I know that map is a reference type in Go (it has a pointer to the map entries memory region in its underlying struct). However, I would like to know what is the size of the underlying struct of the map because I want to know if using a pointer to a map as a function argument would be faster than not using a pointer.
Looking at this blog post it seems that the maptype struct has a lot of fields and that it would take a long time to copy (relative to a pointer).
Looking through the golang standard libraries I have found almost no use of *map[x]x so I guess using just map[x]x should be efficient as a function argument. So this leads me to think that maybe the compiler actually replaces map[x]x by a pointer to the maptype struct. Is that the case? If not what actually is happening that may avoid the copying of the maptype struct with its many fields?
The zero value for a Go map variable is a nil pointer.
var m map[string]int
make intializes a map and sets the map variable to point to a package runtime hmap struct.
m = make(map[string]int)
In Go, all arguments are passed by value. In the case of a map value, a reference type, a map value is a pointer. Therefore, passing a map value as a function or method argument is fast, you are passing a pointer.
The Go map runtime structs are currently located in the src/runtime/map.go Go source file. Since you only see a hmap pointer, their size is unlikely to be relevant.
See GopherCon 2016: Keith Randall - Inside the Map Implementation.