rfc:precise_float_value

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rfc:precise_float_value [2015/08/04 23:35] – yohgakirfc:precise_float_value [2016/06/05 19:02] – Update voting choice bukka
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 ====== PHP RFC: More precise float value handling ====== ====== PHP RFC: More precise float value handling ======
-  * Version: 0.9+  * Version: 1.1
   * Date: 2015-07-30   * Date: 2015-07-30
-  * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>+  * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>, Jakub Zelenka <bukka@php.net>
   * Status: Under Discussion   * Status: Under Discussion
   * First Published at: http://wiki.php.net/rfc/precise_float_value   * First Published at: http://wiki.php.net/rfc/precise_float_value
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 ===== Introduction ===== ===== Introduction =====
  
-This RFC is based on float like JSON numeric discussion and proposes more precise float value handling overall.+This RFC is based on the discussion about displaying float values in json_encode and proposes more precise float value handling overall.
  
-JSON is used to exchange data between systems. Although [[https://tools.ietf.org/html/rfc7159|JSON RFC "6 Numbers"]]  does not require specific implementation for float/int type, float value should be able to handle precise float values as much as possible by default. +JSON is used to exchange data between systems. Although [[https://tools.ietf.org/html/rfc7159|JSON RFC "6 Numbers"]]  does not require specific implementation for float/int type, float value should be handled as precise as possible by default. 
  
-Currently, json_encode() uses PG(precision) which is 14. IEEE 754 double supports more precision and serialize()/var_export() uses PG(serialize_precision)=17 to be more precise. Since json_encode() uses PG(precision), json_encode() removes lower digits of fraction parts and destroys original value even if PHP's float could hold more precise float value. +Currently, json_encode() uses EG(precision) which is set to 14. That means that 14 digits max are used for displaying (printing) the number. IEEE 754 double supports higher precision and serialize()/var_export() uses PG(serialize_precision)=17 to be more precise. Since json_encode() uses EG(precision), json_encode() removes lower digits of fraction parts and destroys original value even if PHP's float could hold more precise float value.
  
 <code php> <code php>
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 </code> </code>
  
-PHP's float type stores "raw" IEEE 754 double and could store accurate fraction value up 17 digits.+PHP's float type stores "raw" IEEE 754 double and could display accurate fraction value up to 17 digits.
  
-Current PHP outputs meaningless values for oversized PG(precision)/PG(serialize_precision).+Current PHP outputs meaningless values for oversized EG(precision)/PG(serialize_precision).
  
 <code php> <code php>
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 float(0.12345678901234567736988623209981597028672695159912109375) float(0.12345678901234567736988623209981597028672695159912109375)
 </code> </code>
 +
 +That is caused by used mode for double to string conversion.
  
 ===== Proposal ===== ===== Proposal =====
  
-  * Introduce PG(precision)=-1 and PG(serialize_precision)=-1 that uses zend_dtoa()'s 0 mode that rounds to nearest value. (-1 is used to indicate 0 mode)+This RFC proposes to introduce EG(precision)=-1 and PG(serialize_precision)=-1 that uses zend_dtoa()'s mode 0 which uses better algorigthm for rounding float numbers (-1 is used to indicate 0 mode).
  
-Since JSON is used extensively for web apps, we may have JSON specific INI setting for better compatibility and ease of use.+The RFC also proposes changing ini for JSON precision to PG(serialize_precision).
  
-  * Add JSON_G(json_precision) - Add JSON module specific precision INI setting.+Followings are sample codes and outputs of the proposed patch. 
 + 
 +NEW behavior 
 +<code php> 
 +<?php 
 +$v = 10.0000000000001; 
 + 
 +ini_set('precision', -1); 
 +ini_set('serialize_precision', -1); 
 + 
 +var_dump($v); 
 +echo var_export($v, true), PHP_EOL; 
 +echo json_encode($v), PHP_EOL; 
 +echo $v, PHP_EOL; 
 +?> 
 +</code> 
 + 
 +  float(10.0000000000001) 
 +  10.0000000000001 
 +  10.0000000000001 
 +  10.0000000000001 
 + 
 + 
 + 
 +OLD behavior 
 +<code php> 
 +<?php 
 +$v = 10.00000000000001; 
 + 
 +ini_set('precision', 14); 
 +ini_set('serialize_precision', 17); 
 + 
 +var_dump($v); 
 +echo var_export($v, true), PHP_EOL; 
 +ini_set('serialize_precision', 14); 
 +echo json_encode($v), PHP_EOL; 
 +ini_set('serialize_precision', 17); 
 +echo $v, PHP_EOL; 
 +?> 
 +</code> 
 + 
 +  float(10) 
 +  10.000000000000011 
 +  10 
 +  10 
 + 
 + 
 +Please note that IEEE float cannot store exactly precise values. e.g. Result of "10/3" - see phpt of the patch. Even with this proposal, there will be rounding errors, but the behavior becomes similar to other languages and values are more precise in many cases.
  
