Adobe position paper on the ECMAScript 4 proposal space -- decimal

Lars Hansen lhansen at adobe.com
Wed Feb 27 05:33:31 PST 2008


Hi Mike,
 
taking your questions in order:
 
Regarding "Premature" / "Knowledgable users"
 
When we say decimal is "premature" we mean it in the sense outlined in
the paper only: The TG had an intuition about what we wanted for the
language (The Right Thing - essentially a type of big red switch) but
then we came to the conclusion that what we wanted is simply not
workable on the web (see below), and therefore we now no longer know
exactly what it is that we want.  Therefore decimal is premature in the
sense we use here: more experience may be necessary before the TG
members know what we want.  As authors of the position paper, Jeff and I
did not and do not mean to imply that decimal arithmetic, considered in
isolation, is immature, far from it.
 
About the big red switch.  There are two variants: One is global to an
execution context, were a program sets the global floating point mode to
be decimal and all arithmetic is carried out in decimal (and all
literals are decimal); the other is lexically scoped, where a "use
decimal" pragma affects operators and literal interpretation in its
lexical scope.
 
(In either case the implementation will use a polymorphic number
representation and perform most arithmetic on integers, of course, but
on overflow the representation becomes decimal floating point rather
than binary floating point if the switch has been turned on.)
 
To take the global case first:
 
Two of the themes that have been very prominent in the work of the TG
have been that ES4 must support existing ES3 content exceptionally well,
and that we must expect that most web sites that migrate to ES4 will not
do so all at once, but will upgrade some of their code to ES4, leaving
other code as unmodified ES3.  For example, one could imagine a page
that has exception handlers and some infrastructure code that are all
ES3 code but which loads ES4 code dynamically if the browser supports
ES4.  Mashups are another example; in this case, the page author has no
control over the code loaded into the page (and for a long time most of
it will be ES3 code).  As a third case, code on one page can call code
on another page (or in a frame or iframe).  In summary, the TG has
considered it to be unacceptable to require a program to consist of all
ES3 code or all ES4 code, mixing is going to be the reality.
 
As a consequence of those compatibility concerns, it is unacceptable for
one piece of code to alter the floating point environment of the other
code by flipping the big red switch to "decimal", because the
consequences of doing so are unknown.  Of course decimal has a larger
exponent range and higher precision than double-precision floating
point; but my experience with ES on the web is that people write code
with all sorts of dependencies, and many TG members believe that there
is important code out there that depends on the number representation
being "double".
 
As a compromise it would be possible to make the big red switch apply
only to ES4 code that runs in an all-ES4 environment, but my sense is
that the browser vendors do not believe that is a particularly
compelling use case (nor does Adobe).
 
The lexically scoped case has other problems.  On the one hand, the
programmer controls when "1" means "decimal(1)" and when "a+b" means
"decimal(a)+decimal(b)".  On the other hand, the question is really how
far this control extends.  What happens when code in the context of "use
decimal" reads a number from a data structure defined outside that
scope, or receives a number from a function defined outside that scope
-- is the number converted to decimal?  What if the code stores a
decimal value in an external data structure or passes it to an external
function that expects a double?  The issues of compatibility with
existing ES3 code reappear in this context.  
 
Furthermore, the intent was also that the "use decimal" pragma should
control the precision and rounding settings in a lexical fashion, but
that precludes those settings being passed off implicitly to separately
defined functions or library functions that (maybe!) should take them
into account.
 
Overall, the lexically scoped case has usability issues, and TG members
(myself as much as anyone) were not convinced that they can be overcome
at this time.  More experience is needed.
 
Maciej has already discussed performance concerns surrounding a general
switch to decimal.  Since a polymorphic number representation ensures
that decimal is used only for floating-point numbers, any concerns
should apply mainly to programs that perform a significant amount of
floating-point arithmetic (programs that have a significant number of
overflows from machine integer but which then chop values back to
integer would be impacted too).  I believe strongly that there are web
applications out there that will see a significant slowdown, but I do
not know how important the problem is in practice.  Speaking for Adobe,
current ActionScript code tends to use strong static typing and very
specific number types, so most existing ActionScript code would not be
impacted much.  Once ActionScript aligns with ES4 the issue is more
open.
 
None of the above speaks to the possibility that "decimal" might be a
distinct data type in the language, of course, along with "double" and
"int" and "uint".  Such a data type would in our opinion not be a poor
fit for ES4, and as your own writings demonstrate, there are good use
cases for it.  We think a few of our customers, and probably a few users
on the web, would find the data type useful.  In fact, Adobe has
implemented prototypical support for decimal in the Tamarin virtual
machine, we have polled some of our users about the need for a distinct
decimal type, and we are evaluating whether to support the data type in
our compilers.  So when we now advocate cutting the type from the
language it is in part because we also advocate cutting the int and uint
types, which also seem premature (less well thought out than they
probably ought to be).  The simplification of going back to the ES3
numerics model without any of these three types is substantial in our
opinion.  If there was a huge demand for a separate decimal type then
the situation might be different; as it is, the need is there but
probably marginal, and it is in our estimation not worth the complexity
cost to the language (also taking into consideration the next two
issues).
 
