If $v1 and $v2 were expressed as 64-bit unsigned integers, I'd be tempted to investigate (untested): 
sub is_same_leading_bit {
  my ($v1, $v2) = (shift, shift);
  my $and = 1 << 63;
  for my $shift(0 .. 63) {
    my $c1 = ($v1 << $shift) & $and;
    my $c2 = ($v2 << $shift) & $and;
    return 1 if $c1 & $c2;
    return 0 if $c1 | $c2;
  }

  return 0; # both args are 0.
}

[download]
But I'd be writing the sub in C and accessing it via Inline::C or XS.

Cheers,
Rob

In reply to Re: reliable get position of leftmost bit in large integers by syphilis
in thread reliable get position of leftmost bit in large integers by wollmers

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