HHVM Cyclical Garbage Collecting is broken

### HHVM Version

```
hhvm --version
HipHop VM 3.14.5 (rel)
Compiler: tags/HHVM-3.14.5-0-gc6bf714e6468213eed377527b431e9cd4cec1432
Repo schema: 9ab3fe4281c731189b558d921eb7317b0eb2b2e6
```

```
php --version
PHP 5.5.32 (cli) (built: Feb 4 2016 23:20:47)
Copyright (c) 1997-2015 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2015 Zend Technologies
with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2015, by Zend Technologies
```
### Standalone code, or other way to reproduce the problem

Two test scripts:

```
# cyclical_test.php
right = $right;
$right->left = $left;
$iter++;
if ($iter % 1000000 === 0) {
$formattedIter = number_format($iter, 0);
$memoryUsage = memory_get_usage() / (1024 * 1024);
echo "Survived {$formattedIter} iterations: {$memoryUsage}MB used\n";
}
}

?>
```

```
# non_cyclical_test.php

```
### Expected result

We expect memory usage to stay low and not explode:

```
$ /usr/bin/php cyclical_test.php
Survived 1,000,000 iterations: 1.1863784790039MB used
Survived 2,000,000 iterations: 1.2109756469727MB used
Survived 3,000,000 iterations: 1.2353897094727MB used
Survived 4,000,000 iterations: 1.2598037719727MB used
Survived 5,000,000 iterations: 1.2842178344727MB used
Survived 6,000,000 iterations: 1.3086318969727MB used
Survived 7,000,000 iterations: 1.3330459594727MB used
Survived 8,000,000 iterations: 1.3574752807617MB used
Survived 9,000,000 iterations: 1.3818740844727MB used
Survived 10,000,000 iterations: 1.4062881469727MB used
Survived 11,000,000 iterations: 1.4307022094727MB used
Survived 12,000,000 iterations: 1.4551467895508MB used
Survived 13,000,000 iterations: 1.4796981811523MB used
Survived 14,000,000 iterations: 1.5053634643555MB used
Survived 15,000,000 iterations: 1.5302047729492MB used
Survived 16,000,000 iterations: 1.5549240112305MB used
Survived 17,000,000 iterations: 1.5798873901367MB used
Survived 18,000,000 iterations: 1.6045608520508MB used
Survived 19,000,000 iterations: 1.6292953491211MB used
Survived 20,000,000 iterations: 1.653923034668MB used
Survived 21,000,000 iterations: 1.6791610717773MB used
Survived 22,000,000 iterations: 1.7042007446289MB used
Survived 23,000,000 iterations: 1.7290725708008MB used
Survived 24,000,000 iterations: 1.7536544799805MB used
Survived 25,000,000 iterations: 1.7786636352539MB used
Survived 26,000,000 iterations: 1.8034439086914MB used
Survived 27,000,000 iterations: 1.8284072875977MB used
Survived 28,000,000 iterations: 1.8535232543945MB used
Survived 29,000,000 iterations: 1.8784408569336MB used
Survived 30,000,000 iterations: 1.9027481079102MB used
Survived 31,000,000 iterations: 1.927360534668MB used
Survived 32,000,000 iterations: 1.9524154663086MB used
Survived 33,000,000 iterations: 1.9762802124023MB used
Survived 34,000,000 iterations: 2.0016708374023MB used
Survived 35,000,000 iterations: 2.025749206543MB used
Survived 36,000,000 iterations: 2.0504379272461MB used
Survived 37,000,000 iterations: 2.0751419067383MB used
Survived 38,000,000 iterations: 2.0998764038086MB used
Survived 39,000,000 iterations: 2.1247634887695MB used
Survived 40,000,000 iterations: 2.1489181518555MB used
Survived 41,000,000 iterations: 2.1744918823242MB used
Survived 42,000,000 iterations: 2.199333190918MB used
Survived 43,000,000 iterations: 2.2241287231445MB used
Survived 44,000,000 iterations: 2.2484664916992MB used
Survived 45,000,000 iterations: 2.2726669311523MB used
Survived 46,000,000 iterations: 2.297233581543MB used
Survived 47,000,000 iterations: 2.322151184082MB used
Survived 48,000,000 iterations: 2.346565246582MB used
Survived 49,000,000 iterations: 2.3715744018555MB used
