Unfortunately for you, interfaces are immutable. We can't provide a means
to create an interface from a pointer, because then the user can modify the
interface using the pointer they constructed it with (as you were planning
to do).
You could use a modifiable reflect.Value for this.
var i int64 = 77
v := reflect.ValueOf(&i).Elem()
At this point, v now has .Type() of int64, and is settable.
Note that to get the value you can't do v.Interface().(int64), as that
allocates. You need to use v.Int().
Of course, reflection has its own performance gotchas. It will solve this
problem but may surface others.
On Tuesday, December 15, 2020 at 12:04:54 PM UTC-8 ben...@gmail.com wrote:
> Nice project!
>
> It's a pity Go doesn't have C-like unions for cases like this (though I
> understand why). In my implementation of AWK in Go, I modelled the value
> type as a pseudo-union struct, passed by value:
>
> type value struct {
> typ valueType // Type of value (Null, Str, Num, NumStr)
> s string // String value (for typeStr)
> n float64 // Numeric value (for typeNum and typeNumStr)
> }
>
> Code here:
> https://github.com/benhoyt/goawk/blob/22bd82c92461cedfd02aa7b8fe1fbebd697d59b5/interp/value.go#L22-L27
>
> Initially I actually used "type Value interface{}" as well, but I switched
> to the above primarily to model the funky AWK "numeric string" concept.
> However, I seem to recall that it had a significant performance benefit
> too, as passing everything by value avoided a number of allocations.
>
> Lua has more types to deal with, but you could try something similar. Or
> maybe include int64 (for bool as well) and string fields, and everything
> else falls back to interface{}? It'd be a fairly large struct, so not sure
> it would help ... you'd have to benchmark it. But I'm thinking something
> like this:
>
> type Value struct {
> typ valueType
> i int64 // for typ = bool, integer
> s string // for typ = string
> v interface{} // for typ = float, other
> }
>
> -Ben
>
> On Wednesday, December 16, 2020 at 6:50:05 AM UTC+13 arn...@gmail.com
> wrote:
>
>> Hi
>>
>> The context for this question is that I am working on a pure Go
>> implementation of Lua [1] (as a personal project). Now that it is more or
>> less functionally complete, I am using pprof to see what the main CPU
>> bottlenecks are, and it turns out that they are around memory management.
>> The first one was to do with allocating and collecting Lua "stack frame"
>> data, which I improved by having add-hoc pools for such objects.
>>
>> The second one is the one that is giving me some trouble. Lua is a
>> so-called "dynamically typed" language, i.e. values are typed but variables
>> are not. So for easy interoperability with Go I implemented Lua values
>> with the type
>>
>> // Go code
>> type Value interface{}
>>
>> The scalar Lua types are simply implemented as int64, float64, bool,
>> string with their type "erased" by putting them in a Value interface. The
>> problem is that the Lua runtime creates a great number of short lived Value
>> instances. E.g.
>>
>> -- Lua code
>> for i = 0, 1000000000 do
>> n = n + i
>> end
>>
>> When executing this code, the Lua runtime will put the values 0 to 1
>> billion into the register associated with the variable "i" (say, r_i). But
>> because r_i contains a Value, each integer is converted to an interface
>> which triggers a memory allocation. The critical functions in the Go
>> runtime seem to be convT64 and mallocgc.
>>
>> I am not sure how to deal with this issue. I cannot easily create a pool
>> of available values because Go presents say Value(int64(1000)) as an
>> immutable object to me, so I cannot keep it around for later use to hold
>> the integer 1001. To be more explicit
>>
>> // Go code
>> i := int64(1000)
>> v := Value(i) // This triggers an allocation (because the interface
>> needs a pointer)
>> // Here the Lua runtime can work with v (containing 1000)
>> j := i + 1
>> // Even though v contains a pointer to a heap location, I cannot
>> modify it
>> v := Value(j) // This triggers another allocation
>> // Here the Lua runtime can work with v (containing 1001)
>>
>>
>> I could perhaps use a pointer to an integer to make a Value out of. This
>> would allow reuse of the heap location.
>>
>> // Go code
>> p :=new(int64) // Explicit allocation
>> vp := Value(p)
>> i :=int64(1000)
>> *p = i // No allocation
>> // Here the Lua runtime can work with vp (contaning 1000)
>> j := i + 1
>> *p = j // No allocation
>> // Here the Lua runtime can work with vp (containing 1001)
>>
>> But the issue with this is that Go interoperability is not so good, as Go
>> int64 now map to (interfaces holding) *int64 in the Lua runtime.
>>
>> However, as I understand it, in reality interfaces holding an int64 and
>> an *int64 both contain the same thing (with a different type annotation): a
>> pointer to an int64.
>>
>> Imagine that if somehow I had a function that can turn an *int64 to a
>> Value holding an int64 (and vice-versa):
>>
>> func Int64PointerToInt64Iface(p *int16) interface{} {
>> // returns an interface that has concrete type int64, and points
>> at p
>> }
>>
>> func int64IfaceToInt64Pointer(v interface{}) *int64 {
>> // returns the pointer that v holds
>> }
>>
>> then I would be able to "pool" the allocations as follows:
>>
>> func NewIntValue(n int64) Value {
>> v = getFromPool()
>> if p == nil {
>> return Value(n)
>> }
>> *p = n
>> return Int64PointerToint64Iface(p)
>> }
>>
>> func ReleaseIntValue(v Value) {
>> addToPool(Int64IPointerFromInt64Iface(v))
>> }
>>
>> func getFromPool() *int64 {
>> // returns nil if there is no available pointer in the pool
>> }
>>
>> func addToPool(p *int64) {
>> // May add p to the pool if there is spare capacity.
>> }
>>
>> I am sure that this must leak an abstraction and that there are good
>> reasons why this may be dangerous or impossible, but I don't know what the
>> specific issues are. Could someone enlighten me?
>>
>> Or even better, would there be a different way of modelling Lua values
>> that would allow good Go interoperability and allow controlling heap
>> allocations?
>>
>> If you got to this point, thank you for reading!
>>
>> Arnaud Delobelle
>>
>> [1] https://github.com/arnodel/golua
>>
>
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