FW: debugging interfaces

Allen Wirfs-Brock Allen.Wirfs-Brock at microsoft.com
Thu Aug 13 12:45:02 PDT 2009

Actually meant to forward this one rather than the previous Brendan message...

From: es5-discuss-bounces at mozilla.org [mailto:es5-discuss-bounces at mozilla.org] On Behalf Of Brendan Eich
Sent: Thursday, August 13, 2009 11:47 AM
To: David-Sarah Hopwood
Cc: es5-discuss at mozilla.org
Subject: Re: debugging interfaces

On Aug 13, 2009, at 11:33 AM, David-Sarah Hopwood wrote:

Jordan Osete wrote:

Then, if we accept the idea of keeping only a reference to the objects
until information about them is explicitly requested, couldn't we do the
same things for the variables and arguments objects ?

Keeping a reference to these would interfere with the use of optimized
calling conventions where arguments are only stored in machine registers
and never reified (when 'arguments' is not referred to). Similarly for
variable frames, which need not be reified unless they are captured by a

Right. Or, even moreso, if possible -- there may be no variable object reified in an implementation that optimizes harder. ES1-5 take pains to avoid any reference to such an object escaping. Firefox 3.5 optimizes various closure cases including non-escaping Algol-like procedures (using a display) and closures capturing write-once variables whose initialisers dominate the closure ("flat closures", what Chez Scheme calls "display closures"). Assignment conversion (a la Chez Scheme) is being explored.

If we impose a model where you have to "compile with -g" to get first-class stack inspection, otherwise you get nothing, developers will always turn on the deoptimizing debugger-friendly option, after being burned by missing the chance to diagnose a bug in flagrante because they were running in optimized mode.

I like Christian's proposal, FWIW. We should see about other VMs implementing it.


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