rfc:integer_semantics

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rfc:integer_semantics [2014/08/19 22:35]
ajf Under Discussion
rfc:integer_semantics [2014/09/20 23:50]
ajf
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 ====== PHP RFC: Integer Semantics ====== ====== PHP RFC: Integer Semantics ======
-  * Version: 0.2.1+  * Version: 0.2.3
   * Date: 2014-08-19   * Date: 2014-08-19
   * Author: Andrea Faulds <ajf@ajf.me>   * Author: Andrea Faulds <ajf@ajf.me>
-  * Status: Under Discussion+  * Status: Implemented
   * First Published at: http://wiki.php.net/rfc/integer_semantics   * First Published at: http://wiki.php.net/rfc/integer_semantics
  
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   - Left bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0, even on CPUs which wrap around   - Left bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0, even on CPUs which wrap around
   - Right bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0 or -1 (depending on sign), even on CPUs which wrap around   - Right bitwise shifts by a number of bits beyond the bit width of an integer will always result in 0 or -1 (depending on sign), even on CPUs which wrap around
 +
 +==== Examples ====
 +
 +For all "was" results, these are only on my machine, as they would differ across platforms. All "now" results are the same on all platforms.
 +
 +<code php>
 +// Was: int(-9223372036854775808)
 +// Now: int(0)
 +var_dump((int)NAN);
 +// Was: int(-9223372036854775808)
 +// Now: int(0)
 +var_dump((int)INF);
 +// Was: int(4611686018427387904)
 +// Now: bool(false) and E_WARNING
 +var_dump(1 << -2);
 +// Was: int(8)
 +// Now: int(0)
 +var_dump(8 >> 64);
 +</code>
  
 ===== Rationale ===== ===== Rationale =====
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 "Negative shifts" do not do what users would reasonably expect them to do: shift in the opposite direction. Rather, a negative shift is usually a shift by the 2's complement unsigned integer representation (in the case of ''-2'', this would be ''18446744073709551614'' when integers are 64-bit). This is also reliant on undefined behaviour in C, and will give different results depending on the processor and integer size. For this reason, we now disallow such shifts. "Negative shifts" do not do what users would reasonably expect them to do: shift in the opposite direction. Rather, a negative shift is usually a shift by the 2's complement unsigned integer representation (in the case of ''-2'', this would be ''18446744073709551614'' when integers are 64-bit). This is also reliant on undefined behaviour in C, and will give different results depending on the processor and integer size. For this reason, we now disallow such shifts.
  
-On Intel CPUs, a bitwise shift by a number of bits that is greater than the bit width of an integer (e.g. ''>> 65'' on a 64-bit machine) will "wrap around" (e.g. ''>> 65'' is effectively ''>> 1''). To ensure cross-platform consistency, we ensure that such shifts shifts will always result in zero (for left shifts), or zero or negative one (for right shifts, depending on the sign of the number being shifted). It is worth noting that shifts of a number of bits greater than the bit width of an integer is also undefined behaviour in C.+On Intel CPUs, a bitwise shift by a number of bits that is greater than the bit width of an integer (e.g. ''>> 65'' on a 64-bit machine) will "wrap around" (e.g. ''>> 65'' is effectively ''>> 1''). To ensure cross-platform consistency, we ensure that such shifts will always result in zero (for left shifts), or zero or negative one (for right shifts, depending on the sign of the number being shifted). It is worth noting that shifts of a number of bits greater than the bit width of an integer is also undefined behaviour in C.
  
 Making NaN and Infinity always become zero when casted to integer means more cross-platform consistency, and is also less surprising than what is currently produces, where NaN produces the minimum integer on my machine (''-9223372036854775808''). Making NaN and Infinity always become zero when casted to integer means more cross-platform consistency, and is also less surprising than what is currently produces, where NaN produces the minimum integer on my machine (''-9223372036854775808'').
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 Integer to float conversion is untouched. Despite some people's misconceptions, the bitwise shift operators do not operate on strings like the other bitwise operators do, so I have not affected how they deal with strings as they didn't in the first place (they cast to integer). This does not touch the behaviour of array key casting (except for Infinity and NaN), although this is something I would like to do Integer to float conversion is untouched. Despite some people's misconceptions, the bitwise shift operators do not operate on strings like the other bitwise operators do, so I have not affected how they deal with strings as they didn't in the first place (they cast to integer). This does not touch the behaviour of array key casting (except for Infinity and NaN), although this is something I would like to do
- 
-===== Open Questions/Possible Future Scope ===== 
- 
-Currently PHP throws an ''E_WARNING'' and gives ''FALSE'' for a division by zero, and under this RFC, so would shifts by negative counts. While not strictly an //integer// semantics issue, should we make division by zero result in Infinity/-Infinity per IEEE 754, and perhaps make it throw no error, or an E_NOTICE? If we did that, should we make this return NaN instead? 
  
 ===== Proposed Voting Choices ===== ===== Proposed Voting Choices =====
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 ===== Patches and Tests ===== ===== Patches and Tests =====
  
-work-in-progress patch is here: https://github.com/TazeTSchnitzel/php-src/compare/php:master...TazeTSchnitzel:integer_semantics+pull request is here: https://github.com/php/php-src/pull/781
  
 The patch is working and has tests, but code review would be appreciated. The patch is working and has tests, but code review would be appreciated.
  
 ===== Implementation ===== ===== Implementation =====
-After the project is implementedthis section should contain  + 
-  - the version(s) it was merged to +Merged into master. It will be in PHP 7. 
-  a link to the git commit(s) + 
-  a link to the PHP manual entry for the feature+It is not yet in the manual. 
 + 
 +===== Vote ===== 
 + 
 +As this is a language changea 2/3 majority is required. 
 + 
 +Voting started 2014-09-14 and ended on 2014-09-21. 
 + 
 +<doodle title="Accept the integer semantics RFC and merge patch into master?" auth="ajf" voteType="single" closed="true"> 
 +   * Yes 
 +   * No 
 +</doodle>
  
 ===== References ===== ===== References =====
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 ===== Changelog ===== ===== Changelog =====
  
 +  * v0.2.3 - Removed "Open Questions/Possible Future Scope" to avoid confusion
 +  * v0.2.2 - Added examples
   * v0.2.1 - Open Questions/Possible Future Scope added   * v0.2.1 - Open Questions/Possible Future Scope added
   * v0.2 - Dropped zend_parse_parameters change, fixed left shift too   * v0.2 - Dropped zend_parse_parameters change, fixed left shift too
   * v0.1.1 - Introduction added   * v0.1.1 - Introduction added
   * v0.1 - Initial version   * v0.1 - Initial version
rfc/integer_semantics.txt · Last modified: 2017/09/22 13:28 (external edit)