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chrono::ceil
has surprising requirementSection: 29.5.8 [time.duration.cast] Status: New Submitter: Jonathan Wakely Opened: 2020-11-18 Last modified: 2020-11-29
Priority: 3
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Discussion:
29.5.8 [time.duration.cast] p7 requires that the return value is "The least result t
representable in ToDuration
for which t >= d
".
chrono::ceil<chrono::microseconds>(chrono::duration<float, milli>(m)).count()
is required to be the smallest integer n
such that (float)n == m*1000.0f
, which might be less
than the mathematically correct value of m x 1000
.
(The specific values below assume float
uses the IEEE binary32 format and default rounding, but
similar problems will exist for other formats, even if the specific values are different.)
For example, if m == 13421772.0f
then the naively expected result is n == 13421772000
, but
the standard requires n == 13421771265
, a significantly lower value. This surprising result is a
consequence of how the chrono::ceil
spec interacts with floating-point arithmetic, due to the fact that
for the integers in the range [13421770753, 13421772799]
, only one can be exactly represented as
32-bit float
. All but that one will be rounded to a different value when converted to float
.
A straightforward implementation of chrono::ceil
will produce (long long)(13421772.0f * 1000)
which is 13421771776
, which is less than the expected result, but compares equal using the t >= d
expression. That expression converts both operands to their common_type
, which is
chrono::duration<float, micro>
. That means we compare (float)13421771776 >= (13421772.0f * 1000)
which is true
. But the spec requires an even worse result. All integers in [13421771265, 13421771776)
are also rounded to that value when converted to float
. That means chrono::microseconds(13421771265)
is "the least result representable in ToDuration
for which t >= d
".
Meeting the "least result" requirement is impractical, and unhelpful. The straightforward result 13421771776
is already lower than the naively expected result (which is surprising for a "ceil" function). To meet the
standard's requirements the implementation would have to do extra work, just to produce an even lower (and even
more surprising) result.
It might be impractical to require the naively expected value to be returned (the additional work might have
unacceptable performance implications), but the standard should at least permit the straightforward result
instead of requiring an even worse one.
The same problem almost certainly exists for chrono::floor
in reverse.
[2020-11-29; Reflector prioritization]
Set priority to 3 during reflector discussions.
Proposed resolution: