Bug 118914 – Setting a large numeric property on an object causes it to allocate a huge backing store

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Mark Hahnenberg]

There are two distinct actions that we're trying to optimize for:

new Array(100000);

and:

a = [];
a[100000] = 42;

In the first case, the programmer has indicated that they expect this Array to be very big, so they should get a contiguous array up until some threshold, above which we perform density calculations to see if it is indeed dense enough to warrant being contiguous.

In the second case, the programmer hasn't indicated anything about the size of the Array, so we should be more conservative and assume it should be sparse until we've proven otherwise.

Currently both of those cases are handled by MIN_SPARSE_ARRAY_INDEX. We should distinguish between them for the purposes of not over-allocating large backing stores like we see on http://www.peekanalytics.com/burgerjoints/

The way that we'll do this is to keep the MIN_SPARSE_ARRAY_INDEX for the first case, and introduce a new heuristic for the second case. If we are putting to an index above a certain threshold (say, 1000) and it is beyond the length of the array, then we will use a sparse map instead. So for example, in the second case above the empty array has a blank indexing type and a length of 0. We put-by-val to an index > 1000 and > a.length, so we'll use a sparse map.

[Mark Hahnenberg]

Created attachment 207135 [details]
Patch

[Mark Hahnenberg]

This patch is neutral on all other benchmarks we track.

[Mark Hahnenberg]

Still need to add test expectations for regression test.

[Geoffrey Garen]

Comment on attachment 207135 [details]
Patch

r=me

[Build Bot]

Comment on attachment 207135 [details]
Patch

Attachment 207135 [details] did not pass mac-ews (mac):
Output: http://webkit-queues.appspot.com/results/1131309

New failing tests:
fast/js/regress/put-by-val-large-index-blank-indexing-type.html

[Build Bot]

Created attachment 207141 [details]
Archive of layout-test-results from webkit-ews-04 for mac-mountainlion

The attached test failures were seen while running run-webkit-tests on the mac-ews.
Bot: webkit-ews-04  Port: mac-mountainlion  Platform: Mac OS X 10.8.3

[Build Bot]

Comment on attachment 207135 [details]
Patch

Attachment 207135 [details] did not pass mac-wk2-ews (mac-wk2):
Output: http://webkit-queues.appspot.com/results/1131351

New failing tests:
fast/js/regress/put-by-val-large-index-blank-indexing-type.html

[Build Bot]

Created attachment 207157 [details]
Archive of layout-test-results from webkit-ews-13 for mac-mountainlion-wk2

The attached test failures were seen while running run-webkit-tests on the mac-wk2-ews.
Bot: webkit-ews-13  Port: mac-mountainlion-wk2  Platform: Mac OS X 10.8.3

[Build Bot]

Comment on attachment 207135 [details]
Patch

Attachment 207135 [details] did not pass win-ews (win):
Output: http://webkit-queues.appspot.com/results/1247234

New failing tests:
dom/xhtml/level1/core/documentinvalidcharacterexceptioncreateentref1.xhtml
dom/svg/level3/xpath/XPathEvaluator_evaluate_INVALID_EXPRESSION_ERR.svg
dom/html/level2/events/dispatchEvent04.html
dom/html/level2/html/HTMLSelectElement20.html
dom/svg/level3/xpath/XPathEvaluator_createExpression_INVALID_EXPRESSION_ERR.svg
dom/svg/level3/xpath/XPathEvaluator_createExpression_NAMESPACE_ERR_02.svg
dom/html/level1/core/documentinvalidcharacterexceptioncreateentref.html
dom/html/level2/core/hc_namednodemapinvalidtype1.html
dom/svg/level3/xpath/XPathEvaluator_evaluate_NAMESPACE_ERR.svg
dom/html/level2/events/dispatchEvent01.html
dom/xhtml/level1/core/hc_attrappendchild4.xhtml
dom/html/level2/events/dispatchEvent03.html
dom/html/level2/events/dispatchEvent02.html
dom/html/level2/core/createDocumentType04.html
dom/html/level1/core/documentinvalidcharacterexceptioncreateentref1.html
dom/html/level1/core/documentinvalidcharacterexceptioncreatepi1.html
dom/html/level2/events/dispatchEvent06.html
css1/basic/comments.html
dom/xhtml/level1/core/documentinvalidcharacterexceptioncreatepi1.xhtml
dom/html/level2/events/dispatchEvent07.html
dom/html/level2/core/setAttributeNS10.html
dom/html/level2/events/dispatchEvent05.html
dom/html/level1/core/hc_attrappendchild2.html
dom/html/level2/core/createAttributeNS06.html
dom/html/level1/core/hc_attrappendchild4.html
dom/xhtml/level1/core/documentinvalidcharacterexceptioncreatepi.xhtml
dom/svg/level3/xpath/XPathEvaluator_createExpression_NAMESPACE_ERR_01.svg
dom/xhtml/level1/core/documentinvalidcharacterexceptioncreateentref.xhtml
dom/xhtml/level1/core/hc_attrappendchild2.xhtml
dom/html/level1/core/documentinvalidcharacterexceptioncreatepi.html

[Build Bot]

Created attachment 207530 [details]
Archive of layout-test-results from APPLE-EWS-5 for win-future

The attached test failures were seen while running run-webkit-tests on the win-ews.
Bot: APPLE-EWS-5  Port: win-future  Platform: CYGWIN_NT-6.1-WOW64-1.7.20-0.266-5-3-i686-32bit

[Mark Hahnenberg]

Committed r153374: <http://trac.webkit.org/changeset/153374>

[Geoffrey Garen]

<rdar://problem/14443495>

[David Kilzer (:ddkilzer)]

<rdar://problem/14443495>