When you do a DIV r32 in 64bit mode the result will be saved in
EAX but this means that you can only do that if the high word
of the dividend is smaller that the divisor.
You can't unconditionally replace uint32(A / uint64(B)) with a
DIV r32 instruction because if A is big (take 2**62) and B is
small (take 13) then the quotient will overflow EAX.
A.
Il giorno martedì 8 novembre 2016 06:18:02 UTC+1, ra...@cockroachlabs.com
ha scritto:
>
> 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> wrote:
>>
>>>
>>>
>>> On Sun, Jan 20, 2013 at 5:22 AM, Jan Mercl <0xj...@gmail.com> wrote:
>>>
>>>> On Sat, Jan 19, 2013 at 10:13 PM, Dominik Honnef <domi...@fork-bomb.org>
>>>> 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 | +1
>> 650-335-5765
>>
>
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