I do x & (0xffffffff - 1<<32).
On Tuesday, June 14, 2016 at 1:44:14 PM UTC-7, Michael Jones wrote:
>
> I have fought this many times.
>
> What I almost always do is cast all variables involved as unsigned so that
> I can express the logical operations as desired. The exception is right
> shift which must be signed if the expected outcome for signed values is to
> happen.
>
> On Tue, Jun 14, 2016 at 1:08 PM, Dave MacFarlane <dri...@gmail.com
> <javascript:>> wrote:
>
>> You're right. I've already written it the ugly way, but at least it
>> has the benefit of being generalizable to fixed.Int52_12,
>> while there is no int128.
>>
>> Would a patch that adds a Mult receiver function to Int52_12 and
>> Int26_6 be welcome in x/image/math/fixed? I've already
>> worked out the stupid shifting/bitwise arithmetic so you don't end up
>> with things like multiplying by fixed.I(1) overflowing. It seems
>> like the type of thing that users of the package shouldn't have to
>> derive from scratch if they just want to multiply 2 fixed point
>> numbers.
>>
>> - Dave
>>
>> On Tue, Jun 14, 2016 at 3:41 PM, Matt Harden <matt....@gmail.com
>> <javascript:>> wrote:
>> > Consider coercing them to int64, multiply, then shift and coerce back to
>> > Int26_6. I suspect that would be faster than splitting with shifts and
>> ANDs,
>> > two int32 multiplies, shift and add.
>> >
>> > On Tue, Jun 14, 2016, 11:42 Dave MacFarlane <dri...@gmail.com
>> <javascript:>> wrote:
>> >>
>> >> I'm not actually trying to do x & -1, that would be pointless, as you
>> say.
>> >> It was just
>> >> the easiest way to demonstrate the behaviour that I didn't understand
>> in a
>> >> minimal
>> >> way. I understand the problem now--I was thinking of 0x as a prefix
>> >> representing a bitmask
>> >> when used as a constant with a bitwise operation, while Go thinks of
>> it as
>> >> a prefix
>> >> representing a hexadecimal number even in that context.
>> >>
>> >> What I *really* want to do is multiply 2 x/image/math/fixed.Int26_6
>> >> variables. I don't
>> >> want to lose the precision that x*y >> 6 would unnecessarily as x or y
>> get
>> >> large, so I wanted
>> >> to extract the first 26 bits, multiply them, and then separately
>> multiply
>> >> the decimal portion and add
>> >> it back shifted into the correct location.
>> >>
>> >> (Int26_6 is defined as `type Int26_6 int32`)
>> >>
>> >> On Tue, Jun 14, 2016 at 2:16 PM, Ian Lance Taylor <ia...@golang.org
>> <javascript:>> wrote:
>> >>>
>> >>> On Tue, Jun 14, 2016 at 10:44 AM, Dave MacFarlane <dri...@gmail.com
>> <javascript:>>
>> >>> wrote:
>> >>> > What I'm not understanding is why that's not the case in this
>> example.
>> >>> > It's
>> >>> > a 32 bit bitwise operation on a 32 bit signed type. Shouldn't
>> >>> > 0xFFFFFFFF be
>> >>> > coerced to a value of -1?
>> >>>
>> >>> Why don't you just write -1?
>> >>>
>> >>> I don't actually understand what you are doing. Given an int32 value
>> >>> x, x & 0xFFFFFFF (assuming that were valid) is always simply x. What
>> >>> else could it be? If you want to mask out the sign bit you should
>> >>> write x & 0x7FFFFFFF.
>> >>>
>> >>> Ian
>> >>>
>> >>>
>> >>> > On Tue, Jun 14, 2016 at 1:23 PM, Jan Mercl <0xj...@gmail.com
>> <javascript:>> wrote:
>> >>> >>
>> >>> >> All binary operators, except shifts, require identical left and
>> right
>> >>> >> types. Untyped values will be coerced to the type of the other
>> side,
>> >>> >> if
>> >>> >> representable as such after the conversion. That's not the case in
>> >>> >> this
>> >>> >> example.
>> >>> >>
>> >>> >> On Tue, Jun 14, 2016, 19:16 Dave MacFarlane <dri...@gmail.com
>> <javascript:>> wrote:
>> >>> >>>
>> >>> >>> Is this supposed to be legal in Go:
>> >>> >>>
>> >>> >>> var x int32 = 3
>> >>> >>>
>> >>> >>> fmt.Printf("%d", x & 0xFFFFFFFF)?
>> >>> >>>
>> >>> >>> The language spec just says the bitwise operator "applies to
>> integers
>> >>> >>> only" and
>> >>> >>> "yields a result of the same type as the first operand" that I can
>> >>> >>> see,
>> >>> >>> but it's giving
>> >>> >>> me a compiler error:
>> >>> >>>
>> >>> >>> ./main.go:10: constant 4294967295 overflows int32
>> >>> >>>
>> >>> >>> with go 1.6.2.
>> >>> >>>
>> >>> >>> Is this a compiler bug, or am I missing something else in the spec
>> >>> >>> that
>> >>> >>> makes it impossible
>> >>> >>> to mask out the high bit in a signed integer type without
>> converting
>> >>> >>> to
>> >>> >>> an unsigned equivalent first?
>> >>> >>>
>> >>> >>> - Dave
>> >>> >>>
>> >>> >>> --
>> >>> >>> You received this message because you are subscribed to the Google
>> >>> >>> Groups
>> >>> >>> "golang-nuts" group.
>> >>> >>> To unsubscribe from this group and stop receiving emails from it,
>> >>> >>> send an
>> >>> >>> email to golang-nuts...@googlegroups.com <javascript:>.
>> >>> >>> For more options, visit https://groups.google.com/d/optout.
>> >>> >>
>> >>> >> --
>> >>> >>
>> >>> >> -j
>> >>> >
>> >>> >
>> >>> >
>> >>> >
>> >>> > --
>> >>> > - Dave
>> >>> >
>> >>> > --
>> >>> > You received this message because you are subscribed to the Google
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>> >>> > an
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>> >>
>> >>
>> >>
>> >>
>> >> --
>> >> - Dave
>> >>
>> >> --
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>>
>>
>> --
>> - Dave
>>
>> --
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>
>
>
> --
> Michael T. Jones
> michae...@gmail.com <javascript:>
> http://www.google.com/profiles/michael.jones
>
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