value of local variable stored in register

value of local variable stored in register

[Neeraj kushwaha]

Hi All,

I am trying to find the value of local variable stored in register.
But I am getting wrong value from gdb.
Here what i am trying

#include<stdio.h>

int main()
{
int *a=0;
volatile register int b=0xabcd;
 register int c=20;
printf("%x\n",b*c);
*a=0;
return 0;
}


------------------------------
-----------------------
GDB info:

Core was generated by `./a.out'.
Program terminated with signal 11, Segmentation fault.
 #0  0x080483e1 in main () at 1.c:9
9       *a=0;
(gdb) info local
a = (int *) 0x0
b = 43981
c = 11305136
(gdb) info reg
eax            0x0      0
ecx            0x0      0
edx            0xac80b0 11305136
 ebx            0xac6ff4 11300852
esp            0xbfc65a00       0xbfc65a00
ebp            0xbfc65a28       0xbfc65a28
esi            0x973ca0 9911456
edi            0x0      0
eip            0x80483e1        0x80483e1 <main+61>
 eflags         0x210282 [ SF IF RF ID ]
cs             0x73     115
ss             0x7b     123
ds             0x7b     123
es             0x7b     123
fs             0x0      0
gs             0x33     51
 (gdb)


The value of local variable is shown as c = 11305136, but actual value is 20.

How to get the correct value of variables which are stored in register.

Regards
Neeraj

Re: value of local variable stored in register

[Rohit Arul Raj]

Hi,

1. You are getting segmentation fault because of trying to write to a
invalid memory location (0x0) through pointer a;
2. It is not always true that if u give the storage class specifier as
"Register", a register will be allocated for the local variable. It
always depends on the compiler.
3. It is possible that for arithmetic operations, the compiler may
move your data to register from memory (operation: b*c & your
arithmetic operations does not support memory operands). In that case
u can look at the assembly generated.

Regards,
Rohit

On Feb 4, 2008 10:23 AM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> Hi All,
>
> I am trying to find the value of local variable stored in register.
> But I am getting wrong value from gdb.
> Here what i am trying
>
> #include<stdio.h>
>
> int main()
> {
> int *a=0;
> volatile register int b=0xabcd;
>  register int c=20;
> printf("%x\n",b*c);
> *a=0;
> return 0;
> }
>
>
> ------------------------------
> -----------------------
> GDB info:
>
> Core was generated by `./a.out'.
> Program terminated with signal 11, Segmentation fault.
>  #0  0x080483e1 in main () at 1.c:9
> 9       *a=0;
> (gdb) info local
> a = (int *) 0x0
> b = 43981
> c = 11305136
> (gdb) info reg
> eax            0x0      0
> ecx            0x0      0
> edx            0xac80b0 11305136
>  ebx            0xac6ff4 11300852
> esp            0xbfc65a00       0xbfc65a00
> ebp            0xbfc65a28       0xbfc65a28
> esi            0x973ca0 9911456
> edi            0x0      0
> eip            0x80483e1        0x80483e1 <main+61>
>  eflags         0x210282 [ SF IF RF ID ]
> cs             0x73     115
> ss             0x7b     123
> ds             0x7b     123
> es             0x7b     123
> fs             0x0      0
> gs             0x33     51
>  (gdb)
>
>
> The value of local variable is shown as c = 11305136, but actual value is 20.
>
> How to get the correct value of variables which are stored in register.
>
> Regards
> Neeraj
>

Re: value of local variable stored in register

[Neeraj kushwaha]

Hi Rohit,

thanks for the reply.

segmentation occurs due to unreferencing null pointer. (this was done
intensionally)
I am running gdb on core dump.

My main intention to get the value of the local variable stored in register.
From the dwarf dump information i can say it was stored in DW_OP_reg2.

