Bug #81627 [Opn]: floats < PHP_INT_MIN are converted to positive ints
Edit report at https://bugs.php.net/bug.php?id=81627&edit=1
ID: 81627
Updated by: cmb@php.net
Reported by: shaohua dot li at inf dot ethz dot ch
-Summary: Incorrect result of php bitwise and with floats
using clang13 -O2
+Summary: floats < PHP_INT_MIN are converted to positive ints
Status: Open
Type: Bug
Package: Scripting Engine problem
Operating System: Ubuntu 20.04.3 LTS
PHP Version: 8.1Git-2021-11-16 (Git)
Block user comment: N
Private report: N
New Comment:
Well, should have (also) read the fine manual[1]:
| If the float is beyond the boundaries of int (usually +/-
| 2.15e+9 = 2^31 on 32-bit platforms and +/- 9.22e+18 = 2^63 on
| 64-bit platforms), the result is undefined, since the float
| doesn't have enough precision to give an exact int result.
According to that, the reported issue is not a bug. On the other
hand, it would allow us to change the behavior to something more
reasonable.
[1] <https://www.php.net/manual/en/language.types.integer.php#language.types.integer.casting.from-float>
Previous Comments:
------------------------------------------------------------------------
[2021-11-17 12:24:12] cmb@php.net
The 32bit implementation of zend_dval_to_lval_slow() has an
explicit comment[1] which says "we're going to make this number
positive". I don't understand the reasoning, but apparently that
is a deliberate design decision. Then again I don't understand
why we don't saturate[2].
[1] <https://github.com/php/php-src/blob/php-7.4.26/Zend/zend_operators.c#L3261-L3262>
[2] <https://github.com/php/php-src/commit/77566edbafb969e166239b3fbc929588c6630ee9>
------------------------------------------------------------------------
[2021-11-17 11:32:36] cmb@php.net
Casting very small floats to int may change the sign:
<https://3v4l.org/Ku5YH>. I don't think this is
particularly
related to clang, nor to branch prediction, but the different
behavior might rather be related to whether
ZEND_DVAL_TO_LVAL_CAST_OK is defined or not[1]. If it is defined,
we cast to zend_long, and for double values outside the range of
zend_long, the behavior is undefined.
On Windows, where ZEND_DVAL_TO_LVAL_CAST_OK is never defined,
zend_dval_to_lval_slow()[2] yields an erroneous result anyway.
[1] <https://github.com/php/php-src/blob/php-7.4.26/Zend/Zend.m4#L158-L187>
[2] <https://github.com/php/php-src/blob/php-7.4.26/Zend/zend_operators.c#L3268-L3280>
------------------------------------------------------------------------
[2021-11-17 11:11:16] shaohua dot li at inf dot ethz dot ch
Yes, I noticed the warning. I'm just worried that shouldn't all compilers/optimizations
emit consistent results even if it's an error?
I also tried gcc11 with -O0 and -O2, on which php emits the same results as clang13 -O2.
------------------------------------------------------------------------
[2021-11-17 10:32:20] requinix@php.net
I assume you do *not* get the float-to-int deprecation warning when $n=58? PHP doesn't support
bitwise AND with floats and will round them to ints, but that comes with branch prediction so -O2
may be running afoul of that.
And that there is the limit of my knowledge on this matter.
------------------------------------------------------------------------
[2021-11-17 08:53:03] shaohua dot li at inf dot ethz dot ch
Hi,
Even if I decouple the two operations into two statements, the issue still exists. Also, for the
robustness, correctness, and consistency of php, the outputs should be the same.
Test script:
----------------
<?php
function test() {
$n = 0;
$a = 0;
while($a <= 0) {
$a &= $a + $a;
$a--;
if (++$n > 59) die("bug\n");
}
}
test();
?>
------------------------------------------------------------------------
The remainder of the comments for this report are too long. To view
the rest of the comments, please view the bug report online at
https://bugs.php.net/bug.php?id=81627
--
Edit this bug report at https://bugs.php.net/bug.php?id=81627&edit=1
Thread (10 messages)