"Hardship on small systems"
 
A 50KB piece of code (your decimal code + various levels of support in
libraries and the on-line compiler) would increase the size of at least
one commercial, compliant, high-performance ES engine I know of by more
than 20%, and it would increase the size of another by more than that
still.  Either way 50KB is quite a bit of code on current mid-range
mobile platforms.  (Percentage-wise an increase will be less than 20%
once those engines is upgraded to ES4, granted.)
 
This becomes a matter of opinion, but the TG has in fact accepted other
restrictions in the proposed language in order to accomodate very small
systems, such as using a constructor function prefix syntax (a la C++)
to initialize nullable and constant instance properties instead of
requiring definite assignment analysis (which would require building
trees for the program in the compiler -- something that is not necessary
for ES3, and which we hope to avoid requiring for ES4).
 
"Hardship to implement"
 
Speaking as somebody who used to work for an OEM browser vendor: Some
OEM buyers are extremely reluctant to take software that includes open
source, public domain, or other "free" software.  This situation may
change over time, obviously, but at one of my previous employers there
was always pressure to write code from scratch rather than incorporate
source code authored elsewhere, for this very reason.  It just seems to
be a fact of life.  (By itself it would not be a reason to reject any
sort of decimal type, of course.  It's just a contributing fractor.)
 
--lars
 



________________________________

	From: Mike Cowlishaw [mailto:MFC at uk.ibm.com] 
	Sent: 27. februar 2008 12:02
	To: Lars Hansen
	Cc: e-TC39 at ecma-international.org; es4-discuss Discuss
	Subject: Re: Adobe position paper on the ECMAScript 4 proposal
space -- decimal
	
	

	Lars, many thanks for posting the interesting Adobe position
paper.  Could you explain a little more about your position on decimal
support?  In particular: 
	

	*	Your description of the standardization of decimal
arithmetic as 'premature':
		
		The requirement for adding decimal arithmetic to
ECMAScript was raised in TC39 in October 1998.  The notes of TC39 WG
meetings show this was discussed in the meetings of 1998/11 and also of
1999/11.  The final TC39 'futures' list shows decimal support as being
on the very short list of 'provisionally agreed' items for ES4, as of
1999/03. 
		
		In the meantime, the decimal arithmetic in the IEEE 754
revision (754r, now in ballot) has been widely adopted, with extensions
being made to many languages, including  ECMA C# and CLI, Java, ISO C,
Python, and SAP ABAP, to better support it.  A number of C compilers -
including GCC - already have the new decimal support, and software
libraries are available from several sources.  Hardware support is now
available in two server architectures:  power.org's  PowerPC (since June
2007) and IBM's z10 mainframes (announced yesterday).
		
		In short, decimal support in ECMAScript is overdue, not
premature.  
		
	*	The 'use decimal' notation means that only knowledgeable
users will get to use it:
		
		This is a valid criticism.  This approach is not ideal,
but at least means that where decimal arithmetic is essential it is
available easily.  In contrast, it is currently not available at all; it
is very hard to replicate server-side calculations on the client using
binary floating-point.
		
		However, the 'big red switch' approach that you advocate
would probably be better - in essence saying that external environments
can override the arithmetic base used within scripting if the scripting
engine supports that.   What approach would Adobe favor for this?  There
have been a number of suggestions (for example, in an HTML context a
meta tag in the <head> of a page could be used, which page builders
could add by default for new pages).   It is not necessary to define the
mechanism for all hosting environments  (and that is probably outside
the scope of TG1 in any case). 
		
	*	Your assertion that implementing decimal may be a
hardship for implementations on smaller systems:
		
		Our current fixed-size decimal library, including all
the 754r operations on both basic decimal formats (including FMA and
many other functions and modes not needed for ECMAScript) are less than
80kBytes when compiled using GCC for Pentium.   An ECMAScript
implementation, using just one format, would probably need at most a
third of this.
		
	*	Your assertion that implementing decimal may be a
hardship for implementations that cannot use existing open-source
libraries:
		
		There are now a number of open-source libraries
implementing the 754r decimal types.  At least two of these are
essentially 'public domain' licenses.  For example, ours is a part of
the International Components for Unicode (ICU) library, whose license
allows unrestricted use of source and derivative works in any project or
product.  The only requirement is acknowledgement of copyright.   (For
the full text of the license, which is just three paragraphs, see
http://source.icu-project.org/repos/icu/icu/trunk/license.html
<http://source.icu-project.org/repos/icu/icu/trunk/license.html>  .)  
		
		We have had no requests or suggestions that would imply
that the ICU license would prevent any implementation from using the
library.  The ICU libraries are widely used in many projects and
products (including a number of Adobe products).
		

	Thanks - Mike 
	
	- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
- - -
	Mike Cowlishaw, IBM Fellow
	IBM UK (MP8), PO Box 31, Birmingham Road, Warwick, CV34 5JL
	mailto:mfc at uk.ibm.com <mailto:mfc at uk.ibm.com>   --
http://www2.hursley.ibm.com/mfcsumm.html
<http://www2.hursley.ibm.com/mfcsumm.html> 
	
	
	
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