Huqiu Liao wrote on 31/03/2016 17:47:
Hi, internals,
I have a question about Assign By Reference and I posted on StackOverflow,
I'd like to know the reason behind it, and I did not get any this kind of
answer, can anyone give me some clues.
---
We have a piece of simple code:
1 <?php
2 $i = 2;
3 $j = &$i;
4 echo (++$i) + (++$i);
On PHP5, it outputs 8, because:
$i is a reference, when we increase $i by ++i, it will change the zval
rather than make a copy, so line 4 will be 4 + 4 = 8. This is Assign By
Reference.
To me, that sounds like a fixed bug. It's noteworthy in the 3v4l results
that HHVM also gives 7 as the answer, and I can't see any logical reason
for the answer to be 8 - regardless of execution order, the result of
the two calls to ++$i should never be the same.
It would be interesting to see the VLD dump in PHP 5, with and without
reference, because looking at the PHP 7 one, it's clear that the two
increments result in two temp vars so it shouldn't make any difference
what type of zval was given as input:
4 2 PRE_INC $2 !0 # !0 is compiled $i, $2 is a
new value returned
3 PRE_INC $3 !0 # !0 is referenced again, but
$3 is unrelated to $2
4 ADD ~4 $2, $3 # the two values are added,
with no reference to !0
Maybe in PHP 5 the opcodes are the same, but $2 and $3 somehow end up as
references to !0, rather than new zvals, as though the code were:
$i = 2;
$i++; $temp1 =& $i;
$i++; $temp2 =& $i;
echo $temp1 + $temp2;
rather than, as seems logical to me, HHVM, and PHP7:
$i = 2;
$i++; $temp1 = $i;
$i++; $temp2 = $i;
echo $temp1 + $temp2;
Regards,
--
Rowan Collins
[IMSoP]
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