Re: Logical shift

[Jonathan Goble <jcgoble3@...>]

On Sun, Oct 13, 2019 at 9:59 PM Sean Conner <sean@conman.org> wrote:
>
> It was thus said that the Great D Burgess once stated:
> > Forgive me if this has been previously noted:
> > The 5.3 manual says
> > >  All bitwise operations convert its operands to integers (see §3.4.3), operate on all bits of those integers, and result in an integer.
> > > Both right and left shifts fill the vacant bits with zeros.
> >
> > In other words an "unsigned" shift or logical shift. No sign extension.
> >
> > Lua 5.3.5  Copyright (C) 1994-2018 Lua.org, PUC-Rio
> > > string.format("0x%016x",(1<<63)>>8)
> > 0x0080000000000000
> >
> > Bash
> > # printf '0x%016x\n' $(( (1<<63)>>8 ))
> > 0xff80000000000000
> >
> > PHP
> > printf '0x%016x\n' $(( (1<<63)>>8 ))
> > 0xff80000000000000
> >
> > Perl
> > # perl -e 'printf "0x%016x\n",(1<<63)>>8'
> > 0x0080000000000000
> >
> > SQLite 3.29.0
> > sqlite> select printf('0x%016x',(1<<63)>>8);
> > 0xff80000000000000
> >
> > *I wish this was highlighted in the manual.*
> >
> > What was the reasoning in choosing logical shift for >> rather than arithmetic?
>
>   From the C89 standard (ISO/IEC 9899:1990), section 6.3.7 (emphasis added):
>
>         The result of El >> E2 is El right-shifted E2 bit positions. If El
>         has an unsigned type or if El has a signed type and a nonnegative
>         value, the value of the result is the integral part of the quotient
>         of El divided by the quantity, 2 raised to the power E2. If El has a
>         signed type and a negative value, the resulting value is
>         implementation-defined.
>         ^^^^^^^^^^^^^^^^^^^^^^
>
>   From that, I would assume that the only way to ensure consistent values
> across all C compilers is to only do unsigned shifts.

The Lua VM implements shifts here:
https://github.com/lua/lua/blob/master/lvm.c#L673-L687

It took me a moment to understand that, but after looking at the intop
macro in lvm.h (https://github.com/lua/lua/blob/master/lvm.h#L61), it
casts the operands to unsigned before performing the operation
(https://github.com/lua/lua/blob/master/llimits.h#L129-L141).

I think it might be useful to offer both types of right shift. Perhaps
the arithmetic shift can be offered as the >>> operator?