Re^5: Introspection into floats/NV

[ikegami]

See Mini-Tutorial: Formats for Packing and Unpacking Numbers

F is for the build's NV
d is for the build's double

The sub requires an IEEE double.

There's no way to know if F is an IEEE double or not.
There's no way to know if d is an IEEE double or not.

But as far as I know, double and thus d is an IEEE double on all machines on which Perl currently runs.

But some people build Perl which uses long double for NV. So d is more reliable for a sub requiring an IEEE double.

But since you're more interested in inspecting an NV, F would make more sense for you. But you would also need to handle formats other than an IEEE double.

[LanX]

> There's no way to know if F is an IEEE double or not.

I'm confused, I'm expecting F to return 32, 64, 80 or 128 bits (at least)

And d always 64 bits

[ikegami]

Not surprising. Most machines support IEEE doubles, and most if not all C compilers on such machines will define double to be an IEEE double. But the C standard does not require this.

That's why I followed up with: "But as far as I know, double and thus d is an IEEE double on all machines on which Perl currently runs." Maybe you missed this?

[LanX]

I'm not a C programmer,

So let me restate my question, provided perl is compiled with other settings. (FP32,FP64,FP80,FP128, etc?)

Will F return another bitlength? ¹

Citing the docs

f  A single-precision float in native format.
d  A double-precision float in native format.

F  A Perl internal floating-point value (NV) in native format
D  A float of long-double precision in native format.
     (Long doubles are available only if your system supports
      long double values. Raises an exception otherwise.              
+(* WRONG see demo)
      Note that there are different long double formats.)

What I read - and can't test² - is that F will produce f,d or D according to the setting.

D will fail* if not internally available, but f and d will coerce° accordingly.

Unfortunately, there is no mention of the FP80³ format you mentioned in another thread.

If I'm wrong, don't you think perldocs of packtut and pack should clarify this?

update 1

1)

From our previous discussion:

> > > There's no way to know if F is an IEEE double or not.

> > I'm confused, I'm expecting F to return 32, 64, 80 or 128 bits (at least)

IEEE double means FP64, if F returns different bit length it's possible to tell that it's not a FP64.

It might be much trickier to tell if its according to IEEE though (i.e. position and format of sign and exponent)

update 2

²) well lets try

*) wrong °)seems to be true

main::(-e:1):    0
  DB<1> say unpack "B*", pack "f", 1/3
10101011101010101010101000111110

  DB<2> say unpack "B*", pack "d", 1/3
0101010101010101010101010101010101010101010101011101010100111111

  DB<3> say unpack "B*", pack "F", 1/3
0101010101010101010101010101010101010101010101011101010100111111

  DB<4> say unpack "B*", pack "D", 1/3
0000000010101000101010101010101010101010101010101010101010101010111111
+0100111111000000000000000000000000000000000000000000000000

  DB<5> use warnings; say unpack "B*", pack "D", 1/3         # NO EXCE
+PTION RAISED
0000000010101000101010101010101010101010101010101010101010101010111111
+0100111111000000000000000000000000000000000000000000000000