Re: [ANN] Lua 5.3.0 (work2) now available

[William Ahern <william@...>]

On Tue, Mar 25, 2014 at 01:31:06PM -0400, Ralph Hempel wrote:
> Dirk Laurie wrote:
> >2014-03-25 18:20 GMT+02:00 Paul Baker<paulbaker8@gmail.com>:
> >
> >>Regarding the new bitwise operators, the Reference Manual states "Both
> >>right and left shifts fill with zeros the vacant bits". Given that
> >>"Lua 5.3 has integers but not unsigned integers" [1], it seems strange
> >>that the>>  operator is unsigned rather than signed. Is there a reason
> >>for this? Personally, I think it would be useful for Lua to support
> >>both types of right shift - perhaps using the same syntax as
> >>JavaScript:>>  and>>>  ?
> >
> >Don't agree.
> >
> >I can see the point of arithmetic shift in C, where it could be useful
> >when programming multiprecision arithmetic ??? but C does not have
> >it. Its inclusion in the bit32 library always felt to me like a loving
> >preservation of a quirk of some early computer (IBM 360 springs to
> >mind).
> 
> I believe that the result of a shift in C must preserve the implied 
> "signedness" of the value being shifted.
> 
> For the case of signed values, a right shift will fill high order bits with 
> the value of the MSB.

Right shifting a signed type with a negative value has
implementation-defined behavior.

Left shifting a signed type with a negative value has undefined-behavior.

See N1570 (C11) 6.5.7 p4 and p5.