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To: "PHP Internals List" <internals@lists.php.net>,
	"Daniel Convissor" <danielc@analysisandsolutions.com>
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Date: Mon, 6 Apr 2009 13:00:47 -0500
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Subject: Re: [PHP-DEV] [PATCH] double to long conversion change
From: php_lists@realplain.com ("Matt Wilmas")

Hi Dan,

----- Original Message -----
From: "Daniel Convissor"
Sent: Thursday, April 02, 2009

> Hi Matt:
>
> On Sat, Mar 14, 2009 at 09:30:42AM -0500, Matt Wilmas wrote:
>>
>> But of course with
>> doubles, precision has been lost long before 2^63 anyway, as far as
>> increments of 1 (it's 1024 at 2^63).
>
> I just ran into these issues in PHP 5.2.8 on 64 bit Linux while running
> examples I'm using to improve our documentation surrounding bitwise
> operators, bindec() and decbin().
>
> The integer float conversions start failing above 2^53, meaning
> 9007199254740993 is the first integer that doesn't work.  As you noted,
> ints divisible by 1024 are fine.  I can provide the test script to anyone
> who needs it.

Sorry I didn't get back to you sooner, but unless I'm missing something, 
you're talking about converting long/int to double/float.  That's the 
opposite of this thread subject, which is how to convert a double to a long 
when it's out of the range of a long. :-)

But, for what you're testing, that's the behavior I'd expect -- once you've 
reached the precision of a double, you'll only get the closest 
representation possible (and of course a 64-bit long is more precise than a 
double since there's no floating point to represent). Also, I assume what 
can be represented by a double is the same across platforms, if it's IEEE 
754.

Just curious though, you're saying that all whole numbers (from long) below 
2^53 are representable? (Powers of 2 should always be OK.)  When writing a 
big literal number on a 32-bit system, I'm seeing much lower than that 
(around 2^40), but in that case conversion is happening from a string, 
digit-by-digit, instead of directly from a 64-bit integer type, so maybe 
that's why...

> A bash script I put together makes it look like things work correctly in
> the shell.  So it seems this is a PHP thing, not a Linux thing.

That's interesting, though I don't know much about shell script stuff (or 
other *nix details ;-)).  Like I said, I figure a double type should behave 
the same everywhere.  Unless the shell/bash uses a *long* double type (twice 
as big as a regular double)? *shrug*

> Am I correct in believing your changes will take care of these issues?
> It seems you're looking to patch 5.3 and HEAD.  Are there thoughts of
> applying this to 5.2?
>
> Thanks,
>
> --Dan
>
> Note: The guy who put the box together says everything on it is built
> against 64 bit libraries.


- Matt