https://bugs.webkit.org/show_bug.cgi?id=118914

Reviewed by Geoffrey Garen.

Source/JavaScriptCore: 

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.

This fix is ~800x speedup on the accompanying regression test :-o

* runtime/ArrayConventions.h:
(JSC::indexIsSufficientlyBeyondLengthForSparseMap):
* runtime/JSObject.cpp:
(JSC::JSObject::putByIndexBeyondVectorLengthWithoutAttributes):
(JSC::JSObject::putByIndexBeyondVectorLengthWithArrayStorage):
(JSC::JSObject::putByIndexBeyondVectorLength):
(JSC::JSObject::putDirectIndexBeyondVectorLengthWithArrayStorage):

LayoutTests: 

Added new regression test for put-by-val-ing to a blank indexing type with a large index.
This fix is ~800x speedup on this regression test :-o

* fast/js/regress/put-by-val-large-index-blank-indexing-type.html: Added.
* fast/js/regress/script-tests/put-by-val-large-index-blank-indexing-type.js: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153374 268f45cc-cd09-0410-ab3c-d52691b4dbfc