Re: [Gm2] SHIFT questions

[Iztok Kobal]

If I read e.g. XDS M2 doc (see 
http://www.excelsior-usa.com/doc/isom2.html) the things for me are more 
or less clear:

"A call of SHIFT shall have two actual parameters. The first parameter 
shall be an expression that is of a packedset type. The second parameter 
shall be an expression of a type with which the signed type is value 
parameter compatible."

Note: value parameter compatibility

That means: compile time (in case of constant 2nd argument) or run-time 
exception (in case of type range check was enabled and variable 2nd 
argument) when 2nd argument was greater than MAXBITS.


Otherwise (2nd argument was variable and no_of_shifts >= MAXBITS and no 
runtime type range check was enabled):

"With the interpretation of a value of type packedset as a sequence of 
bits, the shift operation can be considered as a zero-propagating 
logical-shift-up (left) operation or a logical-shift-down (right) 
operation, according to the value of the second operand, and within the 
bit field defined by the base type of the packedset."

Note: zero-propagating shift.

That means: propagating more than or equal to MAXBITS results in overall 
zeroed result.


This seems reasonable for me.


Regards, Iztok

Gaius Mulley wrote:
> Hi,
>
> during the course of fixing some regression tests I came across
> the problem of what to do if the user attempts to SHIFT by more
> than set size bits.
>
> Ie:
>
>     SHIFT(bitset, MAX(BITSET)+1)   is a no-op   MAX(BITSET)=31
>                                    and shifting by 32 on a 32 bit
>                                    machine yields the same result.
>
> I cannot find how SHIFT should behave in the M2 ISO standard given
> these parameters.  Currently if SHIFTing x where x is>MAX(settype)+1
> will be the same as SHIFTing x MOD (MAX(settype)+1), for any settype.
> Hope this is correct..
>
> regards,
> Gaius
>
>
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