parent: 17086:2879112bff3d tip, linux/amd64
The program is:
package main
import "sync"
func main() {
 println(foo(0))
}
func foo(x int) int {
 // fast path
 if x != 42 {
 return x
 }
 // slow path
 mu.Lock()
 defer mu.Unlock()
 seq++
 return seq
}
var (
 mu sync.Mutex
 seq int
)
The generated code for fast path is:
func foo(x int) int {
 400c40: 64 48 8b 0c 25 f0 ff mov %fs:0xfffffffffffffff0,%rcx
 400c47: ff ff 
 400c49: 48 3b 21 cmp (%rcx),%rsp
 400c4c: 77 05 ja 400c53 <main.foo+0x13>
 400c4e: e8 bd 81 01 00 callq 418e10 <runtime.morestack16>
 400c53: 48 83 ec 08 sub 0ドルx8,%rsp
 400c57: 48 8b 44 24 10 mov 0x10(%rsp),%rax
 400c5c: 48 c7 44 24 18 00 00 movq 0ドルx0,0x18(%rsp)
 400c63: 00 00 
 if x != 42 {
 400c65: 48 83 f8 2a cmp 0ドルx2a,%rax
 400c69: 74 0f je 400c7a <main.foo+0x3a>
 return x
 400c6b: 48 89 44 24 18 mov %rax,0x18(%rsp)
 }
 mu.Lock()
 defer mu.Unlock()
 seq++
 return seq
}
 400c70: e8 eb b7 00 00 callq 40c460 <runtime.deferreturn>
 400c75: 48 83 c4 08 add 0ドルx8,%rsp
 400c79: c3 retq 
If the compiler performs some CFG analysis, it can figure out that 'callq
runtime.deferreturn' is unnecessary in this case.