786743 - Need real variable names for debugging self-hosted JavaScript

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Norbert Lindenberg]

Since all self-hosted JavaScript code currently is minified, Error messages relating to this code can have only minified variable names. In a larger program, the name "a" can occur in many functions, making it hard to track down which variable was meant.

There should be a way to get the original variable names in debug builds. The js2c.py script seems to hint at a more comprehensive debugging solution, but as a first step just using the original source code rather than a minified form in debug builds would help.

Comment 1

[Till Schneidereit [:till]]

Attachment: Disable JS minification in debug builds

This patch disables minification for debug builds. The enticingly-looking bits about debugging in JS2C.py are unfortunately related to V8's debug capabilities and thus not related to debugging the self-hosted code itself.

One thing to consider is a flag that can be set to disable the hiding of source code and stack frames for self-hosted code. That could only ever be a runtime-settable flag instead of a build flag or something that's always activated for debug builds. Otherwise, the difference in behavior would certainly be too big.

Comment 2

[Luke Wagner [:luke]]

Comment on attachment 656884 [details] [diff] [review]
Disable JS minification in debug builds

Logically the change makes sense, but I'm completely unfamiliar with this code (and our build system in general...); could you use the original reviewer of this code?

Comment 3

[Till Schneidereit [:till]]

Comment on attachment 656884 [details] [diff] [review]
Disable JS minification in debug builds

Right, I didn't think the request through. Sorry for the noise, Luke.

Comment 4

[Nicholas Nethercote [inactive]]

I don't know the first thing about this stuff, but I wonder:  how important is the minification for performance?

Comment 5

[Till Schneidereit [:till]]

(In reply to Nicholas Nethercote [:njn] from comment #4)
> I don't know the first thing about this stuff, but I wonder:  how important
> is the minification for performance?

It shouldn't matter for performance at all. It does, however, decrease code size. 

Also, I don't know enough about how the bytecode cloning and source code retaining works to know this, but it might also be important for memory use as all self-hosted functions get cloned into globals using them.

Comment 6

[(not currently active) Ted Mielczarek]

Comment on attachment 656884 [details] [diff] [review]
Disable JS minification in debug builds

Review of attachment 656884 [details] [diff] [review]:
-----------------------------------------------------------------

::: js/src/Makefile.in
@@ +839,5 @@
>    $(srcdir)/builtin/macros.py \
>    $(srcdir)/builtin/js2c.py \
>    $(srcdir)/builtin/embedjs.py
>  
> +selfhosting_embed_flags := $(NULL)

You don't need this, you can += to an unset variable and it will work fine.

::: js/src/builtin/embedjs.py
@@ +36,5 @@
>      return line
>  
>  def main():
> +    files_offset = 0
> +    debug = sys.argv[1] == '-d'

This is fine for now. If this script grows any more options you'll want to switch to using OptionParser.

@@ +38,5 @@
>  def main():
> +    files_offset = 0
> +    debug = sys.argv[1] == '-d'
> +    if debug:
> +        files_offset = 1

Just do sys.argv.pop(1) instead of this fiddling.

Comment 7

[Till Schneidereit [:till]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/2f83e74b38ff

Thanks for the review!

Comment 8

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/2f83e74b38ff