Also if for the arithmetic operations, data is moved to register from
memory, but when the printf function returns all the saved register
value should be restored.

this is the dwarf information extracted from a.out

<1><  276>      DW_TAG_subprogram
               DW_AT_sibling               <341>
               DW_AT_external              yes(1)
               DW_AT_name                  main
               DW_AT_decl_file             1
/home/neeraj/SSG/Release2/gdblocal/2.c
               DW_AT_decl_line             5
               DW_AT_type                  <199>
               DW_AT_low_pc                0x80483a4
               DW_AT_high_pc               0x80483f5
               DW_AT_frame_base            [
0]<lowpc=0x0><highpc=0x4>DW_OP_breg4+4
                                       [ 1]<lowpc=0x4><highpc=0xa>DW_OP_reg1
                                       [ 2]<lowpc=0xa><highpc=0xb>DW_OP_breg4+4
                                       [ 3]<lowpc=0xb><highpc=0xd>DW_OP_breg4+8
                                       [
4]<lowpc=0xd><highpc=0x51>DW_OP_breg5+8
<2><  305>      DW_TAG_variable
               DW_AT_name                  a
               DW_AT_decl_file             1
/home/neeraj/SSG/Release2/gdblocal/2.c
               DW_AT_decl_line             6
               DW_AT_type                  <341>
               DW_AT_location              DW_OP_fbreg -16
<2><  317>      DW_TAG_variable
               DW_AT_name                  b
               DW_AT_decl_file             1
/home/neeraj/SSG/Release2/gdblocal/2.c
               DW_AT_decl_line             7
               DW_AT_type                  <347>
               DW_AT_location              DW_OP_fbreg -20
<2><  329>      DW_TAG_variable
               DW_AT_name                  c
               DW_AT_decl_file             1
/home/neeraj/SSG/Release2/gdblocal/2.c
               DW_AT_decl_line             8
               DW_AT_type                  <199>
               DW_AT_location              DW_OP_reg2
<1><  341>      DW_TAG_pointer_type
               DW_AT_byte_size             4
               DW_AT_type                  <199>
<1><  347>      DW_TAG_volatile_type
               DW_AT_type                  <199>

Regards
Neeraj

On Feb 4, 2008 10:58 AM, Rohit Arul Raj <rohitarulraj@gmail.com> wrote:
> Hi,
>
> 1. You are getting segmentation fault because of trying to write to a
> invalid memory location (0x0) through pointer a;
> 2. It is not always true that if u give the storage class specifier as
> "Register", a register will be allocated for the local variable. It
> always depends on the compiler.
> 3. It is possible that for arithmetic operations, the compiler may
> move your data to register from memory (operation: b*c & your
> arithmetic operations does not support memory operands). In that case
> u can look at the assembly generated.
>
> Regards,
> Rohit
>
>
> On Feb 4, 2008 10:23 AM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> > Hi All,
> >
> > I am trying to find the value of local variable stored in register.
> > But I am getting wrong value from gdb.
> > Here what i am trying
> >
> > #include<stdio.h>
> >
> > int main()
> > {
> > int *a=0;
> > volatile register int b=0xabcd;
> >  register int c=20;
> > printf("%x\n",b*c);
> > *a=0;
> > return 0;
> > }
> >
> >
> > ------------------------------
> > -----------------------
> > GDB info:
> >
> > Core was generated by `./a.out'.
> > Program terminated with signal 11, Segmentation fault.
> >  #0  0x080483e1 in main () at 1.c:9
> > 9       *a=0;
> > (gdb) info local
> > a = (int *) 0x0
> > b = 43981
> > c = 11305136
> > (gdb) info reg
> > eax            0x0      0
> > ecx            0x0      0
> > edx            0xac80b0 11305136
> >  ebx            0xac6ff4 11300852
> > esp            0xbfc65a00       0xbfc65a00
> > ebp            0xbfc65a28       0xbfc65a28
> > esi            0x973ca0 9911456
> > edi            0x0      0
> > eip            0x80483e1        0x80483e1 <main+61>
> >  eflags         0x210282 [ SF IF RF ID ]
> > cs             0x73     115
> > ss             0x7b     123
> > ds             0x7b     123
> > es             0x7b     123
> > fs             0x0      0
> > gs             0x33     51
> >  (gdb)
> >
> >
> > The value of local variable is shown as c = 11305136, but actual value is 20.
> >
> > How to get the correct value of variables which are stored in register.
> >
> > Regards
> > Neeraj
> >
>

Re: value of local variable stored in register

[Rohit Arul Raj]

Hi,

Then, the register value is getting corrupted due to the segmentation fault.
Try to set the break-point before the *a = 0 statement, and try to get
the register info and local variable info.

