525527 - TM: avoid uint16 in JSXML (slow and can interfere with conservative stack scanning)

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Comment 1

[Andreas Gal :gal]

Attachment: patch

Comment 2

[Igor Bukanov]

Comment on attachment 409383 [details] [diff] [review]
patch

>diff --git a/js/src/jsxml.h b/js/src/jsxml.h
>--- a/js/src/jsxml.h
>+++ b/js/src/jsxml.h
>@@ -107,19 +107,18 @@ struct JSXML {
> #ifdef DEBUG_notme
>     JSCList             links;
>     uint32              serial;
> #endif
>     JSObject            *object;
>     void                *domnode;       /* DOM node if mapped info item */
>     JSXML               *parent;
>     JSObject            *name;
>-    uint16              xml_class;      /* discriminates u, below */
>-    uint16              xml_flags;      /* flags, see below */
>-    uint32              align;
>+    uint32              xml_class;      /* discriminates u, below */
>+    uint32              xml_flags;      /* flags, see below */

Nit: comment here that all fields must be at least 4 bytes in size and that for code simplicity we tolerate wasteful uint32 usage for xml_flags and xml_class. Also refer to this bug for reasons.

Comment 3

[Andreas Gal :gal]

No space is wasted here. I eliminated the alignment field. So this patch is strictly faster without wasting a single bit. I am not convinced the 16-bit fields are actually a problem. This patch is mostly preventive. We don't comment JSObject either. I think I will add a comment to the conservative scanning code instead.

Comment 4

[Andreas Gal :gal]

http://hg.mozilla.org/tracemonkey/rev/444068768c1e

Comment 5

[Robert Sayre]

http://hg.mozilla.org/mozilla-central/rev/444068768c1e