Status: Accepted
Owner: [email protected]
CC: [email protected]
Labels: Type-Bug Priority-Medium
New issue 2974 by [email protected]: Repeated redefinition of functions
in an object leads to different maps
http://code.google.com/p/v8/issues/detail?id=2974
Consider following test case:
function f(o) { o.z = 1; }
var loops = 5;
for (var j = 0; j < 5; j++) {
print("\n===================================\n");
o = { };
o.a = function() { };
o.b = function() { };
o.c = function() { };
o.d = function() { };
o.e = function() { };
o.z = 0;
%DebugPrint(o);
for (var i = 0; i < 10000; i++) f(o);
}
This emulates the behavior of running audio-oscillator several times. The
function f is optimized in every loop, and deopted in the following loop
because object o has a different map. The reason is that, when we start out
with map A0, the first run of the loop looks like this regarding map
transitions:
A0: {} -> A1 { a: constant } -> A2 { a: constant, b: constant } -> A3 { a:
constant, b: constant, c: constant } -> ...
In the second loop, we assign o.a again, which causes a to become a field
instead of constant.
A0: {} -> B1 { a: field } -> B2 { a: field, b: constant } -> B3 { a: field,
b: constant, c: constant } -> ...
In the third loop:
A0: {} -> B1 { a: field } -> C2 { a: field, b: field } -> B3 { a: field, b:
field, c: constant } -> ...
So we figure out that we should use fields instead of constants one by one,
causing map changes all the time.
Calling JSObject::AddFastProperty instead of JSObject::AddConstantProperty
in JSObject::AddProperty (as a hack) fixes this problem.
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