Regards,
Rohit

On Feb 4, 2008 12:09 PM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
>
> Hi Rohit,
>
> thanks for the reply.
>
> segmentation occurs due to unreferencing null pointer. (this was done
> intensionally)
> I am running gdb on core dump.
>
> My main intention to get the value of the local variable stored in register.
> From the dwarf dump information i can say it was stored in DW_OP_reg2.
>
> Also if for the arithmetic operations, data is moved to register from
> memory, but when the printf function returns all the saved register
> value should be restored.
>
> this is the dwarf information extracted from a.out
>
> <1><  276>      DW_TAG_subprogram
>                DW_AT_sibling               <341>
>                DW_AT_external              yes(1)
>                DW_AT_name                  main
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             5
>                DW_AT_type                  <199>
>                DW_AT_low_pc                0x80483a4
>                DW_AT_high_pc               0x80483f5
>                DW_AT_frame_base            [
> 0]<lowpc=0x0><highpc=0x4>DW_OP_breg4+4
>                                        [ 1]<lowpc=0x4><highpc=0xa>DW_OP_reg1
>                                        [ 2]<lowpc=0xa><highpc=0xb>DW_OP_breg4+4
>                                        [ 3]<lowpc=0xb><highpc=0xd>DW_OP_breg4+8
>                                        [
> 4]<lowpc=0xd><highpc=0x51>DW_OP_breg5+8
> <2><  305>      DW_TAG_variable
>                DW_AT_name                  a
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             6
>                DW_AT_type                  <341>
>                DW_AT_location              DW_OP_fbreg -16
> <2><  317>      DW_TAG_variable
>                DW_AT_name                  b
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             7
>                DW_AT_type                  <347>
>                DW_AT_location              DW_OP_fbreg -20
> <2><  329>      DW_TAG_variable
>                DW_AT_name                  c
>                DW_AT_decl_file             1
> /home/neeraj/SSG/Release2/gdblocal/2.c
>                DW_AT_decl_line             8
>                DW_AT_type                  <199>
>                DW_AT_location              DW_OP_reg2
> <1><  341>      DW_TAG_pointer_type
>                DW_AT_byte_size             4
>                DW_AT_type                  <199>
> <1><  347>      DW_TAG_volatile_type
>                DW_AT_type                  <199>
>
> Regards
> Neeraj
>
> On Feb 4, 2008 10:58 AM, Rohit Arul Raj <rohitarulraj@gmail.com> wrote:
>
> > Hi,
> >
> > 1. You are getting segmentation fault because of trying to write to a
> > invalid memory location (0x0) through pointer a;
> > 2. It is not always true that if u give the storage class specifier as
> > "Register", a register will be allocated for the local variable. It
> > always depends on the compiler.
> > 3. It is possible that for arithmetic operations, the compiler may
> > move your data to register from memory (operation: b*c & your
> > arithmetic operations does not support memory operands). In that case
> > u can look at the assembly generated.
> >
> > Regards,
> > Rohit
> >
> >
> > On Feb 4, 2008 10:23 AM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> > > Hi All,
> > >
> > > I am trying to find the value of local variable stored in register.
> > > But I am getting wrong value from gdb.
> > > Here what i am trying
> > >
> > > #include<stdio.h>
> > >
> > > int main()
> > > {
> > > int *a=0;
> > > volatile register int b=0xabcd;
> > >  register int c=20;
> > > printf("%x\n",b*c);
> > > *a=0;
> > > return 0;
> > > }
> > >
> > >
> > > ------------------------------
> > > -----------------------
> > > GDB info:
> > >
> > > Core was generated by `./a.out'.
> > > Program terminated with signal 11, Segmentation fault.
> > >  #0  0x080483e1 in main () at 1.c:9
> > > 9       *a=0;
> > > (gdb) info local
> > > a = (int *) 0x0
> > > b = 43981
> > > c = 11305136
> > > (gdb) info reg
> > > eax            0x0      0
> > > ecx            0x0      0
> > > edx            0xac80b0 11305136
> > >  ebx            0xac6ff4 11300852
> > > esp            0xbfc65a00       0xbfc65a00
> > > ebp            0xbfc65a28       0xbfc65a28
> > > esi            0x973ca0 9911456
> > > edi            0x0      0
> > > eip            0x80483e1        0x80483e1 <main+61>
> > >  eflags         0x210282 [ SF IF RF ID ]
> > > cs             0x73     115
> > > ss             0x7b     123
> > > ds             0x7b     123
> > > es             0x7b     123
> > > fs             0x0      0
> > > gs             0x33     51
> > >  (gdb)
> > >
> > >
> > > The value of local variable is shown as c = 11305136, but actual value is 20.
> > >
> > > How to get the correct value of variables which are stored in register.
> > >
> > > Regards
> > > Neeraj
> > >
> >
>