Survived 50,000,000 iterations: 1.1623916625977MB used
Survived 51,000,000 iterations: 1.1868057250977MB used
Survived 52,000,000 iterations: 1.2114181518555MB used
Survived 53,000,000 iterations: 1.2367172241211MB used
Survived 54,000,000 iterations: 1.2617568969727MB used
Survived 55,000,000 iterations: 1.286491394043MB used
Survived 56,000,000 iterations: 1.3120040893555MB used
Survived 57,000,000 iterations: 1.3371047973633MB used
Survived 58,000,000 iterations: 1.3626480102539MB used
Survived 59,000,000 iterations: 1.3877182006836MB used
Survived 60,000,000 iterations: 1.4123916625977MB used
Survived 61,000,000 iterations: 1.4376449584961MB used
Survived 62,000,000 iterations: 1.4625015258789MB used
Survived 63,000,000 iterations: 1.4873580932617MB used
Survived 64,000,000 iterations: 1.5118179321289MB used
Survived 65,000,000 iterations: 1.5366897583008MB used
Survived 66,000,000 iterations: 1.5615768432617MB used
Survived 67,000,000 iterations: 1.5861434936523MB used
Survived 68,000,000 iterations: 1.6105117797852MB used
Survived 69,000,000 iterations: 1.6356430053711MB used
Survived 70,000,000 iterations: 1.6600570678711MB used
Survived 71,000,000 iterations: 1.6847763061523MB used
^C
```

```
$ qhhvm non_cyclical_test.php
Survived 1,000,000 iterations: 2MB used
Survived 2,000,000 iterations: 2MB used
Survived 3,000,000 iterations: 2MB used
Survived 4,000,000 iterations: 2MB used
Survived 5,000,000 iterations: 2MB used
Survived 6,000,000 iterations: 2MB used
Survived 7,000,000 iterations: 2MB used
Survived 8,000,000 iterations: 2MB used
Survived 9,000,000 iterations: 2MB used
Survived 10,000,000 iterations: 2MB used
Survived 11,000,000 iterations: 2MB used
Survived 12,000,000 iterations: 2MB used
Survived 13,000,000 iterations: 2MB used
Survived 14,000,000 iterations: 2MB used
Survived 15,000,000 iterations: 2MB used
Survived 16,000,000 iterations: 2MB used
Survived 17,000,000 iterations: 2MB used
Survived 18,000,000 iterations: 2MB used
Survived 19,000,000 iterations: 2MB used
Survived 20,000,000 iterations: 2MB used
Survived 21,000,000 iterations: 2MB used
Survived 22,000,000 iterations: 2MB used
Survived 23,000,000 iterations: 2MB used
Survived 24,000,000 iterations: 2MB used
Survived 25,000,000 iterations: 2MB used
Survived 26,000,000 iterations: 2MB used
Survived 27,000,000 iterations: 2MB used
Survived 28,000,000 iterations: 2MB used
Survived 29,000,000 iterations: 2MB used
Survived 30,000,000 iterations: 2MB used
Survived 31,000,000 iterations: 2MB used
Survived 32,000,000 iterations: 2MB used
Survived 33,000,000 iterations: 2MB used
Survived 34,000,000 iterations: 2MB used
Survived 35,000,000 iterations: 2MB used
Survived 36,000,000 iterations: 2MB used
Survived 37,000,000 iterations: 2MB used
Survived 38,000,000 iterations: 2MB used
Survived 39,000,000 iterations: 2MB used
Survived 40,000,000 iterations: 2MB used
Survived 41,000,000 iterations: 2MB used
Survived 42,000,000 iterations: 2MB used
Survived 43,000,000 iterations: 2MB used
Survived 44,000,000 iterations: 2MB used
Survived 45,000,000 iterations: 2MB used
Survived 46,000,000 iterations: 2MB used
Survived 47,000,000 iterations: 2MB used
Survived 48,000,000 iterations: 2MB used
Survived 49,000,000 iterations: 2MB used
Survived 50,000,000 iterations: 2MB used
Survived 51,000,000 iterations: 2MB used
Survived 52,000,000 iterations: 2MB used
Survived 53,000,000 iterations: 2MB used
Survived 54,000,000 iterations: 2MB used
Survived 55,000,000 iterations: 2MB used
Survived 56,000,000 iterations: 2MB used
Survived 57,000,000 iterations: 2MB used
Survived 58,000,000 iterations: 2MB used
Survived 59,000,000 iterations: 2MB used
Survived 60,000,000 iterations: 2MB used
