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[atomics.types.operations]p20's note makes incorrect assumptions about atomic representation #2104

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zygoloid opened this issue Jun 7, 2018 · 4 comments
Open

[atomics.types.operations]p20's note makes incorrect assumptions about atomic representation #2104

zygoloid opened this issue Jun 7, 2018 · 4 comments
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sg1 Issue must be reviewed by SG1.

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@zygoloid
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zygoloid commented Jun 7, 2018

[ Note: For example, the effect of compare_exchange_strong is

if (memcmp(this, &expected, sizeof(*this)) == 0)
  memcpy(this, &desired, sizeof(*this));
else
  memcpy(expected, this, sizeof(*this));

— end note ]

This is wrong in a multitude of ways... not least of which is that that's not even atomic. But also this assumes that sizeof(atomic<T>) == sizeof(T), that the layout of an atomic directly contains a T, and so on.
@jensmaurer
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Do we want to deal with this editorially, or have SG1 chime in?

@jensmaurer jensmaurer added the decision-required A decision of the editorial group (or the Project Editor) is required. label Jun 28, 2018
@jwakely
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jwakely commented Jun 28, 2018

IIRC SG1 came up with that example, so I think getting their input would be best.

@jensmaurer jensmaurer added the lwg Issue must be reviewed by LWG. label Jun 28, 2018
@jfbastien
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I can whip up a paper that fixes it if you think that's helpful? Another approach would be to wait for us to say that atomic contains a T, which we're working on for the next mailing. Then we can fix this more easily example (perhaps even in the same paper).

@jensmaurer jensmaurer added sg1 Issue must be reviewed by SG1. and removed lwg Issue must be reviewed by LWG. labels Oct 11, 2018
@jensmaurer jensmaurer removed the decision-required A decision of the editorial group (or the Project Editor) is required. label Nov 6, 2018
@jensmaurer
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Editorial meeting: Yes, need SG1 input.

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@zygoloid @jfbastien @jwakely @jensmaurer