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Re: Negative shift counts

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Noob <root@127.0.0.1> wrote:
 
> I am aware that negative shift counts have undefined behavior.
 
>   int i = 40 << -2; // UB
 
> It seems to me that a natural way to extend shift counts to
> negative values would be to just flip the shift direction.
 
> The above statement would behave "as if" we'd written
 
>   int i = 40 >> 2;

The shift operators follow what much hardware does.  If you happen
to be on a PDP-10, then most likely your negative shifts do what 
you ask, as that is what the hardware does. 

Otherwise, it requires a conditional branch on each one, likely
much slower than the shift itself.  Note also that most hardware
uses only some bits of the shift amount.

Note that the Fortran ISHFT function does shift left or right based
on the sign. Hopefully the compilers optimize for the constant case.

-- glen

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Negative shift counts Noob <root@127.0.0.1> - 2015-04-21 11:33 +0200
  Re: Negative shift counts David Brown <david.brown@hesbynett.no> - 2015-04-21 12:22 +0200
    Re: Negative shift counts Noob <root@127.0.0.1> - 2015-04-21 13:12 +0200
  Re: Negative shift counts Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-04-21 12:02 +0100
  Re: Negative shift counts glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-04-29 20:08 +0000

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