Yes, this is exactly what I was searching for. Thank you!
On Friday, 25 March 2022 at 21:51:27 UTC+3 Ian Lance Taylor wrote:
> On Fri, Mar 25, 2022 at 11:42 AM Maksadbek wrote:
> >
> > I've been reading the map internals in Go and came across into following
> structure in source code:
> >
> > https://github.com/golang/go/blob/master/src/runtime/map.go#L150
> >
> > // A bucket for a Go map.
> > type bmap struct {
> > // tophash generally contains the top byte of the hash value
> > // for each key in this bucket. If tophash[0] < minTopHash,
> > // tophash[0] is a bucket evacuation state instead.
> > tophash [bucketCnt]uint8
> > // Followed by bucketCnt keys and then bucketCnt elems.
> > // NOTE: packing all the keys together and then all the elems together
> makes the
> > // code a bit more complicated than alternating key/elem/key/elem/...
> but it allows
> > // us to eliminate padding which would be needed for, e.g.,
> map[int64]int8.
> > // Followed by an overflow pointer.
> > }
> >
> > But if you scroll down the file and see the usage of this structure,
> actually it has the following structure:
> >
> > // NOT REAL
> > type bmap struct {
> > topbits [8]uint8
> > keys [8]keytype
> > values [8]valuetype
> > pad uintptr
> > overflow uintptr
> > }
> >
> > I can't find out the place in the source code where this structure is
> extended. In order to access those extra fields, authors do some pointer
> arithmetics:
> >
> > https://github.com/golang/go/blob/master/src/runtime/map.go#L445
> > https://github.com/golang/go/blob/master/src/runtime/map.go#L501
> >
> > But in order to access those memory points, they needs to be allocated.
> Where does the allocation happen ?
>
> The allocation occurs in calls to newobject in hmap.newoverflow and
> calls to newarray in makeBucketArray. But you are probably asking
> where the t.bucket struct is defined. That happens in the compiler in
> MapBucketType in cmd/compile/internal/reflectdata/reflect.go.
>
> Ian
>
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