cmd/compile: bad DWARF location for variable

[aarzilli]

$ go version
go version go1.20.1 linux/amd64

Given the following code:

package main

import "fmt"

func cul(row int64) bool {
	id := int((row >> 32) & 0x3fffffff)
	
	return id != 0
}

func main() {
	fmt.Println(cul(0))
}

Compiled with `-gcflags='all=-N -l', main.cul function compiles into:

  test.go:5		0x49c8c0		4883ec18		SUBQ $0x18, SP		
  test.go:5		0x49c8c4		48896c2410		MOVQ BP, 0x10(SP)	
  test.go:5		0x49c8c9		488d6c2410		LEAQ 0x10(SP), BP	
  test.go:5		0x49c8ce		4889442420		MOVQ AX, 0x20(SP)	
  test.go:5		0x49c8d3		c644240700		MOVB $0x0, 0x7(SP)	
  test.go:6		0x49c8d8		488b4c2420		MOVQ 0x20(SP), CX	
  test.go:6		0x49c8dd		48c1f920		SARQ $0x20, CX		
  test.go:6		0x49c8e1		81e1ffffff3f		ANDL $0x3fffffff, CX	
  test.go:8		0x49c8e7		4885c9			TESTQ CX, CX		
  test.go:8		0x49c8ea		0f95c0			SETNE AL		
  test.go:6		0x49c8ed		48894c2408		MOVQ CX, 0x8(SP)	
  test.go:8		0x49c8f2		88442407		MOVB AL, 0x7(SP)	
  test.go:8		0x49c8f6		488b6c2410		MOVQ 0x10(SP), BP	
  test.go:8		0x49c8fb		4883c418		ADDQ $0x18, SP		
  test.go:8		0x49c8ff		90			NOPL			
  test.go:8		0x49c900		c3			RET			

and the variable id has DW_OP_fbreg -0x18 has its location, however at instruction 0x49c8ea, which is the instruction picked for a breakpoint on line :8, the variable is stored on register rcx, not in its stack location (where it gets moved only later at instruction 0x49c8ed).
Originally reported as go-delve/delve#3290.

cc @dr2chase.

[dmitshur]

CC @golang/compiler.

[mknyszek]

@dr2chase assigned to you in triage since we want your input but feel free to unassign.

[dr2chase]

I think this "fixed itself" which is not happy-making. A more permanent fix would be, when assigning line numbers in -N compilation, to take note of assignments that should have happened earlier and be sure that they are actually completed to memory. I'd like to leave this open but bump it to the next release, since that fix is actually a little complicated.

[ZekeLu]

In case it helps, here is another case extracted from this stackoverflow question :

 1	package main
 2	
 3	func main() {
 4		a := f(1)
 5		b := 1
 6	
 7		c := a < b
 8		_ = c
 9	}
10	
11	func f(i int) int {
12		if i > 0 {
13			return i
14		}
15		return -i
16	}

And here is the output of a delve debug session:

(dlv) l
> main.main() ./main.go:7 (hits goroutine(1):1 total:1) (PC: 0x4608de)
     2:	
     3:	func main() {
     4:		a := f(1)
     5:		b := 1
     6:	
=>   7:		c := a < b
     8:		_ = c
     9:	}
    10:	
    11:	func f(i int) int {
    12:		if i > 0 {
(dlv) disass
TEXT main.main(SB) /home/zeke/src/temp/76380802/main.go
	main.go:3	0x4608c0	493b6610		cmp rsp, qword ptr [r14+0x10]
	main.go:3	0x4608c4	7639			jbe 0x4608ff
	main.go:3	0x4608c6	4883ec28		sub rsp, 0x28
	main.go:3	0x4608ca	48896c2420		mov qword ptr [rsp+0x20], rbp
	main.go:3	0x4608cf	488d6c2420		lea rbp, ptr [rsp+0x20]
	main.go:4	0x4608d4	b801000000		mov eax, 0x1
	main.go:4	0x4608d9	e842000000		call $main.f
=>	main.go:7	0x4608de*	4883f801		cmp rax, 0x1
	main.go:4	0x4608e2	4889442418		mov qword ptr [rsp+0x18], rax
	main.go:5	0x4608e7	48c744241001000000	mov qword ptr [rsp+0x10], 0x1
	main.go:7	0x4608f0	0f9c44240f		setl byte ptr [rsp+0xf]
	main.go:9	0x4608f5	488b6c2420		mov rbp, qword ptr [rsp+0x20]
	main.go:9	0x4608fa	4883c428		add rsp, 0x28
	main.go:9	0x4608fe	c3			ret
	main.go:3	0x4608ff	90			nop
	main.go:3	0x460900	e83bccffff		call $runtime.morestack_noctxt
	main.go:3	0x460905	ebb9			jmp $main.main
(dlv) locals
a = 824633745824
b = 824633843808

[aarzilli]

Another example, this one on arm64:

package main

import "fmt"

func main() {
        SimpleMethodWithArgument(0)
}

//go:noinline
func SimpleMethodWithArgument(i int) {
        a := 5
        y := 10
        x := a + y + i // Breakpoint here
        fmt.Printf("a: %v, y: %v\n", a, y)
        _ = x
        return
}

SimpleMethodWithArguments compiles to:

test.go:10	S	0x4977c0	4c8d6424e8	LEAQ -0x18(SP), R12
test.go:10		0x4977c5	4d3b6610	CMPQ 0x10(R14), R12
test.go:10		0x4977c9	0f8615010000	JBE 0x4978e4
test.go:10		0x4977cf	55	PUSHL BP
test.go:10		0x4977d0	4889e5	MOVQ SP, BP
test.go:10	SP	0x4977d3	4881ec90000000	SUBQ $0x90, SP
test.go:10		0x4977da	48898424a0000000	MOVQ AX, 0xa0(SP)
test.go:11	S	0x4977e2	48c744243805000000	MOVQ $0x5, 0x38(SP)
test.go:12	S	0x4977eb	48c74424280a000000	MOVQ $0xa, 0x28(SP)
test.go:13	S	0x4977f4	488d480f	LEAQ 0xf(AX), CX
test.go:13		0x4977f8	48894c2430	MOVQ CX, 0x30(SP)
test.go:14	S	0x4977fd	440f117c2470	MOVUPS X15, 0x70(SP)

at 0x4977d3, the first statement, the stack variables are not yet initialized.