Bug #14218
closed
Addition of negative Object#hash values can overflow
Description
Script to reproduce:
p 1_000_000.times.count{a=Object.new.hash; b=Object.new.hash; a < 0 && b < 0 && a + b > 0}
p 1_000_000.times.count{a=Object.new.hash; b=Object.new.hash; 0 + a + b != 0 + b + a}
Expected value: As hash values are regular integers according to the documentation, the above script should produce a pair of zeroes.
Actual value: there are consistently about 58 000 cases out of a million (5.8%) in which the first loop detects a discrepancy. The second loop detects a discrepancy in about 78 000 cases out of a million.
Testing environment:
- ruby 2.4.1p111 (2017-03-22 revision 58053) [x64-mingw32] - originally discovered on and reproduced
- ruby 2.4.3p205 (2017-12-14 revision 61247) [x64-mingw32] - reproduced
- Ruby 2.4.3 i386-mingw32, same OS / hardware - NOT reproduced, IIRC
Linux seems to consistently not reproduce:
- ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux] - Ubuntu for Windows, same hardware
- ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux] - different machine, same as for the three cases below it
- ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
- ruby 1.8.7 (2014-01-28 patchlevel 376) [x86_64-linux],
- ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]
Both irb and ruby reproduce the issue.
Further observations:
The cases that produce the discrepancy are always pairs of negative int32s such that 32-bit addition overflows. The errorneous results are always positive and correct modulo 2**32.
Changing Object.new.hash to Object.new.hash.to_i does not change the observed behavior. Changing it to Object.new.hash + 0 does remove the inconsistencies, as does retyping the hash values into the interpreter. Removing 0+ from both sides of the comparison also removes the second inconsistency. No inconsistency observed with integer multiplication.
Relevant discussion on StackOverflow chat begins here: https://chat.stackoverflow.com/transcript/message/40546901#40546901
Updated by 
Updated by