[PATCH]: Fix compiler segfault failure in cd_dce pass

[Richard Biener]

On Wed, Nov 21, 2012 at 10:35 PM, Xinliang David Li <davidxl@google.com> wrote:
> In compiling one of the very large C++ source, the compiler hit a
> segfault in cddce -- the ssa_name of a vuse operand has a null def
> stmt.
>
> The def stmt was a PHI node, and later got removed by the phicprop
> pass (in eliminate_degenerated_phis because it seems to have zero
> uses)
>
> <bb 17>:
>  .MEM_343 =  PHI (.......)
>   VUSE (.MEM_343);
>   std::__throw_runtime_error (..)
>
>
> From the IR dump, it looks normal -- the PHI's def has a real use, but
> why does it got removed? Closer investigation shows that there is
> actually no use operand created for MEM_343 and the VUSE is a dangling
> reference. How did this happen?
>
> Here is how it happens:
>
> The phi node was created during SSA update after PRE, The
> make_ssa_name happens to pick up the dead ssa_name MEM_343 just got
> released because of unreachable code elimination (note that PRE can
> make condition be folded). After MEM_343 is release, the virtual use
> in bb 17 becomes dangled ( bb 17 was not deleted together with its
> predecessor that defines MEM_343 is because  of the tail block merging
> pass in PRE).  During post PRE ssa_update, the compiler sees that the
> tree value is the same '.MEM_343' so it does not bother to recreate
> the use, thus makes the phi look like dead.
>
>
> (As you can see, the condition to trigger the failure is really really
> rare, that is why there is no reduced test case for it --- multidelta
> is crunching for 4 days, and the best it got is still > 50k lines).
>
>
> The following simple patch solve the problem. Bootstrtapped and there
> is no regression. Ok to install for trunk?

That's not the correct fix - the fix is to fix PRE not to do this.
Btw, I fixed a
similar issue a few weeks ago - are you sure the problem still persists?

Thanks,
Richard.

>
>
> Index: tree-into-ssa.c
> ===================================================================
> --- tree-into-ssa.c     (revision 193698)
> +++ tree-into-ssa.c     (working copy)
> @@ -1767,7 +1767,7 @@ maybe_replace_use (use_operand_p use_p)
>    else if (is_old_name (use))
>      rdef = get_reaching_def (use);
>
> -  if (rdef && rdef != use)
> +  if (rdef)
>      SET_USE (use_p, rdef);
>  }
>
> 2012-11-21  Xinliang David Li  <davidxl@google.com>
>
> * tree-into-ssa.c (make_replace_use): force use replacement
> in SSA update.
>
>
> David