Hello Christian,

Thursday, October 30, 2008, 10:55:48 AM, you wrote:

> Hi,

>>   Modified files:              (Branch: PHP_5_3)
>>     /php-src/ext/standard     math.c 
>>   Log:
>>   Fixed bug #42294 (Unified solution for round() based on C99 round)
>>   [DOC] New implementation of round() to work-around inconsistencies for 
>> win32
>>   and 64 bit platforms.
>>   
>>   This solution is very roughly based on BSD's implmentation of round(), 
>> which
>>   itself is an implementation of C99 standard. We take the absolute value of 
>> number
>>   we want to round time the 10 to the power of the number of decimal spaces 
>> we are 
>>   rounding to. The resulting value is rounded up and the pre-rounded value 
>> is 
>>   subtracted from it. If the difference is greater then 0.5000000001 we 
>> round up,
>>   otherwise we round down.

> Apparently, nobody reads internals anymore. :-(

> I made my initial comment on the bug report over a year ago. Since then
> I've dug quite a bit into floating point arithmetics and the actual
> problems behind round(). This lead to:

> http://wiki.php.net/rfc/rounding

> Which I posted quite a while ago and nearly nobody was interested:

> http://news.php.net/php.internals/40070

> [Now please don't simply apply the patch there, I've done some
> additional research since.]

> The solution I proposed over a year ago is actually wrong and it does
> not solve all the problems of round()'s g, see my RFC for that.

> The general problems with round are actually:

> 1) Internal FPU precision on x86.
> 2) Specification problem (which rounding mode should actually be used?)
> 3) Dividing/multiplying by >= 10^23 is not exact.
> 4) round (1.255, 2) should give 1.26 but gives 1.25. The FUZZ stuff
>    tries to resolve this issue (but not the other three) but I've
>    come to the conclusion that the FUZZ is actually the wrong solution
>    for the problem.

> Since I didn't get any reaction on the RFC on internals, I pinged Lukas
> and Johannes (as they are RMs for PHP 5.3) in private in order to ask
> whether it was possible to include my proposal in 5.3 (I don't have ZE2
> Karma and my patch also needs to change zend_strtod).

> Lukas and Johannes were concerned about the interopability of my
> solution of problem (1). So I did some tests on different platforms and
> operating systems and Lukas and Johannes asked around for other people
> to do tests, too. I've summarized results of these tests and proposed
> solution for correct cross-platform floating point arithmetics here:

> http://www.christian-seiler.de/projekte/fpmath/

> I've mailed patches for PHP 5.3 and HEAD to Johannes and Lukas for
> ZendEngine2 that only address the above issue (1) and do the following:

> 1) Define some macros for math-related functions that will ensure the
> function itself always uses double precision. Add configure checks for
> these macros.

> 2) Modified zend_strtod and the add/sub/div/mul functions in
> zend_operators.c to make use of those macros. This ensures that PHP will
> always use double precision for calculations and math is thus portable.

> 3) Added a test that determines if the calculations precision is
> actually correct.

> The patches for 5.3 and HEAD are found here:

> http://www.christian-seiler.de/temp/php/2008-10-28-fpu/php-float-precision-5.3.patch
> http://www.christian-seiler.de/temp/php/2008-10-28-fpu/php-float-precision-6.patch

> My next step (as discussed with Johannes and Lukas) would have been to
> apply the Non-ZE2-part of my patch to ext/standard/math.c in 5.3 and
> HEAD (I don't have separate patches for that yet but they are quite
> trivial to adapt to the new macros).

> Now the question is: Where do we go from here? Your commit does not
> solve all the problems of PHP's round but is at least better than the
> previous implementation since at least the platform-dependency on
> whether or not to use the FUZZ is removed. The other problems, however,
> remain.

Imo you should still go ahead with 5.3 and HEAD, ignoring the issue on 5.2.
It is pretty sad that your patch is necessary, but lets face reality, it is
necessary. The only thing I am wondering about is whether the configure
time resolution is a good idea. People will still switch processors,
wouldn't that break the whole idea? Not that it would ever affect me or
most developers. But users who typically install rpms might get affected.


Best regards,
 Marcus


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