Re: value of local variable stored in register

[Rohit Arul Raj]

Hi,

Since the live range of value 'c' is dead after printf, the compiler
may be discarding it.
Try to use something like b = c + 200; after printf statement, so that
we extend the live range of variable 'c'.

Regards,
Rohit

On Feb 4, 2008 12:32 PM, Rohit Arul Raj <rohitarulraj@gmail.com> wrote:
> Hi,
>
> Then, the register value is getting corrupted due to the segmentation fault.
> Try to set the break-point before the *a = 0 statement, and try to get
> the register info and local variable info.
>
> Regards,
> Rohit
>
>
> On Feb 4, 2008 12:09 PM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> >
> > Hi Rohit,
> >
> > thanks for the reply.
> >
> > segmentation occurs due to unreferencing null pointer. (this was done
> > intensionally)
> > I am running gdb on core dump.
> >
> > My main intention to get the value of the local variable stored in register.
> > From the dwarf dump information i can say it was stored in DW_OP_reg2.
> >
> > Also if for the arithmetic operations, data is moved to register from
> > memory, but when the printf function returns all the saved register
> > value should be restored.
> >
> > this is the dwarf information extracted from a.out
> >
> > <1><  276>      DW_TAG_subprogram
> >                DW_AT_sibling               <341>
> >                DW_AT_external              yes(1)
> >                DW_AT_name                  main
> >                DW_AT_decl_file             1
> > /home/neeraj/SSG/Release2/gdblocal/2.c
> >                DW_AT_decl_line             5
> >                DW_AT_type                  <199>
> >                DW_AT_low_pc                0x80483a4
> >                DW_AT_high_pc               0x80483f5
> >                DW_AT_frame_base            [
> > 0]<lowpc=0x0><highpc=0x4>DW_OP_breg4+4
> >                                        [ 1]<lowpc=0x4><highpc=0xa>DW_OP_reg1
> >                                        [ 2]<lowpc=0xa><highpc=0xb>DW_OP_breg4+4
> >                                        [ 3]<lowpc=0xb><highpc=0xd>DW_OP_breg4+8
> >                                        [
> > 4]<lowpc=0xd><highpc=0x51>DW_OP_breg5+8
> > <2><  305>      DW_TAG_variable
> >                DW_AT_name                  a
> >                DW_AT_decl_file             1
> > /home/neeraj/SSG/Release2/gdblocal/2.c
> >                DW_AT_decl_line             6
> >                DW_AT_type                  <341>
> >                DW_AT_location              DW_OP_fbreg -16
> > <2><  317>      DW_TAG_variable
> >                DW_AT_name                  b
> >                DW_AT_decl_file             1
> > /home/neeraj/SSG/Release2/gdblocal/2.c
> >                DW_AT_decl_line             7
> >                DW_AT_type                  <347>
> >                DW_AT_location              DW_OP_fbreg -20
> > <2><  329>      DW_TAG_variable
> >                DW_AT_name                  c
> >                DW_AT_decl_file             1
> > /home/neeraj/SSG/Release2/gdblocal/2.c
> >                DW_AT_decl_line             8
> >                DW_AT_type                  <199>
> >                DW_AT_location              DW_OP_reg2
> > <1><  341>      DW_TAG_pointer_type
> >                DW_AT_byte_size             4
> >                DW_AT_type                  <199>
