[patch] avoid ICE due to NULL pointer dereference in ipa-comdats.c

[Jeff Law]

On 09/17/14 13:59, Sebastian Pop wrote:
> Hi,
>
> I got an ICE while building libstdc++ of a cross compiler x86 to aarch64.  I
> have a testcase that ICEs on current GCC trunk.  I was trying to painfully
> reduce it with creduce, and it is still several thousand lines of c++. Frustrated
> that it does not reduce anymore, I decided to have a look with gdb at why the
> compiler was iceing: the code dereferences a NULL pointer that we get by looking
> up the value of a symbol in a map.  Around that place, there is another pattern
> that first makes sure that the pointer we get from the map is non NULL: I copied
> that code around and that seemed to have solved the ICE.
>
> Regtested on x86-64-linux, and also checked that my aarch64 cross compilers are
> now building correctly libstdc++.
>
> Ok to commit?
Well, the obvious question is whether or not we ought to be able to 
lookup the symbol in that particular map.

You don't mention the other similar pattern of code which checks for a 
NULL return value from map.get, so I had to guess it was this one just 
above the one you want to change:

      /* Get current lattice value of SYMBOL.  */
       val = map.get (symbol);
       if (val)
         group = *val;

       /* If it is bottom, there is nothing to do; do not clear AUX
          so we won't re-queue the symbol.  */
       if (group == error_mark_node)
         continue;

I couldn't convince myself this code was correct if map.get returns a 
NULL pointer either!  It certainly isn't obvious if just continuing 
either loop is correct if the symbol isn't found.

I really think Jan needs to chime in here.

Jeff