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Bug #73923 Different floating point results with on modern machines (-ffp-contract)
Submitted: 2017-01-12 20:38 UTC Modified: 2017-04-01 19:45 UTC
From: christophm30 at gmail dot com Assigned:
Status: Open Package: Date/time related
PHP Version: Irrelevant OS: all
Private report: No CVE-ID:
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 [2017-01-12 20:38 UTC] christophm30 at gmail dot com
Description:
------------
As described in https://github.com/facebook/hhvm/issues/7594
there are new FMA (floating-point multiply-add) instructions,
which are emitted when the target system supports them
(GCC needs -O2 or higher).
The advantage of those instructions is that the provide more accurate results,
but that also means different results.

In HHVM this resulted in a self-test to fail.
As the calculated (and unexpected) value could be reproduced with
PHP7 on an arm64 machine (Ubuntu Xenial) as well,
I want to bring this up here in the PHP bugtracker as well.



Test script:
---------------
From https://github.com/facebook/hhvm/blob/master/hphp/test/slow/ext_date/date_sunrise_test.php:

date_default_timezone_set("Asia/Calcutta");
var_dump( date_sunrise(mktime(0, 0, 0, 12, 26, 2012), SUNFUNCS_RET_DOUBLE) );

We got 9.6226631046689 instead of 9.622663104669.


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 [2017-04-01 19:45 UTC] tpunt@php.net
-Package: date_time +Package: Date/time related
 
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