https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93844
Bug ID: 93844
Summary: [debug] Incorrect scope for local variables
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: go
Assignee: ian at airs dot com
Reporter: vries at gcc dot gnu.org
CC: cmang at google dot com
Target Milestone: ---
Consider the following c example hello.c (based on
gdb/testsuite/gdb.go/hello.go):
...
#include <stdio.h>
const char *st = "Shall we?";
int
main (void)
{
printf ("%s\n", st);
printf ("%s\n", "Before assignment");
{
const char *st = "Hello, world!";
printf ("%s\n", st);
}
return 0;
}
...
when compiling and executing, we get first the value of the global variable,
then the one of the local variable:
...
$ gcc hello.c -g
$ ./a.out
Shall we?
Before assignment
Hello, world!
...
Likewise, when debugging:
...
Temporary breakpoint 2, main () at hello.c:8
8 printf ("%s\n", st);
(gdb) p st
$3 = 0x4005d4 "Shall we?"
(gdb) n
Shall we?
9 printf ("%s\n", "Before assignment");
(gdb) p st
$4 = 0x4005d4 "Shall we?"
(gdb) n
Before assignment
11 const char *st = "Hello, world!";
(gdb) p st
$5 = 0x0
(gdb) n
12 printf ("%s\n", st);
(gdb) p st
$6 = 0x4005f0 "Hello, world!"
(gdb)
...
Now, consider the equivalent program in go:
...
package main
import "fmt"
var st = "Shall we?"
func main () {
fmt.Println (st)
fmt.Println ("Before assignment")
st := "Hello, world!"
fmt.Println (st)
}
...
When compiling and executing, again we get first the value of the global
variable, then the one of the local variable:
...
$ gccgo hello.go -g
$ ./a.out
Shall we?
Before assignment
Hello, world!
...
But when debugging, we fail to print the value of the global variable:
...
Thread 1 "a.out" hit Temporary breakpoint 1, main.main () at hello.go:7
7 func main () {
(gdb) p st
$1 = 0x0 ""
(gdb) n
8 fmt.Println (st)
(gdb) p st
$2 = 0x0 ""
(gdb) n
Shall we?
9 fmt.Println ("Before assignment")
(gdb) p st
$3 = 0x0 ""
(gdb) n
Before assignment
10 st := "Hello, world!"
(gdb) p st
$4 = 0x0 ""
(gdb) n
11 fmt.Println (st)
(gdb) p st
$5 = 0x404e06 "Hello, world!"
...
This is due to lack of scoping of the local variable.
In the C case, the st local variable is scoped by a DW_TAG_lexical_block:
...
<1><3c3>: Abbrev Number: 18 (DW_TAG_variable)
<3c4> DW_AT_name : st
<1><3d7>: Abbrev Number: 19 (DW_TAG_subprogram)
<3d8> DW_AT_name : main
<2><3f4>: Abbrev Number: 20 (DW_TAG_lexical_block)
<3><405>: Abbrev Number: 21 (DW_TAG_variable)
<406> DW_AT_name : st
...
But in the Go case, that scoping is missing:
...
<1><155>: Abbrev Number: 10 (DW_TAG_variable)
<156> DW_AT_name : main.st
(DW_OP_addr: 60e1f0)
<1><1ceb>: Abbrev Number: 34 (DW_TAG_subprogram)
<1cec> DW_AT_name : main.main
<2><1d05>: Abbrev Number: 35 (DW_TAG_variable)
<1d06> DW_AT_name : st
...
