[Rd] 1954 from NA

[brodie gaslam]

This is because the NA in question is NA_real_, which
is encoded in double precision IEEE-754, which uses
64 bits.  The "1954" is just part of the NA.  The NA
must also conform to the NaN encoding for double precision
numbers, which requires that the "beginning" portion of
the number be "0x7ff0" (well, I think it should be "0x7ff8"
but that's a different story), as you can see here:

    x.word[hw] = 0x7ff0;
    x.word[lw] = 1954;

Both those components are part of the same double precision
value.  They are just accessed this way to make it easy to
set the high bits (63-32) and the low bits (31-0).

So NA is not just 1954, its 0x7ff0 0000 & 1954 (note I'm
mixing hex and decimals here).

In IEEE 754 double precision encoding numbers that start
with 0x7ff are all NaNs.  The rest of the number except for
the first bit which designates "quiet" vs "signaling" NaNs can
be anything.  R has taken advantage of that to designate the
R NA by setting the lower bits to be 1954.

Note I'm being pretty loose about endianess, etc. here, but
hopefully this conveys the problem.

In terms of your proposal, I'm not entirely sure what you gain.
You're still attempting to generate a 64 bit representation
in the end.  If all you need is to encode the fact that there
was an NA, and restore it later as a 64 bit NA, then you can do
whatever you want so long as the end result conforms to the
expected encoding.

In terms of using 'short' here (which again, I don't see the
need for as you're using it to generate the final 64 bit encoding),
I see two possible problems.  You're adding the dependency that
short will be 16 bits.  We already have the (implicit) assumption
in R that double is 64 bits, and explicit that int is 32 bits.
But I think you'd be going a bit on a limb assuming that short
is 16 bits (not sure).  More important, if short is indeed 16 bits,
I think in:

    x.word[hw] = 0x7ff0;

You overflow short.

Best,

B.



On Sunday, May 23, 2021, 8:56:18 AM EDT, Adrian Dușa <dusa.adrian using unibuc.ro> wrote: 





Dear R devs,

I am probably missing something obvious, but still trying to understand why
the 1954 from the definition of an NA has to fill 32 bits when it normally
doesn't need more than 16.

Wouldn't the code below achieve exactly the same thing?

typedef union
{
    double value;
    unsigned short word[4];
} ieee_double;


#ifdef WORDS_BIGENDIAN
static CONST int hw = 0;
static CONST int lw = 3;
#else  /* !WORDS_BIGENDIAN */
static CONST int hw = 3;
static CONST int lw = 0;
#endif /* WORDS_BIGENDIAN */


static double R_ValueOfNA(void)
{
    volatile ieee_double x;
    x.word[hw] = 0x7ff0;
    x.word[lw] = 1954;
    return x.value;
}

This question has to do with the tagged NA values from package haven, on
which I want to improve. Every available bit counts, especially if
multi-byte characters are going to be involved.

Best wishes,
-- 
Adrian Dusa
University of Bucharest
Romanian Social Data Archive
Soseaua Panduri nr. 90-92
050663 Bucharest sector 5
Romania
https://adriandusa.eu

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