Survived 61,000,000 iterations: 2MB used
Survived 62,000,000 iterations: 2MB used
Survived 63,000,000 iterations: 2MB used
Survived 64,000,000 iterations: 2MB used
Survived 65,000,000 iterations: 2MB used
Survived 66,000,000 iterations: 2MB used
Survived 67,000,000 iterations: 2MB used
Survived 68,000,000 iterations: 2MB used
Survived 69,000,000 iterations: 2MB used
Survived 70,000,000 iterations: 2MB used
Survived 71,000,000 iterations: 2MB used
Survived 72,000,000 iterations: 2MB used
Survived 73,000,000 iterations: 2MB used
Survived 74,000,000 iterations: 2MB used
Survived 75,000,000 iterations: 2MB used
Survived 76,000,000 iterations: 2MB used
Survived 77,000,000 iterations: 2MB used
Survived 78,000,000 iterations: 2MB used
Survived 79,000,000 iterations: 2MB used
Survived 80,000,000 iterations: 2MB used
Survived 81,000,000 iterations: 2MB used
Survived 82,000,000 iterations: 2MB used
Survived 83,000,000 iterations: 2MB used
Survived 84,000,000 iterations: 2MB used
Survived 85,000,000 iterations: 2MB used
Survived 86,000,000 iterations: 2MB used
Survived 87,000,000 iterations: 2MB used
Survived 88,000,000 iterations: 2MB used
Survived 89,000,000 iterations: 2MB used
Survived 90,000,000 iterations: 2MB used
Survived 91,000,000 iterations: 2MB used
Survived 92,000,000 iterations: 2MB used
Survived 93,000,000 iterations: 2MB used
Survived 94,000,000 iterations: 2MB used
Survived 95,000,000 iterations: 2MB used
^C
```

```
$ /usr/bin/php non_cyclical_test.php
Survived 1,000,000 iterations: 0.22219848632812MB used
Survived 2,000,000 iterations: 0.22237396240234MB used
Survived 3,000,000 iterations: 0.22237396240234MB used
Survived 4,000,000 iterations: 0.22237396240234MB used
Survived 5,000,000 iterations: 0.22237396240234MB used
Survived 6,000,000 iterations: 0.22237396240234MB used
Survived 7,000,000 iterations: 0.22237396240234MB used
Survived 8,000,000 iterations: 0.22237396240234MB used
Survived 9,000,000 iterations: 0.22237396240234MB used
Survived 10,000,000 iterations: 0.22237396240234MB used
Survived 11,000,000 iterations: 0.22237396240234MB used
Survived 12,000,000 iterations: 0.22237396240234MB used
Survived 13,000,000 iterations: 0.22237396240234MB used
Survived 14,000,000 iterations: 0.22237396240234MB used
Survived 15,000,000 iterations: 0.22237396240234MB used
Survived 16,000,000 iterations: 0.22237396240234MB used
Survived 17,000,000 iterations: 0.22237396240234MB used
Survived 18,000,000 iterations: 0.22237396240234MB used
Survived 19,000,000 iterations: 0.22237396240234MB used
Survived 20,000,000 iterations: 0.22237396240234MB used
Survived 21,000,000 iterations: 0.22237396240234MB used
Survived 22,000,000 iterations: 0.22237396240234MB used
Survived 23,000,000 iterations: 0.22237396240234MB used
Survived 24,000,000 iterations: 0.22237396240234MB used
Survived 25,000,000 iterations: 0.22237396240234MB used
Survived 26,000,000 iterations: 0.22237396240234MB used
Survived 27,000,000 iterations: 0.22237396240234MB used
Survived 28,000,000 iterations: 0.22237396240234MB used
Survived 29,000,000 iterations: 0.22237396240234MB used
Survived 30,000,000 iterations: 0.22237396240234MB used
Survived 31,000,000 iterations: 0.22237396240234MB used
Survived 32,000,000 iterations: 0.22237396240234MB used
Survived 33,000,000 iterations: 0.22237396240234MB used
Survived 34,000,000 iterations: 0.22237396240234MB used
Survived 35,000,000 iterations: 0.22237396240234MB used
Survived 36,000,000 iterations: 0.22237396240234MB used
Survived 37,000,000 iterations: 0.22237396240234MB used
Survived 38,000,000 iterations: 0.22237396240234MB used
Survived 39,000,000 iterations: 0.22237396240234MB used
Survived 40,000,000 iterations: 0.22237396240234MB used
