←back to thread

C++ proposal: There are exactly 8 bits in a byte

(www.open-std.org)
288 points Twirrim | 3 comments | 17 Oct 24 22:21 UTC | HN request time: 0.619s | source
Show context
favorited ◴[17 Oct 24 23:54 UTC] No.41875023[source]▶
Previously, in JF's "Can we acknowledge that every real computer works this way?" series: "Signed Integers are Two’s Complement" <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p09...>
replies(1): >>41875200 #
jsheard ◴[18 Oct 24 00:25 UTC] No.41875200[source]▶
Maybe specifying that floats are always IEEE floats should be next? Though that would obsolete this Linux kernel classic so maybe not.

https://github.com/torvalds/linux/blob/master/include/math-e...

replies(9): >>41875213 #>>41875351 #>>41875749 #>>41875859 #>>41876173 #>>41876461 #>>41876831 #>>41877394 #>>41877730 #
jcranmer ◴[18 Oct 24 04:54 UTC] No.41876461[source]▶
I'm literally giving a talk next week who's first slide is essentially "Why IEEE 754 is not a sufficient description of floating-point semantics" and I'm sitting here trying to figure out what needs to be thrown out of the talk to make it fit the time slot.

One of the most surprising things about floating-point is that very little is actually IEEE 754; most things are merely IEEE 754-ish, and there's a long tail of fiddly things that are different that make it only -ish.

replies(4): >>41876497 #>>41876510 #>>41876547 #>>41877838 #
1. chungy ◴[18 Oct 24 05:19 UTC] No.41876547[source]▶
The IEEE 754 standard has been updated several times, often by relaxing previous mandates in order to make various hardware implementations become compliant retroactively (eg, adding Intel's 80-bit floats as a standard floating point size).

It'll be interesting if the "-ish" bits are still "-ish" with the current standard.

replies(1): >>41879384 #
2. stephencanon ◴[18 Oct 24 13:40 UTC] No.41879384[source]▶
The first 754 standard (1985) was essentially formalization of the x87 arithmetic; it defines a "double extended" format. It is not mandatory:

> Implementations should support the extended format corresponding to the widest basic format supported.

_if_ it exists, it is required to have at least as many bits as the x87 long double type.¹

The language around extended formats changed in the 2008 standard, but the meaning didn't:

> Language standards or implementations should support an extended precision format that extends the widest basic format that is supported in that radix.

That language is still present in the 2019 standard. So nothing has ever really changed here. Double-extended is recommended, but not required. If it exists, the significand and exponent must be at least as large as those of the Intel 80-bit format, but they may also be larger.

---

¹ At the beginning of the standardization process, Kahan and Intel engineers still hoped that the x87 format would gradually expand in subsequent CPU generations until it became what is now the standard 128b quad format; they didn't understand the inertia of binary compatibility yet. So the text only set out minimum precision and exponent range. By the time the standard was published in 1985, it was understood internally that they would never change the type, but by then other companies had introduced different extended-precision types (e.g. the 96-bit type in Apple's SANE), so it was never pinned down.

replies(1): >>41886225 #
3. adrian_b ◴[19 Oct 24 07:17 UTC] No.41886225[source]▶
The first 754 standard has still removed some 8087 features, mainly the "projective" infinity and it has slightly changed the definition of the remainder function, so it was not completely compatible with 8087.

Intel 80387 was made compliant with the final standard and by that time there were competing FPUs also compliant with the final standard, e.g. Motorola 68881.