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source/atomics.tex
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| operations may result in failed comparisons for values that compare equal with | ||
| \tcode{operator==} if the underlying type has padding bits, trap bits, or alternate | ||
| \begin{note} | ||
| Under cases where \tcode{memcpy} and \tcode{memcmp} semantics of the compare-and-exchange |
| // Padding here. | ||
| unsigned biff = 0xC0DEFEFE; | ||
| }; | ||
| atomic<padded> pad = ATOMIC_VAR_INIT({}); |
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Drop the Cism of ATOMIC_VAR_INIT here and use = padded{} or ({})?
(I've also asked on LWG whether it's intended that atomic<padded> pad = {}; should work. But regardless of whether it should, that is not guaranteed today.)
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You need ATOMIC_VAR_INIT until Nico's fix P0883R1 makes it in.
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Leaving as is then? We can change it later if that's appropriate.
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Yes, I think you should leave it as-is and editorially remove uses of ATOMIC_VAR_INIT when P0833R1 is voted in. This was discussed in CWG and people decided to keep ATOMIC_VAR_INIT in my paper for now. Of course the example isn't normative, so 🤷♂️
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We don't need P0833R1. That permits
atomic<padded> pad = {};
but either of the forms I suggested work fine without P0833, because they use the atomic(T) constructor, not the default constructor.
Still... if CWG wants the ATOMIC_VAR_INIT, then let's leave it alone.
| double celestia = 0.; | ||
| short luna; // padded | ||
| }; | ||
| atomic<pony> princesses = ATOMIC_VAR_INIT({}); |
tkoeppe
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Sorry, I force-pushed this one rather than making a new commit, because I had already rebased on the new master (even though for this branch that wasn't even necessary). I'll try to preserve commits in the future. |
…d-exchange Added a cross-reference for "padding bits".
zygoloid
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Added a cross-reference for "padding bits".
Fixes #2121.