#34599 [Opn->Bgs]: Accuracy of round() function affected by compiler optimizations

From: Date: Thu, 22 Sep 2005 14:18:54 +0000
Subject: #34599 [Opn->Bgs]: Accuracy of round() function affected by compiler optimizations
References: 1  Groups: php.bugs 
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 ID:               34599
 Updated by:       tony2001@php.net
 Reported By:      osolo at wndtabs dot com
-Status:           Open
+Status:           Bogus
 Bug Type:         *Math Functions
 Operating System: Linux
 PHP Version:      4.4.0
 New Comment:

Floating point values have a limited precision. Hence a value might 
not have the same string representation after any processing. That also
includes writing a floating point value in your script and directly 
printing it without any mathematical operations.

If you would like to know more about "floats" and what IEEE
754 is read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
 
Thank you for your interest in PHP.




Previous Comments:
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[2005-09-22 16:04:20] osolo at wndtabs dot com

Description:
------------
The round() function will return inconsistent results when PHP is
compiled under certain Linux distro/CPU/compiler combinations.

For example, if when compiling PHP on Gentoo 1.12.0_pre6/cc
3.4.4/Pentium 4, we get:
  round(0.245,2) => 0.24
Instead of the expected 0.25.  

I've traced the problem to the PHP_ROUND_WITH_FUZZ macro in
ext/standard/math.c.  This macro starts out as:

#define PHP_ROUND_WITH_FUZZ(val, places) { \ 
  double tmp_val=val, f = pow(10.0, (double) places); \
  ...

Changing the first line to:
  volatile double tmp_val=val; double f = pow(10.0, (double) places);
\

I suspect the problem is that the optimizer is keeping tmp_val in a
floating point register that has a difference precision than a C
double.  Declaring it volatile keeps it in the variable and makes
things more consisntant.





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