Re: "Built upon 754"

["Ian McIntosh" <ianm@xxxxxxxxxx>]

1788 has needs that are provided by some other but not all other floating-point designs:
- It needs representations of +Infinity and -Infinity, or some alternate way of indicating those are the bounds.
- It needs the ability to round towards either +Infinity or -Infinity or preferably both.
There are other floating-point formats that provide both of those (eg, Intel 80-bit double extended). There are also recent (eg, Cell SPU single precision, with no Infinities) and many older formats (eg, IBM hex) that do/did not.

While 1788 mentions the possibility of the bounds being a 754 format, it doesn't require that, so isn't exactly "built on 754". Instead it's "built on several features 754 provides".

There are other things 1788 does not need, like having exceptions set flags, not 754, and possibly trap as in most systems using 754. Implementations may benefit from the ability to suppress traps, which is not provided by some alternate formats (eg, IBM hex). It also isn't helped by 0 times Infinity = NaN, when what it needs is 0 times Infinity = 0 because an Infinity bound represents some almost infinite finite number.

In some future revision we might want to be explicit about what floating-point features 1788 needs or benefits from, with 754 mentioned only as a good example of formats providing those features.

- Ian McIntosh IBM Canada Lab Compiler Back End Support and Development


John Pryce ---2016-01-27 12:19:28 PM---Ulrich On 27 Jan 2016, at 17:08, Ulrich Kulisch <ulrich.kulisch@xxxxxxx> wrote:

From: John Pryce <j.d.pryce@xxxxxxxxxx>
To: Ian McIntosh/Toronto/IBM@IBMCA
Date: 2016-01-27 12:19 PM
Subject: Re: "Built upon 754"

---

Ulrich
On 27 Jan 2016, at 17:08, Ulrich Kulisch <ulrich.kulisch@xxxxxxx> wrote:
> So I think that idealy the two standards IEEE 754 and IEEE 1788 also should be kept strictly separate. I would even say that any mentioning of  IEEE 754 and its exceptions in IEEE 1788 is a possible source of confusion. If such mentionings  like +0, -0, NaN, IEEE 754 type, IEEE conformant type, and other hints to IEEE 754 would be eliminated in the text of  IEEE 1788 I would fully agree with it. 
But as I asked: cite me one specific sentence in 1788 that is "built on" 754, then we can discuss.
John P