cmd/compile: DWARF duplicated DW_TAG_formal_parameter for some functions

[dev747368]

What version of Go are you using (go version)?

$ go version
go version go1.20.4 linux/amd64

I can duplicate this starting with 1.18. Binary produced with go 1.17 doesn't seem to show this issue.

Cross compiling to arm64 (instead of amd64) also shows the same issue.

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOARCH="amd64"
GOOS="linux"

What did you do?

package main

import "fmt"

func main() {
	fmt.Println("Hello")
}

readelf -wi hello | less
search for the "internal/poll.(*FD).Write" DW_TAG_subprogram entry

What did you expect to see?

Expected to see a subprogram entry with 4 DW_TAG_formal_parameter params (fd, p, ~r0, ~r1)

What did you see instead?

Th ~r0 and ~r1 formal params elements were duplicated:

 <1>: Abbrev Number: 3 (DW_TAG_subprogram)
       DW_AT_name        : internal/poll.(*FD).Write
       DW_AT_low_pc      : 0x4783e0
       DW_AT_high_pc     : 0x4788e9
       DW_AT_frame_base  : 1 byte block: 9c       (DW_OP_call_frame_cfa)
       DW_AT_decl_file   : 0x5
       DW_AT_external    : 1
 <2>: Abbrev Number: 18 (DW_TAG_formal_parameter)
       DW_AT_name        : fd
       DW_AT_variable_parameter: 0
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x7da43>
       DW_AT_location    : 0x1492a (location list)
 <2>: Abbrev Number: 18 (DW_TAG_formal_parameter)
       DW_AT_name        : p
       DW_AT_variable_parameter: 0
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x6af98>
       DW_AT_location    : 0x14970 (location list)
 <2>: Abbrev Number: 17 (DW_TAG_formal_parameter)     ***1st ~r0***
       DW_AT_name        : ~r0
       DW_AT_variable_parameter: 1
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x698f1>
       DW_AT_location    : 0 byte block:  ()
 <2>: Abbrev Number: 17 (DW_TAG_formal_parameter)     ***2nd ~r0***
       DW_AT_name        : ~r0
       DW_AT_variable_parameter: 1
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x698f1>
       DW_AT_location    : 0 byte block:  ()
 <2>: Abbrev Number: 17 (DW_TAG_formal_parameter)    ***1st ~r1***
       DW_AT_name        : ~r1
       DW_AT_variable_parameter: 1
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x740bc>
       DW_AT_location    : 0 byte block:  ()
 <2>: Abbrev Number: 17 (DW_TAG_formal_parameter)    ***2nd ~r1***
       DW_AT_name        : ~r1
       DW_AT_variable_parameter: 1
       DW_AT_decl_line   : 369
       DW_AT_type        : <0x740bc>
       DW_AT_location    : 0 byte block:  ()
.....

[cherrymui]

cc @thanm @golang/compiler

[mauri870]

Hi, I've been investigating this issue with the duplicated parameters, it seems to start with 1.18 and the issue still exists on tip.

I tried git bisect (1.17 good, 1.18 bad) and it complained that something broke between a merge and a commit. I could not find any related changes tho, might be that the good and bad revisions are not direct descendants of each other.

Here is a testcase for this issue:

// src/cmd/link/internal/ld/dwarf_test.go
func TestIssue61357(t *testing.T) {
	testenv.MustHaveGoBuild(t)

	mustHaveDWARF(t)
	t.Parallel()

	const prog = `
package main

import "fmt"

func main() {
	fmt.Println("Hello")
}
`
	_, ex := gobuildAndExamine(t, prog, NoOpt)

	die := findSubprogramDIE(t, ex, "internal/poll.(*FD).Write")

	found := processParams(die, ex)

	expected := "[fd:0:1 p:1:1 ~r0:2:2 ~r1:3:2]"
	if found != expected {
		t.Errorf("param check failed, wanted:\n%s\ngot:\n%s\n",
			expected, found)
	}
}

For go 1.17 (good revision) it returns:

[fd:0:1 p:1:1 ~r1:2:2 ~r2:3:2]

but I noticed that on newer versions it should be r0 and r1 instead.

For master this test yields [fd:0:1 p:1:1 ~r0:2:2 ~r1:3:2].

Also noticed that it is this particular line that generates the duplicated parameters.

[mauri870]

Did some testing on windows as well with dwarfdump, the debug information appears to be duplicated on the same way:

DW_TAG_formal_parameter
  DW_AT_name                  ~r0
  DW_AT_variable_parameter    yes(1)
  DW_AT_decl_line             0x0000016e
  DW_AT_type                  <GOFF=0x0006ebee>
  DW_AT_location              len 0x0000: 
DW_TAG_formal_parameter
  DW_AT_name                  ~r0
  DW_AT_variable_parameter    yes(1)
  DW_AT_decl_line             0x0000016e
  DW_AT_type                  <GOFF=0x0006ebee>
  DW_AT_location              len 0x0000: 
DW_TAG_formal_parameter
  DW_AT_name                  ~r1
  DW_AT_variable_parameter    yes(1)
  DW_AT_decl_line             0x0000016e
  DW_AT_type                  <GOFF=0x0007999a>
  DW_AT_location              len 0x0000: 
DW_TAG_formal_parameter
  DW_AT_name                  ~r1
  DW_AT_variable_parameter    yes(1)
  DW_AT_decl_line             0x0000016e
  DW_AT_type                  <GOFF=0x0007999a>
  DW_AT_location              len 0x0000: 

[gopherbot]

Change https://go.dev/cl/512197 mentions this issue: cmd/compile: fix duplicated dwarf parameters for some functions