FAQ
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

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