verify that WeakMap works.

gistfile1.txt

[2018-02-19 15:22:01] yangguo@yangguo2:~/v8$ g out.gn/x64.debug/d8 --expose-gc --allow-natives-syntax
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Reading symbols from out.gn/x64.debug/d8...done.
Unable to determine compiler version.
Skipping loading of libstdc++ pretty-printers for now.
Non-google3 binary detected.
(gdb) r
Starting program: /usr/local/google/home/yangguo/v8/out.gn/x64.debug/d8 --expose-gc --allow-natives-syntax
V8 version 6.6.0 (candidate)
d8> var x = (function() { var o1 = {z:12}; var o2 = {z:13}; var m = new WeakMap(); m.set(o1, o2); return [o1, m]; })()
undefined
d8> %DebugPrint(x[1])
DebugPrint: 0x1d7ba0c0d559: [JSWeakMap]
 - map: 0x3dd5bd084191 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x12cc36091ed9 <Object map = 0x3dd5bd0841e1>
 - elements: 0x38bb5bb82251 <FixedArray[0]> [HOLEY_ELEMENTS]
 - table: 0x1d7ba0c0d581 <HashTable[11]>
 - properties: 0x38bb5bb82251 <FixedArray[0]> {}
0x3dd5bd084191: [Map]
 - type: JS_WEAK_MAP_TYPE
 - instance size: 40
 - inobject properties: 0
 - elements kind: HOLEY_ELEMENTS
 - unused property fields: 0
 - enum length: invalid
 - stable_map
 - back pointer: 0x38bb5bb822e1 <undefined>
 - instance descriptors (own) #0: 0x38bb5bb82231 <DescriptorArray[2]>
 - layout descriptor: (nil)
 - prototype: 0x12cc36091ed9 <Object map = 0x3dd5bd0841e1>
 - constructor: 0x12cc36091df1 <JSFunction WeakMap (sfi = 0x38bb5bbaae91)>
 - dependent code: 0x38bb5bb82251 <FixedArray[0]>
 - construction counter: 0

[object WeakMap]

d8> ^C[New Thread 0x7f199a51c700 (LWP 249498)]
[New Thread 0x7f199ad1d700 (LWP 249497)]
[New Thread 0x7f199b51e700 (LWP 249496)]
[New Thread 0x7f199bd1f700 (LWP 249495)]
[New Thread 0x7f199c520700 (LWP 249494)]
[New Thread 0x7f199cd21700 (LWP 249493)]
[New Thread 0x7f199d522700 (LWP 249492)]
[New Thread 0x7f199dd23700 (LWP 249491)]

Program received signal SIGINT, Interrupt.
0x00007f199e21e6ad in read () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) job 0x1d7ba0c0d581
0x1d7ba0c0d581: [HashTable]
 - map: 0x206db52826b1 <Map(HOLEY_ELEMENTS)>
 - length: 11
           0: 1
           1: 0
           2: 4
         3-6: 0x38bb5bb822e1 <undefined>
           7: 0x1d7ba0c0d4e1 <Object map = 0x3dd5bd08c021>
           8: 0x1d7ba0c0d539 <Object map = 0x3dd5bd08c021>
        9-10: 0x38bb5bb822e1 <undefined>
(gdb) job 0x1d7ba0c0d539
0x1d7ba0c0d539: [JS_OBJECT_TYPE]
 - map: 0x3dd5bd08c021 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x12cc360847d1 <Object map = 0x3dd5bd0822a1>
 - elements: 0x38bb5bb82251 <FixedArray[0]> [HOLEY_ELEMENTS]
 - properties: 0x38bb5bb82251 <FixedArray[0]> {
    #z: 13 (data field 0)
 }
(gdb) c
Continuing.
x[0] = null
null
d8> gc()
undefined
d8> %DebugPrint(x[1]) 
DebugPrint: 0xe781cb08451: [JSWeakMap]
 - map: 0x3dd5bd084191 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x12cc36091ed9 <Object map = 0x3dd5bd0841e1>
 - elements: 0x38bb5bb82251 <FixedArray[0]> [HOLEY_ELEMENTS]
 - table: 0xe781cb08479 <HashTable[11]>
 - properties: 0x38bb5bb82251 <FixedArray[0]> {}
0x3dd5bd084191: [Map]
 - type: JS_WEAK_MAP_TYPE
 - instance size: 40
 - inobject properties: 0
 - elements kind: HOLEY_ELEMENTS
 - unused property fields: 0
 - enum length: invalid
 - stable_map
 - back pointer: 0x38bb5bb822e1 <undefined>
 - instance descriptors (own) #0: 0x38bb5bb82231 <DescriptorArray[2]>
 - layout descriptor: (nil)
 - prototype: 0x12cc36091ed9 <Object map = 0x3dd5bd0841e1>
 - constructor: 0x12cc36091df1 <JSFunction WeakMap (sfi = 0x38bb5bbaae91)>
 - dependent code: 0x38bb5bb82251 <FixedArray[0]>
 - construction counter: 0

[object WeakMap]
d8> ^C
Program received signal SIGINT, Interrupt.
0x00007f199e21e6ad in read () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) job 0xe781cb08479
0xe781cb08479: [HashTable]
 - map: 0x206db52826b1 <Map(HOLEY_ELEMENTS)>
 - length: 11
           0: 0
           1: 1
           2: 4
         3-6: 0x38bb5bb822e1 <undefined>
         7-8: 0x38bb5bb82321 <the_hole>
        9-10: 0x38bb5bb822e1 <undefined>