Re: [patch] Fix ICE in set_lattice_value

[Richard Guenther <richard dot guenther at gmail dot com>]

On Thu, Jul 19, 2012 at 9:30 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> Hmm, the point is of couse to not allow transitions that could form a
>> cycle, which is why the reverse transition is not allowed.
>>
>> Let me have a closer look here.
>
> You can reproduce on your favorite platform by locally copying the system.ads
> from gcc/ada/rts in your build tree and turning ZCX_By_Default to False.

That helps reproducing it.  The issue is that this really is a transition in the
wrong direction.  We iterate

Visiting statement:
D.2928_263 = (sizetype) i.29_262;
which is likely UNDEFINED
Lattice value changed to UNDEFINED.  Adding SSA edges to worklist.
...
Visiting statement:
D.2930_265 = (sizetype) j.30_264;
which is likely CONSTANT
Lattice value changed to CONSTANT 0.  Adding SSA edges to worklist.
...
Visiting statement:
elem_266 = &arr[D.2928_263][D.2930_265];
which is likely CONSTANT
Lattice value changed to CONSTANT Lattice value changed to CONSTANT
0x00000000000000000 (0x0fffffffffffffffc).  Adding SSA edges to
worklist.

... 2nd iteration:

Visiting statement:
D.2928_263 = (sizetype) i.29_262;
which is likely CONSTANT
Lattice value changed to CONSTANT 0.  Adding SSA edges to worklist.
...
Visiting statement:
elem_266 = &arr[D.2928_263][D.2930_265];
which is likely CONSTANT

so the error is that in the 1st iteration when visiting elem_266 we do not
treat D.2928_263 optimistically enough.  We could either derive a
lattice-value of &arr[0][0] or simply make it UNDEFINED (which sounds
like the easier solution).

So I'm testing the following

Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c  (revision 189646)
+++ gcc/tree-ssa-ccp.c  (working copy)
@@ -648,6 +649,11 @@ likely_value (gimple stmt)
             the undefined operand may be promoted.  */
          return UNDEFINED;

+       case ADDR_EXPR:
+         /* If any part of an address is UNDEFINED, like the index
+            of an ARRAY_EXPR, then treat the result as UNDEFINED.  */
+         return UNDEFINED;
+
        default:
          ;
        }

Richard.

> --
> Eric Botcazou