Sorry to revive an old thread. One thing to note is that dividing a 64-bit
by a 32-bit is much faster than dividing a 64-bit by a 64-bit, at least on
recent Intel platforms. Unfortunately there is no valid way to express that
kind of calculation in Go..
Is the compiler/optimizer smart enough to figure out it can do that if you
do something like `uint32(a / uint64(b))` where a is `uint64` and b is
`uint32`? (the result of that expression can be computed using the 32-bit
version of DIV)
On Saturday, January 19, 2013 at 6:10:34 PM UTC-5, Michael Jones wrote:
>
> Integer division is slow.
>
> It is the unloved arithmetic operation in CPU design because of its low
> frequency of occurrence in the instruction stream. It is the one--if you
> remember childhood schooling--that was "not like the others" in its
> mechanism: there was lots of guessing using leading digits, trial divisors,
> and so on. This awkwardness exists even in base 2 (though much less) and
> means that any division circuit my have to do at least one "fixup" step. (A
> good design does either zero or one fixup, but that one means more work and
> plausibly an extra cycle on what is already a long cycle-count activity.)
>
> As one specific example, when adding and subtracting were 1 cycle and
> multiply was 4, a 32-bit integer division required 19 cycles.
>
>
> On Sat, Jan 19, 2013 at 1:25 PM, minux <minu...@gmail.com <javascript:>>
> wrote:
>
>>
>>
>> On Sun, Jan 20, 2013 at 5:22 AM, Jan Mercl <0xj...@gmail.com
>> <javascript:>> wrote:
>>
>>> On Sat, Jan 19, 2013 at 10:13 PM, Dominik Honnef <domi...@fork-bomb.org
>>> <javascript:>> wrote:
>>>
>>> Cannot reproduce here (Xeon on dont-know-how-much GHz)
>>>
>>> jnml@fsc-r630:~/src/tmp/20130119$ cat a_test.go
>>> package main
>>>
>>> import (
>>> "testing"
>>> )
>>>
>>> func BenchmarkBase(b *testing.B) {
>>> var x int32
>>> for i := 1; i < b.N; i++ {
>>> }
>>> _ = x
>>> }
>>>
>>> func BenchmarkInt(b *testing.B) {
>>> var x int
>>> for i := 1; i < b.N; i++ {
>>> x /= 42
>>> }
>>> _ = x
>>> }
>>>
>> division by a constant should be turned into multiplication by magic
>> constants
>> by the Go compiler just to avoid the costly divide instruction.
>>
>> you can verify that by 'go tool 6g -S'.
>>
>> --
>>
>>
>>
>
>
>
> --
> Michael T. Jones | Chief Technology Advocate | m...@google.com
> <javascript:> | +1 650-335-5765
>
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