debug/10005: gcc -O -g does not produce location information for aliases of a variable

[werner at almesberger dot net]

>Number:         10005
>Category:       debug
>Synopsis:       gcc -O -g does not produce location information for aliases of a variable
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 08 14:36:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Werner Almesberger
>Release:        3.3 20030303 (prerelease)
>Organization:
>Environment:
System: Linux ar 2.4.18 #5 Mon Mar 18 09:25:24 ART 2002 i686 unknown
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ./configure --prefix=/usr/local/gcc20030303/ --enable-languages=c
>Description:
gcc -O -g does not produce location information for
aliases of a variable, so a debugger may fail to
change the variable.

Of course, one could argue that with -O, all bets are off as
far as the accuracy of debugging information is concerned, but
this still seems to be a fairly bad case.
>How-To-Repeat:
Example (with Red Hat's gcc 3.1 20011127 on ia32; gcc 3.3 yields
different code, but the problem remains):

$ cat <<EOF >foo.c
int bar;

int main(int argc)
{
    bar = (int) &argc;
    bar = argc;
label:
    return argc;
}
EOF 
$ gcc -O -g foo.c
$ readelf -w a.out
...
     DW_AT_name        : (indirect string, offset: 0x3a): argc
...
     DW_AT_location    : 2 byte block: 91 8     (DW_OP_fbreg: 8; )
...
     DW_AT_name        : (indirect string, offset: 0x55): label
...
     DW_AT_low_pc      : 0x80483a9 134513577
...
$ objdump -d a.out 
...
08048398 <main>:
...
 80483a1:       8b 45 08                mov    0x8(%ebp),%eax
 80483a4:       a3 24 95 04 08          mov    %eax,0x8049524
 80483a9:       c9                      leave
...

So suppose we place a breakpoint at "label" (0x80483a9), change
"argc" (supposedly at 8(%ebp)), and let the program continue.
In this case, "main" will still return the original value, and
ignore the change.
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: