cmd/compile: implement global new(T) without calling newobject

[josharian]

package p

var x = new(int)

This generates init code to call newobject (including a write barrier, which I thought wasn't supposed to happen any more?). Instead, we should make a BSS int symbol, and then make x contain a relocation to that symbol.

cc @randall77 @mvdan

[randall77]

No, we still need a write barrier when initializing a global location.

The write barriers I changed recently removes them when initializing the contents of the results of a newobject call. Not sure if you're referring to that.

Yes, we can and should allocate the int statically. Small caveat: watch out for #29068.