Spec proposal

Lars T Hansen lth at acm.org
Thu Dec 13 12:50:51 PST 2007


On Dec 13, 2007 5:07 PM, Michael O'Brien <mob at mbedthis.com> wrote:
> I understand that the cut off for proposals is long past. But I believe
> this is an important issue that will force non-standard implementations
> .... bad.
>
> Proposal:
>
> Running ECMAScript on embedded devices is more and more common. It is
> being used in mobile phone widget engines, mobile browsers and it a wide
> variety of embedded infrastructure hardware. However many such devices
> do not have floating point and this has forced non-standard language
> extensions to allow other default numeric types. For example: most
> feature phones (> 500M devices) do not have floating point and have very
> modest CPU resources. Furthermore, the proposed ES4 decimal standard is
> currently implemented in software as there is no common hardware support
> for it (yet). Consequently, it is fairly large and slow and will
> certainly put a strain on feature phones!
>
> We should allow the default number type to be an integer or 64 bit
> integer. Many of embedded devices have very good 64bit integer support
> that is quite fast. 64 bit integer support is important if floating
> point or decimal is not available to provide a wider scale of numeric
> values.
>
> To do this, I'd like to propose that:
>
> 1. We change the "use decimal" pragma to "use number NUMBER_TYPE". This
> would allow for other number types. E.g.
>
>     use number decimal
>     use number int
>
> 2. We add support for int as the default number
>
> 3. We add "long" and ulong types that are by definition 64 bit. Also
> support "use number long"

I have a couple of comments.

First, the "use decimal" pragma has been changed radically from older
proposals and no longer allows the program to select "decimal
everywhere" semantics.  (What it does allow is a little bit in flux.
At a minimum it allows controlling rounding and precision.)

Second, several good implementations of ES3 fall back on double
arithmetic sparingly in practice by using clever representations; for
example, "1" is represented as an int, "1 + 2" uses int arithmetic
only (with an overflow check).  The same trick could be used if the
default were decimal.  An implementation could extend this principle
up to 53 bits of precision, after which it would have to go to
floating point or become very clever.  "53 bits ought to be enough for
anyone."  (Making all its values decimal would give the program in
excess of 100 bits of integer precision ;-)

My beef with ECMAScript at this point is that it has no integer divide
operator and no easy way to signal to the implementation that that's
what desired.  In practice, "/" forces us into floating point.

--lars



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