cmd/compile: emits unnecessary deferreturn #5712
Description
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 $0x8,%rsp
400c57: 48 8b 44 24 10 mov 0x10(%rsp),%rax
400c5c: 48 c7 44 24 18 00 00 movq $0x0,0x18(%rsp)
400c63: 00 00
if x != 42 {
400c65: 48 83 f8 2a cmp $0x2a,%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 $0x8,%rsp
400c79: c3 retq
If the compiler performs some CFG analysis, it can figure out that 'callq
runtime.deferreturn' is unnecessary in this case.