🔬26. Native integers and UInt in Perl 6

As soon as we touched some native integers yesterday, let us look a bit closer at them. The topic is deep, so we limit ourselves with a brief understanding of interconnections between the different integer data types in Perl 6.

UInt and UInt64

The simplest is UInt. This data type is defined in src/core/Int.pm and is literary a one-liner:

my subset UInt of Int where {not .defined or $_ >= 0};

The where clause restricts the values to be non-negative. Thus, the range of a UInt variable is 0..^Inf:

say UInt.Range; # 0..^Inf

There’s another type, UInt64. It is defined similarly but puts additional restriction to the value: it should not exceed 2⁶⁴.

my Int $UINT64_UPPER = nqp::pow_I(2, 64, Num, Int);
subset UInt64 of Int where { 0 <= $_ < $UINT64_UPPER }

By the way, don’t forget that you can use superscripts in Perl 6 directly:

say 2⁶⁴; # 18446744073709551616

OK, let us confirm the borders of the UInt64 class by calling its Range method:

$ perl6 -e'say UInt64.Range'
-Inf^..^Inf

This result is wrong (U stands for unsigned), but I am sure that if you follow my blog you know where to fix it. Of course, the problem sits in the Range method that we were examining yesterday. Here is the fragment in src/core/Int.pm we need:

default { # some other kind of Int
    .^name eq 'UInt'
        ?? Range.new( 0, Inf, :excludes-max )
        !! Range.new( -Inf, Inf, :excludes-min, :excludes-max )
}

Both UInt and UInt64 are the children of Int but only of the them is handled properly. Let us add the missing check like this, for example:

when .^name eq 'UInt'   { Range.new(0, Inf, :excludes-max) }
when .^name eq 'UInt64' { Range.new(0, 2⁶⁴ - 1) }
default                 { Range.new( -Inf, Inf, :excludes-min, :excludes-max) }

Compile and enjoy the result:

say Int.Range;    # -Inf^..^Inf
say UInt.Range;   # 0..^Inf
say UInt64.Range; # 0..18446744073709551615

This is fine but I am still not satisfied with a big chain of when tests. Would UInt and UInt64 be classes, not subsets, you could add individual Range methods to each of them.

Native ints

Another big cluster of integer type definitions can be found in src/core/natives.pm. Let me quote the big part of that file here:

my native   int is repr('P6int') is Int { }
my native  int8 is repr('P6int') is Int is nativesize( 8) { }
my native int16 is repr('P6int') is Int is nativesize(16) { }
my native int32 is repr('P6int') is Int is nativesize(32) { }
my native int64 is repr('P6int') is Int is nativesize(64) { }

my native   uint is repr('P6int') is Int is unsigned { }
my native  uint8 is repr('P6int') is Int is nativesize( 8) is unsigned { }
my native   byte is repr('P6int') is Int is nativesize( 8) is unsigned { }
my native uint16 is repr('P6int') is Int is nativesize(16) is unsigned { }
my native uint32 is repr('P6int') is Int is nativesize(32) is unsigned { }
my native uint64 is repr('P6int') is Int is nativesize(64) is unsigned { }

These native types are all Ints and are represented by P6int data type. If you dig into MoarVM, you will find a directory nqp/MoarVM/src/6model/reprs that contains many C and C header files, including P6int.c and P6int.h. A brief look tells us that this type is universally used for all the native types listed above:

/* Representation used by P6 native ints. */
struct MVMP6intBody {
    /* Integer storage slot. */
    union {
        MVMint64 i64;
        MVMint32 i32;
        MVMint16 i16;
        MVMint8 i8;
        MVMuint64 u64;
        MVMuint32 u32;
        MVMuint16 u16;
        MVMuint8 u8;
    } value;
};

The is nativesize and is unsigned are the traits (src/core/traits.pm) that set some attributes of the NativeHOW object:

multi sub trait_mod:<is>(Mu:U $type, :$nativesize!) {
    $type.^set_nativesize($nativesize);
}
multi sub trait_mod:<is>(Mu:U $type, :$unsigned!) {
    $type.^set_unsigned($unsigned);
}

And let’s make a break for today.

6 thoughts on “🔬26. Native integers and UInt in Perl 6

  1. Thanks for spotting the UInt64 anomaly. This appears to date back to at least 2011 and was basically undocumented, untested and only used in one place. I have now removed the UInt64 subtype in https://github.com/rakudo/rakudo/commit/cf15435575 . If you want to know the range of a 64-bit integer, please use uint64.Range.

    With regards to performance: if you’re worried about the performance of xxx:U.Range, then don’t use it in a hot path: get the values you need out of the range and use those. xxx:U.Range was intended to be purely informational and is therefore implemented for readability rather than performance.

    Liked by 1 person

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