Newsgroups: php.internals
Path: news.php.net
Xref: news.php.net php.internals:46721
Mailing-List: contact internals-help@lists.php.net; run by ezmlm
Received-SPF: error (pb1.pair.com: domain smashlabs.com from 67.15.58.61 cause and error)
Received-SPF: none (no valid SPF record)
Message-ID: <4B4E658C.7080908@smashlabs.com>
Date: Wed, 13 Jan 2010 18:30:04 -0600
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.0.9) Gecko/20061207 Thunderbird/1.5.0.9 Mnenhy/0.7.4.666
MIME-Version: 1.0
To: internals <internals@lists.php.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Subject: Int to float precision loss in 64bit environment
From: ralph@smashlabs.com (Ralph Schindler)

Hey list:


I noticed an issue today running our ZF unit tests, it primarily came 
down to this issue:

On 32 bit environments ..

~# php -r "var_dump(PHP_INT_MAX+1 > PHP_INT_MAX);"

.. will return true, whereas in 64bit environments, this returns false. 
In talking with Stas, it seems that since PHP_INT_MAX+1 is pushed into a 
(float) and since float values are stuffed into 52bit mantissa / 11 bits 
for exponents, we are loosing some precision.  That lost precisions is 
causing the above to fail in one platform, and work in another.

Is this noted somewhere?  Is there a workaround? Or, is this something 
that can be fixed for 64 bit platforms (somehow)?

This is not a big issue for us since (it seems) it's strictly a value 
that is being created for unit testing and I am not sure if it's 
something people use in code, or even see in the wild in general. I can 
get around it by ensuring the actual value is large enough to change the 
minimal precision value using PHP_INT_MAX+1025.

It't not until you've pushed past a 1024 buffer/(integer loss in 
precision?) until things start working again as expected:

~# php -r "var_dump(PHP_INT_MAX+1 > PHP_INT_MAX);"
bool(false)
~# php -r "var_dump(PHP_INT_MAX+1024 > PHP_INT_MAX);"
bool(false)
~# php -r "var_dump(PHP_INT_MAX+1025 > PHP_INT_MAX);"
bool(true)


Any information would be great.

Thanks,
Ralph Schindler