[GitHub] [arrow] lidavidm commented on pull request #10350: ARROW-12814: [C++][Gandiva] Implements ABS, FLOOR, PI, SQRT, SIGN, LSHIFT, RSHIFT, CEIL, TRUNC, LN and LOG2 functions

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lidavidm commented on pull request #10350:
URL: https://github.com/apache/arrow/pull/10350#issuecomment-865118448


   It looks like the difference is that here the shift amount is reduced modulo the type precision (what x86 generally does but *not* what ARM does, e.g. see this post about [MSVC on ARM](https://devblogs.microsoft.com/cppblog/hello-arm-exploring-undefined-unspecified-and-implementation-defined-behavior-in-c/#shift-operators); allows negative shifts) and in the other PR, such shifts (including negative shifts) are disallowed (following the C++ specification). 
   
   The Arrow Compute behavior reflects the presence of having checked/unchecked variants; it seems the Gandiva variants must always produce a result? But it would be good to have them agree.
   
   Looking at some other systems:
   - T-SQL (MS SQL Server) has no bit-shift operators,
   - Postgres doesn't specify a behavior.
   - Python disallows negative shift values (overflow doesn't apply due to infinite-precision integers)


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