On Monday, June 29, 2020 at 8:16:14 PM UTC-5 Ian Lance Taylor wrote:

> On Mon, Jun 29, 2020 at 5:32 PM Bill Morgan 
> <arthurwil...@gmail.com> wrote: 
> > 
> > for this code: 
> > 
> > m := make(map[int]int, 9) 
> > 
> > I think the compiler creates a maptype that is stored at 
> type.*+60480(SB) that it uses for the call to runtime.makemap: 
> > 
> > m := make(map[int]int, 9) 
> > 0x10d0b81 488d05f8ec0000 LEAQ type.*+60480(SB), AX 
> > 0x10d0b88 48890424 MOVQ AX, 0(SP) 
> > 0x10d0b8c 48c744240809000000 MOVQ $0x9, 0x8(SP) 
> > 0x10d0b95 48c744241000000000 MOVQ $0x0, 0x10(SP) 
> > 0x10d0b9e 6690 NOPW 
> > 0x10d0ba0 e8fbdff3ff CALL runtime.makemap(SB) 
> > 
> > Where is the code in the compiler that creates the maptype? I'm 
> wondering how the bucket size is computed and how the rest of the maptype 
> structure is filled out. 
> https://golang.org/src/cmd/compile/internal/gc/reflect.go#L189 
> > Is there a way to print out the type info stored in the binary? 
> I'm not sure if this is what you want, but you can use reflect.TypeOf: 
> https://play.golang.org/p/FEUIoqc6SMU . 
> Ian 
Thanks Ian, that is pretty cool. I think that is printing out the 
map[int]int type though instead of the *maptype that is generated by the 
compiler and passed to runtime.makemap. 

I think this is loading a pointer to a maptype into AX to pass as the first 
arg to runtime.makemap. The maptype seems to be stored at type.*+60480(SB). 

LEAQ type.*+60480(SB), AX 

here's the type I'm asking about: where is this guy created in the 

type maptype struct {
typ _type
key *_type
elem *_type
bucket *_type // internal type representing a hash bucket
// function for hashing keys (ptr to key, seed) -> hash
hasher func(unsafe.Pointer, uintptr) uintptr
keysize uint8 // size of key slot
elemsize uint8 // size of elem slot
bucketsize uint16 // size of bucket
flags uint32

Here are the runtime.makemap comments and signature

// makemap implements Go map creation for make(map[k]v, hint).
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If h.buckets != nil, bucket pointed to can be used as the first bucket.
func makemap(t *maptype, hint int, h *hmap) *hmap {

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