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Bug #72335 Misoptimize due to type narrowing
Submitted: 2016-06-04 21:31 UTC Modified: 2016-06-06 12:03 UTC
From: nikic@php.net Assigned: dmitry (profile)
Status: Closed Package: opcache
PHP Version: master-Git-2016-06-04 (Git) OS:
Private report: No CVE-ID: None
 [2016-06-04 21:31 UTC] nikic@php.net
Description:
------------
We perform (long|double)->double narrowing even if chains of operations that only eventually result in a double are involved. This can lead to wrong results because parts of the calculation that would have been performed on integers are performed on doubles instead, with the corresponding loss in precision.

We need to either not narrow for such chains or ensure that the transformation is safe.

Test script:
---------------
<?php

function test() {
    $b = false;
    $x = (1<<53)+1;
    do {
        $x = 1.0 * ($x - (1<<53));
    } while ($b);
    return $x;
}
var_dump(test());


Expected result:
----------------
float(1)

Actual result:
--------------
float(0)

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History

AllCommentsChangesGit/SVN commitsRelated reports
 [2016-06-06 05:40 UTC] laruence@php.net
we should check if there is any precision losing while converting long to double.
 [2016-06-06 06:33 UTC] laruence@php.net
Added negative case:
<?php

function test() {
    $b = false;
    $x = -(1<<53) -1;
    do {
        $x = 1.0 * ($x + (1<<53));
    } while ($b);
    return $x;
}
var_dump(test());
 [2016-06-06 12:03 UTC] dmitry@php.net
-Status: Open +Status: Closed -Assigned To: +Assigned To: dmitry
 
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