golang汇编用于内存分配

Question

//  example1.go

packge main

func main() {
    s := make([]byte, 1024, 1024)
   _ = s
}

s will be allocated in stack and lookup assemble code not call runtime.makeslice

// example2.go
packge main

func main() {
    cap := 1024
    s := make([]byte, 1024, cap)
    _ = s
}

s will be allocated in heap and lookup assemble code there is runtime.makeslice why this ???

// example3.go
package main

func main() {
    a := 100
    if a>1 {
        a = 1000
    }
    b := interface{}(a)
    _ = b
}

lookup the assemble code and see the compiler is very clever to opt that but in example2.go why don't do this

Answer 1

I think it's the variable nature of your cap variable - if you change this to const cap = 1024 the runtime.makeslice call disappears. It seems the Go compiler* is currently unable to infer that cap doesn't mutate, i.e. not change in value. As you state; in the last example a is evaluated to 1000 at compile time.

* go version go1.7.1 linux/amd64