  
 ===== Backward Incompatible Changes ===== ===== Backward Incompatible Changes =====
  
-None because old INI default is kept.+Setting mode 0 as default can mean that the rounding will be more precise which also means that the rounding might be different in var_export()/serialize(). 
 + 
 +The BC break could happen only if someone would rely on exact output but that shouldn't be the case. All our existing tests passes when 0 mode is used. 
 + 
 +None when old INI value is used.
  
 ===== Proposed PHP Version(s) ===== ===== Proposed PHP Version(s) =====
  
-PHP 7.0.0 - 0 mode and JSON_G(json_precision) +  * PHP 7.1
-PHP 5.6.0 - 0 mode. JSON uses PG(serialize_precision) for float precision.+
  
 ===== RFC Impact ===== ===== RFC Impact =====
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 ==== To Existing Extensions ==== ==== To Existing Extensions ====
 PHP overall PHP overall
- * 0 mode float outputs values rounded to nearest. +  * 0 mode (EG(precision)= -1) float outputs values rounded to nearest.
- +
-Standard module +
- * serialize(), var_export() +
- +
-JSON +
- * json_encode() - PHP7.0: Use 0 mode. PHP5.6: Use PG(serialize_precision) +
- +
-WDDX? +
- +
-XML_RPC?+
  
 +Standard module and JSON
 +  * serialize(), var_export(), json_encode - Use PG(serialize_precision) and 0 mode by default.
  
 ==== To Opcache ==== ==== To Opcache ====
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 serialize_precision serialize_precision
-  * hardcoded default values : 17 Unmodified 
-  * php.ini-development values : 17 Unmodified 
-  * php.ini-production values : 17 Unmodified 
- 
-json_precision(NEW - PHP7 only) 
   * hardcoded default values : -1   * hardcoded default values : -1
   * php.ini-development values : -1   * php.ini-development values : -1
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 PHP uses "raw" IEEE 754 value internally regardless of precision settings. Therefore, this RFC does not affect internal computation. PHP uses "raw" IEEE 754 value internally regardless of precision settings. Therefore, this RFC does not affect internal computation.
 +
  
 ===== Future Scope ===== ===== Future Scope =====
  
-  * Remove PG(serialize_precision) setting. If we are going to remove PG(serialize_precision) in the future, we may be better to have JSON_G(json_precision) as one may need specific precision for JSON data.+WDDX 
 +  * wddx_serialize_vars/value() - Use PG(serialize_precision) and 0 mode. It uses EG(precision) currently.
  
-===== Proposed Voting Choices =====+XML_RPC 
 +  * xmlrpc_encode() - Use PG(serialize_precision) and 0 mode. It uses EG(precision) currently.
  
-Requires a 50%+1 majority (see [[voting]]) 
  
-Introduce 0 mode to PG(precision)/PG(serialize_precision) for PHP 7.0+===== Proposed Voting Choices =====
  
-Add JSON_G(json_precision) for JSON module only setting for PHP 7.0+Requires a 2/3 majority
  
-Add PG(precision)/PG(serialize_precision) 0 mode support for PHP 5.6 +There will be two votings
  
 +  * whether to introduce mode 0 and use it as default for serialize_precision
 +  * should PG(serialize_precision) be used instead of EG(precision) in json_encode.
  
 ===== Patches and Tests ===== ===== Patches and Tests =====
 +
 +The initial PR can be found here:
  
   * https://github.com/php/php-src/pull/1455   * https://github.com/php/php-src/pull/1455
 +
 +Note that the PR is currently outdate but it will be updated if the RFC is accepted.
  
 ===== Implementation ===== ===== Implementation =====
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 ===== References ===== ===== References =====
  
-https://marc.info/?l=php-internals&m=143786181229714&w=2+  * https://marc.info/?l=php-internals&m=143786181229714&w=2 
 +  * https://bugs.php.net/bug.php?id=71473 
 +  * http://blog.reverberate.org/2016/02/06/floating-point-demystified-part2.html
  
 ===== Rejected Features ===== ===== Rejected Features =====
 Keep this updated with features that were discussed on the mail lists. Keep this updated with features that were discussed on the mail lists.
 +
rfc/precise_float_value.txt · Last modified: 2017/09/22 13:28 by 127.0.0.1

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