How golang calls runtime.panic* functions?

Getting Help
1

I am studying the assembly output of unoptimized go build. There is a function that returns nil no matter what. During runtime go panics as expected due to nil. How does go call the runtime.panic* functions as defined in assembly source? I dont see any calls to them, just some couple of tests.

Source code:

package main

type Some struct {
	a int
} 

func getSome() *Some {
	some := 23
	if some == 40 {
		return &Some{}
	} else {
		return nil	
	}
}

func main() {
	some := getSome()
	some.a = 13
}

Assembly fragment:

0000000000457660 <main.getSome>:
  457660:	55                   	push   %rbp
  457661:	48 89 e5             	mov    %rsp,%rbp
  457664:	48 83 ec 10          	sub    $0x10,%rsp
  457668:	48 c7 44 24 08 00 00 	movq   $0x0,0x8(%rsp)
  45766f:	00 00 
  457671:	48 c7 04 24 17 00 00 	movq   $0x17,(%rsp)
  457678:	00 
  457679:	eb 00                	jmp    45767b <main.getSome+0x1b>
  45767b:	48 c7 44 24 08 00 00 	movq   $0x0,0x8(%rsp)
  457682:	00 00 
  457684:	31 c0                	xor    %eax,%eax
  457686:	48 83 c4 10          	add    $0x10,%rsp
  45768a:	5d                   	pop    %rbp
  45768b:	c3                   	ret
  45768c:	cc                   	int3
  45768d:	cc                   	int3
  45768e:	cc                   	int3
  45768f:	cc                   	int3
  457690:	cc                   	int3
  457691:	cc                   	int3
  457692:	cc                   	int3
  457693:	cc                   	int3
  457694:	cc                   	int3
  457695:	cc                   	int3
  457696:	cc                   	int3
  457697:	cc                   	int3
  457698:	cc                   	int3
  457699:	cc                   	int3
  45769a:	cc                   	int3
  45769b:	cc                   	int3
  45769c:	cc                   	int3
  45769d:	cc                   	int3
  45769e:	cc                   	int3
  45769f:	cc                   	int3

00000000004576a0 <main.main>:
  4576a0:	49 3b 66 10          	cmp    0x10(%r14),%rsp
  4576a4:	76 20                	jbe    4576c6 <main.main+0x26>
  4576a6:	55                   	push   %rbp
  4576a7:	48 89 e5             	mov    %rsp,%rbp
  4576aa:	48 83 ec 08          	sub    $0x8,%rsp
  4576ae:	e8 ad ff ff ff       	call   457660 <main.getSome>
  4576b3:	48 89 04 24          	mov    %rax,(%rsp)
  4576b7:	84 00                	test   %al,(%rax)
  4576b9:	48 c7 00 0d 00 00 00 	movq   $0xd,(%rax)
  4576c0:	48 83 c4 08          	add    $0x8,%rsp
  4576c4:	5d                   	pop    %rbp
  4576c5:	c3                   	ret
  4576c6:	e8 f5 ce ff ff       	call   4545c0 <runtime.morestack_noctxt.abi0>
  4576cb:	eb d3                	jmp    4576a0 <main.main>
2

Godbolt uses color to show which parts of the assembly-code belong to which Go-code

I pasted your code here:

Maybe this helps?

3
  • A panic function (e.g., runtime.Panic, runtime.Panicf) is called with an argument, which can be any value. This argument becomes the “panic value” associated with the error.
4

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.