On Mon, 21 Mar 2022 10:35:25 +0000 Jonathan Wakely via Liaison wrote:
>
>There is some overlap with parts of
>http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2551r0.pdf
>regarding what "has_denorm" means for the new C++ traits. But the answer to
>"what does the C macro mean?" and "what does the C++ trait mean?" should
>not be the same, as they're actually asking different questions, and so the
>proposed changes in this paper do not impact C++.
I have looked at p2551r0 and p1841r2 and I do not find "has_denorm" in either.
The new denorm_min introduced by P1841 is supposed to make it unnecessary, but doesn't really do so, which is the subject of question 1 in P2551. Question 4 in P2551 is also related.
The only version of C++ that I have is WG21 N4860. In it, I do find "has_denorm"
and it appears to be equivalent to C's *_HAS_SUBNORM.
Yes, and P1841 adds a new way to get that same info, but requires you to check whether std::denorm_min<T>::value == std::norm_min<T>::value, which is awkward. That's what P2251 Q1 is about.
It appears that neither standard can give those a useful value if the treatment
of subnormals can be changed at runtime (as can be done on ARM chips).
They also do not cover the two case where:
operands are flushed to zero, but results are not flushed.
results are flushed, but operands are not flushed.
C implementations should define the macros as: -1 indeterminable.
C++ implementations should define as: denorm_indeterminate
While that is the correct value, it is not useful.
Aside: References to 559 should be to 60559.
There should be "norm_max" for the maximum normalized number.
That's std::numeric_limits<T>::max() in all published standards, and std::finite_max<T>::value with the P1841 additions.
The definition of "epsilon" in C++ differs from that in C. C added
"normalized" to the definition. It mattes for the case where
long double is implemented as a pair of doubles.
Does "round_error" cover subnormals numbers flushed to zero?
---
Fred J. Tydeman Tydeman Consulting
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