This should only be valid for a loop that doesn’t include any function calls,
or if the compiler can prove that said function calls don’t involve any
synchronization primitives anywhere in the possible call stack.
So maybe it’s optimized in
for … {
total += *p
}
but I wouldn’t bet on it in
for … {
total += *p
foo()
}
//jb
On 30 Nov 2018, at 19:01, robert engels
<reng...@ix.netcom.com<mailto:reng...@ix.netcom.com>> wrote:
I would think the compiler would be free to optimize this as a single load
outside the loop - as Go has no volatile declaration (to inform the compiler
that some other thread might of changed the value of *p)
On Nov 30, 2018, at 11:58 AM, robert engels
<reng...@ix.netcom.com<mailto:reng...@ix.netcom.com>> wrote:
It’s as simple as:
var count = 1
var p = &count
func X() {
total :=0
for i:=0; i <1000000;i++ {
total = total + *p
}
}
no ?
On Nov 30, 2018, at 11:48 AM, Mark Volkmann
<r.mark.volkm...@gmail.com<mailto:r.mark.volkm...@gmail.com>> wrote:
I think I failed to come up with a good code example. But suppose I need to do
something with a pointer inside a loop that really does require dereferencing
it and the pointer never changes inside the loop. In that case will the Go
compiler optimize that so the dereference doesn't happen in each loop iteration?
On Fri, Nov 30, 2018 at 11:36 AM Jan Mercl
<0xj...@gmail.com<mailto:0xj...@gmail.com>> wrote:
On Fri, Nov 30, 2018 at 6:16 PM Mark Volkmann
<r.mark.volkm...@gmail.com<mailto:r.mark.volkm...@gmail.com>> wrote:
> Will the Go compiler optimize the pointer dereference so it doesn't happen in
> every loop iteration? If not, is it common practice to do that outside the
> loop like this?
>
> myValue := *myPtr
> for _, v := range values {
> fmt.Printf("%v %v\n", myValue, v)
> }
There's no pointer dereference inside the loop. But there's an allocation
inside the loop, on every iteration, that puts a pointer to a copy of myValue
into an interface{} that's passed to fmt.Printf. The allocation can be probably
avoided:
myValue := interface{}(*myPtr)
for _, v := range values {
fmt.Printf("%v %v\n", myValue, v)
}
But the %v verb handle pointers in many cases well. If that holds for the type
of *myPtr then simply:
for _, v := range values {
fmt.Printf("%v %v\n", myPtr, v)
}
is what could be good enough.
--
-j
--
R. Mark Volkmann
Object Computing, Inc.
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