Survived 41,000,000 iterations: 0.22237396240234MB used
Survived 42,000,000 iterations: 0.22237396240234MB used
Survived 43,000,000 iterations: 0.22237396240234MB used
Survived 44,000,000 iterations: 0.22237396240234MB used
Survived 45,000,000 iterations: 0.22237396240234MB used
Survived 46,000,000 iterations: 0.22237396240234MB used
Survived 47,000,000 iterations: 0.22237396240234MB used
Survived 48,000,000 iterations: 0.22237396240234MB used
Survived 49,000,000 iterations: 0.22237396240234MB used
Survived 50,000,000 iterations: 0.22237396240234MB used
Survived 51,000,000 iterations: 0.22237396240234MB used
Survived 52,000,000 iterations: 0.22237396240234MB used
Survived 53,000,000 iterations: 0.22237396240234MB used
Survived 54,000,000 iterations: 0.22237396240234MB used
Survived 55,000,000 iterations: 0.22237396240234MB used
Survived 56,000,000 iterations: 0.22237396240234MB used
Survived 57,000,000 iterations: 0.22237396240234MB used
Survived 58,000,000 iterations: 0.22237396240234MB used
Survived 59,000,000 iterations: 0.22237396240234MB used
Survived 60,000,000 iterations: 0.22237396240234MB used
Survived 61,000,000 iterations: 0.22237396240234MB used
Survived 62,000,000 iterations: 0.22237396240234MB used
Survived 63,000,000 iterations: 0.22237396240234MB used
Survived 64,000,000 iterations: 0.22237396240234MB used
Survived 65,000,000 iterations: 0.22237396240234MB used
Survived 66,000,000 iterations: 0.22237396240234MB used
Survived 67,000,000 iterations: 0.22237396240234MB used
Survived 68,000,000 iterations: 0.22237396240234MB used
Survived 69,000,000 iterations: 0.22237396240234MB used
Survived 70,000,000 iterations: 0.22237396240234MB used
Survived 71,000,000 iterations: 0.22237396240234MB used
Survived 72,000,000 iterations: 0.22237396240234MB used
Survived 73,000,000 iterations: 0.22237396240234MB used
Survived 74,000,000 iterations: 0.22237396240234MB used
Survived 75,000,000 iterations: 0.22237396240234MB used
Survived 76,000,000 iterations: 0.22237396240234MB used
Survived 77,000,000 iterations: 0.22237396240234MB used
Survived 78,000,000 iterations: 0.22237396240234MB used
Survived 79,000,000 iterations: 0.22237396240234MB used
Survived 80,000,000 iterations: 0.22237396240234MB used
Survived 81,000,000 iterations: 0.22237396240234MB used
Survived 82,000,000 iterations: 0.22237396240234MB used
Survived 83,000,000 iterations: 0.22237396240234MB used
Survived 84,000,000 iterations: 0.22237396240234MB used
Survived 85,000,000 iterations: 0.22237396240234MB used
Survived 86,000,000 iterations: 0.22237396240234MB used
Survived 87,000,000 iterations: 0.22237396240234MB used
Survived 88,000,000 iterations: 0.22237396240234MB used
Survived 89,000,000 iterations: 0.22237396240234MB used
Survived 90,000,000 iterations: 0.22237396240234MB used
Survived 91,000,000 iterations: 0.22237396240234MB used
Survived 92,000,000 iterations: 0.22237396240234MB used
Survived 93,000,000 iterations: 0.22237396240234MB used
Survived 94,000,000 iterations: 0.22237396240234MB used
Survived 95,000,000 iterations: 0.22237396240234MB used
Survived 96,000,000 iterations: 0.22237396240234MB used
Survived 97,000,000 iterations: 0.22237396240234MB used
^C
```
### Actual result

What we actually see is that memory usage continuously increases under HHVM, until the kernel kills the process:

```
$ qhhvm cyclical_test.php
Survived 1,000,000 iterations: 92MB used
Survived 2,000,000 iterations: 184MB used
Survived 3,000,000 iterations: 276MB used
Survived 4,000,000 iterations: 368MB used
Survived 5,000,000 iterations: 460MB used
Survived 6,000,000 iterations: 550MB used
Survived 7,000,000 iterations: 642MB used
Survived 8,000,000 iterations: 734MB used
Survived 9,000,000 iterations: 826MB used
Survived 10,000,000 iterations: 916MB used
Survived 11,000,000 iterations: 1008MB used
Survived 12,000,000 iterations: 1100MB used
Survived 13,000,000 iterations: 1192MB used
Survived 14,000,000 iterations: 1284MB used
Survived 15,000,000 iterations: 1374MB used
Survived 16,000,000 iterations: 1466MB used
Survived 17,000,000 iterations: 1558MB used
Survived 18,000,000 iterations: 1650MB used
Survived 19,000,000 iterations: 1740MB used
Survived 20,000,000 iterations: 1832MB used
Survived 21,000,000 iterations: 1924MB used
Survived 22,000,000 iterations: 2016MB used
Survived 23,000,000 iterations: 2108MB used
Survived 24,000,000 iterations: 2198MB used
Survived 25,000,000 iterations: 2290MB used
Survived 26,000,000 iterations: 2382MB used
Survived 27,000,000 iterations: 2474MB used
Survived 28,000,000 iterations: 2564MB used
Survived 29,000,000 iterations: 2656MB used
Survived 30,000,000 iterations: 2748MB used
Survived 31,000,000 iterations: 2840MB used
Survived 32,000,000 iterations: 2932MB used
Survived 33,000,000 iterations: 3022MB used
Survived 34,000,000 iterations: 3114MB used
Survived 35,000,000 iterations: 3206MB used
Survived 36,000,000 iterations: 3298MB used
Survived 37,000,000 iterations: 3388MB used
Survived 38,000,000 iterations: 3480MB used
Survived 39,000,000 iterations: 3572MB used
Survived 40,000,000 iterations: 3664MB used
Survived 41,000,000 iterations: 3756MB used
Survived 42,000,000 iterations: 3846MB used
Survived 43,000,000 iterations: 3938MB used
Survived 44,000,000 iterations: 4030MB used
Survived 45,000,000 iterations: 4122MB used
Survived 46,000,000 iterations: 4212MB used
Killed
```

Even with the (undocumented) garbage collector turned on, it still explodes

```
$ qhhvm -vEval.EnableGC=1 cyclical_test.php
Survived 1,000,000 iterations: 92MB used
Survived 2,000,000 iterations: 184MB used
Survived 3,000,000 iterations: 276MB used
Survived 4,000,000 iterations: 368MB used
Survived 5,000,000 iterations: 460MB used
Survived 6,000,000 iterations: 550MB used
Survived 7,000,000 iterations: 642MB used
Survived 8,000,000 iterations: 734MB used
Survived 9,000,000 iterations: 826MB used
Survived 10,000,000 iterations: 916MB used
Survived 11,000,000 iterations: 1008MB used
Survived 12,000,000 iterations: 1100MB used
Survived 13,000,000 iterations: 1192MB used
Survived 14,000,000 iterations: 1284MB used
Survived 15,000,000 iterations: 1374MB used
Survived 16,000,000 iterations: 1466MB used
Survived 17,000,000 iterations: 1558MB used
Survived 18,000,000 iterations: 1650MB used
Survived 19,000,000 iterations: 1740MB used
Survived 20,000,000 iterations: 1832MB used
Survived 21,000,000 iterations: 1924MB used
Survived 22,000,000 iterations: 2016MB used
Survived 23,000,000 iterations: 2108MB used
Survived 24,000,000 iterations: 2198MB used
Survived 25,000,000 iterations: 2290MB used
Survived 26,000,000 iterations: 2382MB used
Survived 27,000,000 iterations: 2474MB used
Survived 28,000,000 iterations: 2564MB used
Survived 29,000,000 iterations: 2656MB used
Survived 30,000,000 iterations: 2748MB used
Survived 31,000,000 iterations: 2840MB used
Survived 32,000,000 iterations: 2932MB used
Survived 33,000,000 iterations: 3022MB used
Survived 34,000,000 iterations: 3114MB used
Survived 35,000,000 iterations: 3206MB used
Survived 36,000,000 iterations: 3298MB used
Survived 37,000,000 iterations: 3388MB used
Survived 38,000,000 iterations: 3480MB used
Survived 39,000,000 iterations: 3572MB used
Survived 40,000,000 iterations: 3664MB used
Survived 41,000,000 iterations: 3756MB used
Survived 42,000,000 iterations: 3846MB used
Killed
```