How to check availability of 64-bit integers (for `use integer`)?-CodePudding

I want to write a program that needs 40-bit integers. The machine where I'm writing it has 64 bit integers, but I'd like to check inside the program whether there are 64-bit integers available.

How could I do that efficiently?

On a 64-bit machine, this seems to work:

~0 >= 2**63

Is that safe (read: "portable") on different architectures?

Thinking about the problem, I also wondered if the Perl compiler, or interpreter could make these results questionable for a future version of Perl:

  DB<2> sub bittest { use integer; return ((1 << $_[0]) >> $_[0]) != 0; }

  DB<3> x bittest 31
0  1
  DB<4> x bittest 63
0  1
  DB<5> x bittest 64
0  ''

CodePudding user response：

~0 is 18446744073709551615 on 64-bit systems, so

~0 == 18446744073709551615
~0 == 0xFFFFFFFFFFFFFFFF       # (16 F's)

are efficient tests to see if you are on a 64-bit system.

I've only ever used Perl on 32-bit and 64-bit systems, but in case there are ever 39 and 41-bit systems, to use 40-bit integers you just need ~0 to be at least 2**40 - 1, or:

~0 >= 0xFFFFFFFFFF             # (10 F's)

CodePudding user response：

If you need integers in the computer sense (IV/UV), you need them to at least 40 bits in size.

~0 >= 2**40-1

use Config qw( %Config );

$Config{uvsize} >= 8

uvsize refes to the size in bytes of such integers.

You need this if you use any of the following:

the numbers as operands to bitwise operators
the numbers as operands to ../...
pack 'Q'/unpack 'Q'
hex literals larger than 0xFFFF_FFFF

If you need integers in the mathematical sense, floats (NV) with at least 40 bits of precision would also do.

use Config qw( %Config );

$Config{uvsize} >= 8 || eval($Config{nv_overflows_integers_at}) >= 2**40

Every integer up and including eval($Config{nv_overflows_integers_at}) can be represented exactly as an NV.

Note that every build of Perl should support at least 53 bits of precision. For example, eval($Config{nv_overflows_integers_at}) evalutes to 9007199254740992 on my builds, which corresponds to 53 bits as per log(9007199254740992)/log(2).