*syphilis has asked for the wisdom of the Perl Monks concerning the following question:*

Hi,

**UPDATE:** I've now filed a bug report about this.

I've been chewing on this over the last couple of days, and it doesn't look right to me.

It's an issue that arises only when NV-precision is greater than IV-precision.

So, if ivsize is 8 bytes, then in order to experience the issue, you'll generally need to use a quadmath build (nvtype of __float28, NV-precision of 113 bits).

With such a perl, I get:

Do we agree that perl is buggy here ?

If you have a perl with ivsize of 4, you can also demonstrate the same issue.

With ivsize of 4, NV-precision of 113 bits:

Cheers,

Rob

I've been chewing on this over the last couple of days, and it doesn't look right to me.

It's an issue that arises only when NV-precision is greater than IV-precision.

So, if ivsize is 8 bytes, then in order to experience the issue, you'll generally need to use a quadmath build (nvtype of __float28, NV-precision of 113 bits).

With such a perl, I get:

In contrast, POSIX::fmod returns "1" for the same operation.C:\_32>perl -wle "$m = (2**113) - 1; $n = 2; print $m % $n;" 0

Here's the section of perlop documentation that I'm looking at:C:\_32>perl -MPOSIX -wle "$m = (2**113) - 1; $n = 2; print fmod($m, $n +);" 1

AFAICT, the parts of that documentation that are pertinent to my given example are:Binary "%" is the modulo operator, which computes the division remaind +er of its first argument with respect to its second argument. Given integ +er operands $m and $n: If $n is positive, then "$m % $n" is $m minus the largest multiple of $n less than or equal to $m. If $n is negative, th +en "$m % $n" is $m minus the smallest multiple of $n that is not less tha +n $m (that is, the result will be less than or equal to zero). If the operands $m and $n are floating point values and the absolute value of $n (that is "abs($n)") is less than "(UV_MAX + 1)", only the integer portion of $m and $n will be used in the operation (Note: here "UV_MAX +" means the maximum of the unsigned integer type). If the absolute value of the right operand ("abs($n)") is greater than or equal to "(UV_MAX + 1)", "%" computes the floating-point remainder $r in the equation "($r = $m - $i*$n)" where $i is a certain integer that makes +$r have the same sign as the right operand $n (not as the left operand $m like C function "fmod()") and the absolute value less than that of $n. Note that when "use integer" is in scope, "%" gives you direct access +to the modulo operator as implemented by your C compiler. This operator i +s not as well defined for negative operands, but it will execute faster.

And to me that implies that the correct calculation for the given example is to do:If $n is positive, then "$m % $n" is $m minus the largest multiple of +$n less than or equal to $m. .... If the operands $m and $n are floating point values and the absolute v +alue of $n (that is "abs($n)") is less than "(UV_MAX + 1)", only the integer portion of $m and $n will be used in the operation (Note: here "UV_MAX +" means the maximum of the unsigned integer type)

This agrees with the value produced by POSIX::fmod(), but disagrees with the value produced by the "%" operator.C:\_32>perl -wle "$r = 10384593717069655257060992658440191.0 - (519229 +6858534827628530496329220095.0 * 2); print $r;" 1

Do we agree that perl is buggy here ?

If you have a perl with ivsize of 4, you can also demonstrate the same issue.

With ivsize of 4, NV-precision of 113 bits:

With ivsize of 4, NV-precision of 64 bits (long double):C:\_32>perl -wle "$m = (2**113) - 1; $n = 2; print $m % $n;" 0 C:\_32>perl -MPOSIX -wle "$m = (2**113) - 1; $n = 2; print fmod($m, $n +);" 1

And now to really muddy the waters !! One might be expecting the trend to continue when ivsize is 4, and NV-precision is 53-bits .... but not so:C:\>perl -wle "$m = (2**64) - 1; $n = 2; print $m % $n;" 0 C:\>perl -MPOSIX -wle "$m = (2**64) - 1; $n = 2; print fmod($m, $n);" 1

This time the "%" operator decides to act in accordance with (my reading of) the perlop documentation.C:\_32>perl -wle "$m = (2**53) - 1; $n = 2; print $m % $n;" 1 C:\_32>perl -MPOSIX -wle "$m = (2**53) - 1; $n = 2; print fmod($m, $n) +;" 1

Cheers,

Rob

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Re: The "%" operator and its documentation.
by Anonymous Monk on Jun 14, 2021 at 10:42 UTC |

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