Using a pointer receiver (as in your noEscape example) just pushes the
problem up the stack. When you try to call it, e.g.
func parent() bool {
var opts options
return noEscape('0', &opts)
}
you find that &opts escapes to the heap in the parent function instead.
I haven't opened an issue yet (I was hoping to get confirmation that it was
a bug first) but will do so today unless someone posts a definitive answer
here.
Thanks...
On Thursday, September 6, 2018 at 10:33:17 AM UTC-5, Tristan Colgate wrote:
>
> I think this has to do with the pointer reciever, vs the pass by value:
>
> func noEscape(r rune, opts *options) bool {
> f := opts.isDigit
> return f(r)
> }
>
> opts here does not escape, but in:
>
> func escapes(r rune, opts options) bool {
> f := opts.isDigit
> return f(r)
> }
>
> opts is copied, so it is the copy of opts that the compiler believes
> escapes. Perhaps this is because opts could be used by a defer (there is
> none though, the compiler could/should notice that).
>
> In the following, opts2 even escapes and gets heap allocated.
>
> func escapes(r rune, opts *options) bool {
> var res bool
> {
> opts2 := *opts
>
> f := opts2.isDigit
> res = f(r)
> }
> return res
> }
>
> Did you open an issue? I'm curious if there is a reason the escape
> analysis can't pick this up.
>
>
> On Wed, 5 Sep 2018 at 18:06 <paul...@gmail.com <javascript:>> wrote:
>
>> I wonder if this is to do with method values. According to the spec
>> <https://golang.org/ref/spec#Method_values>, when you declare a method
>> value like x.M:
>>
>> The expression x is evaluated and saved during the evaluation of the
>>> method value; the saved copy is then used as the receiver in any calls,
>>> which may be executed later.
>>
>>
>> So using the method value opts.isDigit in index1 does in fact result in
>> &opts being copied. Maybe this causes opts to escape to the heap (although
>> I don't know why the copy would need to live beyond the scope of index1).
>> This would also explain why opts does not escape in index2 where
>> opts.isDigit() is just a normal method call.
>>
>> I tested this theory with two new functions (neither of which call
>> IndexFunc -- that doesn't seem to be part of the problem). One function
>> calls the isDigit method directly and the other uses a method value.
>> They're functionally equivalent but opts only escapes in the second
>> function.
>>
>>
>> // isDigit called directly: opts does not escape to heap
>> func isDigit1(r rune, opts options) bool {
>> return opts.isDigit(r)
>> }
>>
>> // isDigit called via method value: opts escapes to heap
>> func isDigit2(r rune, opts options) bool {
>> f := opts.isDigit
>> return f(r)
>> }
>>
>>
>> Does anyone have any insight/views on a) whether this is really what's
>> happening and b) whether this is the desired behaviour? I don't see why
>> using method values in this way should cause a heap allocation but perhaps
>> there's a reason for it.
>>
>>
>> On Tuesday, September 4, 2018 at 4:46:09 PM UTC-5, Paul D wrote:
>>>
>>> I'm trying to reduce allocations (and improve performance) in some Go
>>> code. There's a recurring pattern in the code where a struct is passed to a
>>> function, and the function passes one of the struct's methods to
>>> strings.IndexFunc. For some reason, this causes the entire struct to escape
>>> to the heap. If I wrap the method call in an anonymous function, the struct
>>> does not escape and the benchmarks run about 30% faster.
>>>
>>> Here is a minimal example. In the actual code, the struct has more
>>> fields/methods and the function in question actually does something. But
>>> this sample code illustrates the problem. Why does the opts argument escape
>>> to the heap in index1 but not in the functionally equivalent index2? And is
>>> there a robust way to ensure that it stays on the stack?
>>>
>>>
>>> type options struct {
>>> zero rune
>>> }
>>>
>>> func (opts *options) isDigit(r rune) bool {
>>> r -= opts.zero
>>> return r >= 0 && r <= 9
>>> }
>>>
>>> // opts escapes to heap
>>> func index1(s string, opts options) int {
>>> return strings.IndexFunc(s, opts.isDigit)
>>> }
>>>
>>> // opts does not escape to heap
>>> func index2(s string, opts options) int {
>>> return strings.IndexFunc(s, func(r rune) bool {
>>> return opts.isDigit(r)
>>> })
>>> }
>>>
>>>
>>> FYI I'm running Go 1.10.3 on Linux. Thanks...
>>>
>>>
>>> --
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