debug/10000: gcc -g generates location expressions which are not valid in prologue

[werner@almesberger.net]

>Number:         10000
>Category:       debug
>Synopsis:       gcc -g generates location expressions which are not valid in prologue
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Mar 08 14:16:01 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 -g generates location expressions which are not valid
in prologue

This may actually be more a misfeature of DWARF2 than a gcc bug.
Theoretically, DWARF2 would allow us to express exactly what is
valid and at which location, even during the prologue, but this
seems to be quite cumbersome.

>How-To-Repeat:

Example (with Red Hat's gcc 3.1 20011127 on ia32):

$ cat <<EOF >foo.c
int main(int argc)
{
    return 0;
}
EOF
$ gcc -g foo.c
$ readelf -w a.out
...
     DW_AT_name        : (indirect string, offset: 0x3f): main
...
     DW_AT_low_pc      : 0x8048398 134513560
...
     DW_AT_frame_base  : 1 byte block: 55       (DW_OP_reg5; )
 <2><40>: Abbrev Number: 3 (DW_TAG_formal_parameter)
     DW_AT_name        : (indirect string, offset: 0x3a): argc
...
     DW_AT_location    : 2 byte block: 91 8     (DW_OP_fbreg: 8; )
...
$ objdump -d a.out
...
08048398 <main>:
 8048398:       55                      push   %ebp
 8048399:       89 e5                   mov    %esp,%ebp
 804839b:       83 ec 08                sub    $0x8,%esp
 804839e:       83 e4 f0                and    $0xfffffff0,%esp
...

Quite obviously, 8(%ebp) only points to argc when we've reached
address 804839b, but the debugging information does not provide
any indication more precise than DW_AT_low_pc.

>Fix:

The work-around gdb uses seems to work reasonably well, but I'm
a bit worried about whether unusual linkage styles might throw
it off. (After all, this has to work with the Linux kernel.)
>Release-Note:
>Audit-Trail:
>Unformatted: