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Re: emulate 32-bit on perl 64

by sundialsvc4 (Monsignor)
on Jan 11, 2012 at 15:58 UTC ( #947373=note: print w/ replies, xml ) Need Help??


in reply to emulate 32-bit on perl 64

Probably the key improvement here is the use of hexadecimal notation:   0xffffffff instead of the corresponding, “magic” decimal.   As long as you are very careful to use unsigned arithmetic and specify bitmasks that are no larger than the integer-size you know that you are using, code like this ought to be transportable.   (It might not be readable, heh, but it ought to be int-length agnostic.)

I would almost edit my comment to say, don’t use “arithmetic” at all when you are bit-twiddling, as in code like this:

$num = $num - 4294967296 if $num > 4294967295; $num = $num + 4294967296 if $num < -2147483648;
  That’s nasty...   And I recognize at least one of those numbers, -2147483648, as an old, familiar 32-bit friend.   I also detect the presence of signed arithmetic, dependent upon the thirty-first bit being the sign-bit.   I suggest that this code should be rewritten to use bit-masking operators ... and to do so, perhaps, in a very specific way, as follows:

If you want to mash-off, say, all but the rightmost 31 bits of an integer quantity, you should take:   qty := qty and ( not 0x7fffffff );.   (I am not using Perl notation here, for clarity.)   The subexpression, (not 0x7fffffff), will evaluate to an int in which all bits are 1 except for the rightmost 31 bits, and it will do so correctly no matter what the sizeof(int) may be.

So, I suggest that you need to go through code like this (fortunately, it is a small piece of code), work out what it is doing and then rewrite it.   Construct a very thorough set of Test::More cases that you can run on both a 32-bit and a 64-bit system to prove that your rewrite is thoroughly correct.


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Re^2: emulate 32-bit on perl 64
by Eliya (Vicar) on Jan 11, 2012 at 16:55 UTC
    As long as you are very careful to use unsigned arithmetic and specify bitmasks that are no larger than the integer-size you know that you are using, code like this ought to be transportable.

    Unfortunately, it's not always quite as simple in the general case.  For example, if the 32-bit code relies on specific overflow/wrap-over effects. Consider the following very simple case:

    my $result = (0xffffffff + 2) & 0xffffffff; print $result;

    Although the result is masked down to 32 bits, it differs depending on the perl being used:

    32-bit perl: 4294967295 64-bit perl: 1

    (I'm not saying it cannot be done, but it requires taking into account the specific overflow behavior, IV/UV to NV conversions of 32-bit perls, etc. — simple 0xffffffff AND-masking isn't enough.)

      Erm, *what*? Why does 32-bit perl (I just tested on MirBSD/i386, it does behave as you said) not wraparound? Urgh!

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