> > <1><  347>      DW_TAG_volatile_type
> >                DW_AT_type                  <199>
> >
> > Regards
> > Neeraj
> >
> > On Feb 4, 2008 10:58 AM, Rohit Arul Raj <rohitarulraj@gmail.com> wrote:
> >
> > > Hi,
> > >
> > > 1. You are getting segmentation fault because of trying to write to a
> > > invalid memory location (0x0) through pointer a;
> > > 2. It is not always true that if u give the storage class specifier as
> > > "Register", a register will be allocated for the local variable. It
> > > always depends on the compiler.
> > > 3. It is possible that for arithmetic operations, the compiler may
> > > move your data to register from memory (operation: b*c & your
> > > arithmetic operations does not support memory operands). In that case
> > > u can look at the assembly generated.
> > >
> > > Regards,
> > > Rohit
> > >
> > >
> > > On Feb 4, 2008 10:23 AM, Neeraj kushwaha <kushneeraj@gmail.com> wrote:
> > > > Hi All,
> > > >
> > > > I am trying to find the value of local variable stored in register.
> > > > But I am getting wrong value from gdb.
> > > > Here what i am trying
> > > >
> > > > #include<stdio.h>
> > > >
> > > > int main()
> > > > {
> > > > int *a=0;
> > > > volatile register int b=0xabcd;
> > > >  register int c=20;
> > > > printf("%x\n",b*c);
> > > > *a=0;
> > > > return 0;
> > > > }
> > > >
> > > >
> > > > ------------------------------
> > > > -----------------------
> > > > GDB info:
> > > >
> > > > Core was generated by `./a.out'.
> > > > Program terminated with signal 11, Segmentation fault.
> > > >  #0  0x080483e1 in main () at 1.c:9
> > > > 9       *a=0;
> > > > (gdb) info local
> > > > a = (int *) 0x0
> > > > b = 43981
> > > > c = 11305136
> > > > (gdb) info reg
> > > > eax            0x0      0
> > > > ecx            0x0      0
> > > > edx            0xac80b0 11305136
> > > >  ebx            0xac6ff4 11300852
> > > > esp            0xbfc65a00       0xbfc65a00
> > > > ebp            0xbfc65a28       0xbfc65a28
> > > > esi            0x973ca0 9911456
> > > > edi            0x0      0
> > > > eip            0x80483e1        0x80483e1 <main+61>
> > > >  eflags         0x210282 [ SF IF RF ID ]
> > > > cs             0x73     115
> > > > ss             0x7b     123
> > > > ds             0x7b     123
> > > > es             0x7b     123
> > > > fs             0x0      0
> > > > gs             0x33     51
> > > >  (gdb)
> > > >
> > > >
> > > > The value of local variable is shown as c = 11305136, but actual value is 20.
> > > >
> > > > How to get the correct value of variables which are stored in register.
> > > >
> > > > Regards
> > > > Neeraj
> > > >
> > >
> >
>

Re: value of local variable stored in register

[Eli Zaretskii]

> Date: Mon, 4 Feb 2008 10:23:26 +0530
> From: "Neeraj kushwaha" <kushneeraj@gmail.com>
> 
> The value of local variable is shown as c = 11305136, but actual value is 20.
> 
> How to get the correct value of variables which are stored in register.

 (gdb) info address c

This will show where the variable c is stored.  If it says it's in a
register, print the value of that register.

Re: value of local variable stored in register

[Jim Blandy]

On Feb 4, 2008 12:31 PM, Eli Zaretskii <eliz@gnu.org> wrote:
>  (gdb) info address c
>
> This will show where the variable c is stored.  If it says it's in a
> register, print the value of that register.

