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Section: 32.5 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Right now, the compare_exchange_weak loop should rapidly converge on the
padding contents. But compare_exchange_strong will require a bit more
compiler work to ignore padding for comparison purposes.
Note that this isn't a problem for structs with no padding, and we do
already have one portable way to ensure that there is no padding that
covers the key use cases: Have elements be the same type. I suspect that
the greatest need is for a structure of two pointers, which has no
padding problem. I suspect the second need is a structure of a pointer
and some form of an integer. If that integer is intptr_t, there will be
no padding.
Related but separable issue: For unused bitfields, or other unused
fields for that matter, we should probably say it's the programmer's
responsibility to set them to zero or otherwise ensure they'll be
ignored by memcmp.
Proposed resolution: Using
atomic<struct-with-padding>::compare_exchange_strong should be either:
I propose Option 1 for C++0x for expediency, though I'm not sure how to
say it. I would be happy with Option 2, which I believe would mean that
compare_exchange_strong would be implemented to avoid comparing padding
bytes, or something equivalent such as always zeroing out padding when
loading/storing/comparing. (Either implementation might require compiler
support.)
[ Summit: ]
Move to open. Blocked until concepts for atomics are addressed.
[ Post Summit Anthony adds: ]
The resolution of LWG 923(i) should resolve this issue as well.
[ 2009-10 Santa Cruz: ]
NAD EditorialResolved. Addressed by N2992.
Proposed resolution: