Hi, recently I was trying to figure out how GC marks stack objects and
found
some places of the implementation detail over my head.
source:
```
package main
import "runtime"
func main() {
x := make([]byte, 256*1024*1024)
runtime.GC() ← t1
x[0] = 2
runtime.GC() ← t2
println("x released")
runtime.GC()
println("over")
}
```
The GC trace info was like this:
```
➜ gc GODEBUG=gctrace=1 ./largeslice
gc 1 @0.009s 0%: 0.051+0.27+0.005 ms clock, 0.40+0/0.18/0.034+0.040 ms cpu,
256->256->256 MB, 257 MB goal, 8 P (forced)
gc 2 @0.010s 1%: 0.033+0.21+0.004 ms clock, 0.26+0/0.16/0.078+0.032 ms cpu,
256->256->256 MB, 512 MB goal, 8 P (forced)
gc 3 @0.012s 1%: 0.019+0.091+0.003 ms clock, 0.15+0/0.15/0.047+0.024 ms
cpu, 256->256->0 MB, 512 MB goal, 8 P (forced)
x released
gc 4 @0.013s 1%: 0.014+0.11+0.004 ms clock, 0.11+0/0.13/0.058+0.036 ms cpu,
0->0->0 MB, 4 MB goal, 8 P (forced)
over
```
It seems that the slice x could be garbage collected after the last use of
it,
which feels so intelligent and a little confusing, since fucntion call
did't return yet,
and made me wondering how Golang dynamically identify these stack objects.
```
// https://github.com/golang/go/blob/master/src/runtime/stack.go#L1246
func getStackMap(frame *stkframe, cache *pcvalueCache, debug bool) (locals,
args bitvector, objs []stackObjectRecord)
{
...
stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
...
```
AFAIK stack pointer maps are got via getStackmap function, which
reads FUNCDATA info from gopclntab(not very sure about this) section.
My question is that will GC get different stack maps at t1 and t2 ?
And if so how Golang implemented it, as GC stack scanning could happen
at any time, which means for different pc there maybe lot of possiblites
of stack maps.
Thanks
Ge
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