Fix compile warning in symtab.c

  My compiler (g++ 7.4.0) can't tell that *bsc_ptr and *slot_ptr are
only used in the cases when it does get initialized. Just initialize
the vars earlier to avoid the warning, there does not seem to be a
downside to it.

../../gdb/symtab.c: In function ‘block_symbol lookup_static_symbol(const char*, domain_enum)’:
../../gdb/symtab.c:1366:11: warning: ‘slot’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     xfree (slot->value.not_found.name);
     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../gdb/symtab.c:2578:29: note: ‘slot’ was declared here
   struct symbol_cache_slot *slot;
                             ^~~~
../../gdb/symtab.c:1405:3: warning: ‘bsc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (bsc == NULL)
   ^~
../../gdb/symtab.c:2577:30: note: ‘bsc’ was declared here
   struct block_symbol_cache *bsc;
                              ^~~
../../gdb/symtab.c: In function ‘block_symbol lookup_global_symbol(const char*, const block*, domain_enum)’:
../../gdb/symtab.c:1366:11: warning: ‘slot’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     xfree (slot->value.not_found.name);
     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../gdb/symtab.c:2658:29: note: ‘slot’ was declared here
   struct symbol_cache_slot *slot;
                             ^~~~
../../gdb/symtab.c:1409:14: warning: ‘bsc’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       ++bsc->collisions;
         ~~~~~^~~~~~~~~~
../../gdb/symtab.c:2657:30: note: ‘bsc’ was declared here
   struct block_symbol_cache *bsc;
                              ^~~

gdb/ChangeLog:

2019-08-22  Christian Biesinger  <cbiesinger@google.com>

	* symtab.c (symbol_cache_lookup): Always initialize *bsc_ptr and *slot_ptr.
---
 gdb/symtab.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)
  

>>>>> "Christian" == Christian Biesinger via gdb-patches <gdb-patches@sourceware.org> writes:

Christian> My compiler (g++ 7.4.0) can't tell that *bsc_ptr and *slot_ptr are
Christian> only used in the cases when it does get initialized. Just initialize
Christian> the vars earlier to avoid the warning, there does not seem to be a
Christian> downside to it.

I think this patch is fine, but I don't understand the connection
between the patch and the warnings.

Christian> ../../gdb/symtab.c: In function ‘block_symbol lookup_static_symbol(const char*, domain_enum)’:
Christian> ../../gdb/symtab.c:1366:11: warning: ‘slot’ may be used uninitialized in this function [-Wmaybe-uninitialized]

It seems to me that if the patch fixes this warning, then it must just
be a gcc bug?

Anyway, this is ok.

Tom
  

On Fri, Aug 23, 2019 at 1:03 PM Tom Tromey <tom@tromey.com> wrote:
>
> >>>>> "Christian" == Christian Biesinger via gdb-patches <gdb-patches@sourceware.org> writes:
>
> Christian> My compiler (g++ 7.4.0) can't tell that *bsc_ptr and *slot_ptr are
> Christian> only used in the cases when it does get initialized. Just initialize
> Christian> the vars earlier to avoid the warning, there does not seem to be a
> Christian> downside to it.
>
> I think this patch is fine, but I don't understand the connection
> between the patch and the warnings.

GCC can't tell that slot/bsc are only used in the codepath when
symbol_cache_lookup returns "entry not found", so this patch always
initializes them.

> Christian> ../../gdb/symtab.c: In function ‘block_symbol lookup_static_symbol(const char*, domain_enum)’:
> Christian> ../../gdb/symtab.c:1366:11: warning: ‘slot’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>
> It seems to me that if the patch fixes this warning, then it must just
> be a gcc bug?

Yeah, arguably this is a bug that gcc's control flow analysis isn't
smart enough to detect this.

> Anyway, this is ok.

Thanks, pushed.

Christian