Q:

Stack overflow using hhvm for benchmarkgame adapted to strict Hack

Since Hack is faster in strict mode, I tried to translate the first benchmarkgame to dito, but get stuck on a stack overflow that never occur in the un-strict version (different with recursive calls using objects?). Any reasons why? How could I increase the stack size? And googling for "php stack overflow" is useless, as you can imagine. ^^
The problem happens on the first recursive call.
My code:
<?hh // strict

class Tree {
  public function bottomUpTree(?num $item, ?num $depth) : array<?num>
  {
    if ($depth === null) return array(null,null,$item);
    if ($item !== null) {
      $item2 = $item + $item;
      $depth--;
      return array( // <--- Stack overflow here
        $this->bottomUpTree($item2-1,$depth),
        $this->bottomUpTree($item2,$depth),
        $item);
    }
    else {
      throw new Exception("Fatal");
    }
  }

  public function itemCheck(array<?int, ?int> $treeNode) : int {
    if ($treeNode !== null && $treeNode[2] !== null) {
      $someNumber = $treeNode[2];
      $a = $someNumber + 10;
      return $someNumber
        + ($treeNode[0][0] === null ? $this->itemCheck($treeNode[0]) : $treeNode[0][2])
        - ($treeNode[1][0] === null ? $this->itemCheck($treeNode[1]) : $treeNode[1][2]);
    }
    else {
      throw new Exception("Fatal");
    }
  }

  public function run($argc, $argv) {

    $minDepth = 4;

    $n = ($argc == 2) ? $argv[1] : 1;
    $maxDepth = max($minDepth + 2, $n);
    $stretchDepth = $maxDepth + 1;

    $stretchTree = $this->bottomUpTree(0, $stretchDepth);
    printf("stretch tree of depth %d\t check: %d\n",
      $stretchDepth, $this->itemCheck($stretchTree));
    unset($stretchTree);

    $longLivedTree = $this->bottomUpTree(0, $maxDepth);

    $iterations = 1 << ($maxDepth);
    do {
      $check = 0;
      for($i = 1; $i <= $iterations; ++$i)
      {
        $t = $this->bottomUpTree($i, $minDepth);
        $check += $this->itemCheck($t);
        unset($t);
        $t = $this->bottomUpTree(-$i, $minDepth);
        $check += $this->itemCheck($t);
        unset($t);
      }

      printf("%d\t trees of depth %d\t check: %d\n", $iterations<<1, $minDepth, $check);

      $minDepth += 2;
      $iterations >>= 2;
    }
    while($minDepth <= $maxDepth);

    printf("long lived tree of depth %d\t check: %d\n",
      $maxDepth, $this->itemCheck($longLivedTree));
  }
}

$tree = new Tree();
$tree->run($argc, $argv);

A:

Your recursion will never hit a base case and stop as the values you're passing down while recursing will never be null.
You probably want to exit if $depth is 0 as well as if it is null.