The output of 'info address' isn't very helpful if c has a non-trivial
DWARF location expression.  I've been intending to fix this for quite
some time now, but have never gotten around to it. :(

Re: value of local variable stored in register

[Eli Zaretskii]

> Date: Tue, 5 Feb 2008 11:42:32 -0800
> From: "Jim Blandy" <jimb@red-bean.com>
> Cc: "Neeraj kushwaha" <kushneeraj@gmail.com>, gdb@sourceware.org
> 
> On Feb 4, 2008 12:31 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> >  (gdb) info address c
> >
> > This will show where the variable c is stored.  If it says it's in a
> > register, print the value of that register.
> 
> The output of 'info address' isn't very helpful if c has a non-trivial
> DWARF location expression.

Can you show an example?

Also, is this problem relevant to the case in point (the OP presented
a test program)?

Re: value of local variable stored in register

[Jim Blandy]

On Feb 5, 2008 12:35 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> > The output of 'info address' isn't very helpful if c has a non-trivial
> > DWARF location expression.
>
> Can you show an example?
>
> Also, is this problem relevant to the case in point (the OP presented
> a test program)?

Even simple programs can have this trouble.  In the example below, GDB
can't print the address of parameter 'x', even though its home is a
fixed offset from the base address of the frame.

$ cat recur.c
#include <stdio.h>

int
recur (int x)
{
  if (x > 0)
    return 2 * recur (x - 2);
  else
    return 1;
}

int
main (int argc, char **argv)
{
  printf ("%d\n", recur (7));
}
$ gcc -g recur.c -o recur
$ ~/gdb/pub/nat/gdb/gdb recur
GNU gdb 6.7.50.20080111-cvs
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-linux-gnu"...
(gdb) break recur
Breakpoint 1 at 0x804838a: file recur.c, line 6.
(gdb) run
Starting program: /home/jimb/play/recur

Breakpoint 1, recur (x=7) at recur.c:6
6         if (x > 0)
(gdb) info address x
Symbol "x" is a variable with complex or multiple locations (DWARF2).
(gdb) shell readelf -wi recur
The section .debug_info contains:

  Compilation Unit @ offset 0x0:
   Length:        369
   Version:       2
   Abbrev Offset: 0
   Pointer Size:  4
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0
     DW_AT_high_pc     : 0x80483e7
     DW_AT_low_pc      : 0x8048384
     DW_AT_producer    : GNU C 4.1.2 20070626 (Red Hat 4.1.2-13)
     DW_AT_language    : 1      (ANSI C)
     DW_AT_name        : recur.c
     DW_AT_comp_dir    : /home/jimb/play
...
 <1><10b>: Abbrev Number: 5 (DW_TAG_subprogram)
     DW_AT_sibling     : <137>
     DW_AT_external    : 1
     DW_AT_name        : recur
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 5
     DW_AT_prototyped  : 1
     DW_AT_type        : <b8>
     DW_AT_low_pc      : 0x8048384
     DW_AT_high_pc     : 0x80483b1
     DW_AT_frame_base  : 0      (location list)
 <2><12a>: Abbrev Number: 6 (DW_TAG_formal_parameter)
     DW_AT_name        : x
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 4
     DW_AT_type        : <b8>
     DW_AT_location    : 2 byte block: 91 0     (DW_OP_fbreg: 0)
...
(gdb)

Re: value of local variable stored in register

[Eli Zaretskii]

> Date: Tue, 5 Feb 2008 16:26:46 -0800
> From: "Jim Blandy" <jimb@red-bean.com>
> Cc: kushneeraj@gmail.com, gdb@sourceware.org
> 
> (gdb) info address x
> Symbol "x" is a variable with complex or multiple locations (DWARF2).

That's a bad misfeature.  How about displaying all the possible
locations, if we cannot say which one is used at the time the command
is issued?

Re: value of local variable stored in register

[Jim Blandy]

On Feb 5, 2008 8:17 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> > (gdb) info address x
> > Symbol "x" is a variable with complex or multiple locations (DWARF2).
>
> That's a bad misfeature.  How about displaying all the possible
> locations, if we cannot say which one is used at the time the command
> is issued?

I agree.  Yes, that's reasonable.  It entails filling in
loclist_describe_location and locexpr_describe_location.