Re: [patch] Fix ICE during RTL expansion at -O1

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

On Sun, Apr 14, 2013 at 9:46 AM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> This is a quadratic algorithm and as such not ok.  We already have
>> aliasing_component_refs_p in tree-ssa-alias.c which is supposed to be
>> the non-quadratic replacement.  It's not used via decl_refs_may_alias_p,
>> so that may be the thing to fix.
>
> aliasing_component_refs_p isn't powerful enough, it eliminates the quadratic
> aspect by assuming that all offsets are constants, so it misses cases like
> (*p)[i].f1 vs a[j].f2.  Moreover it assumes TBAA and we don't need it here.

Note that looking at the access path _is_ assuming TBAA constraints as
soon as the base objects are not the same (in the above case '*p' and 'a'
are not the same and p could alias a in a way that all f1 and f2 overlap).

> I can rewrite nonoverlapping_component_refs_of_decl_p to make it non-quadratic
> and catch the same cases I think, patch attached (without the vect testsuite
> adjustments, but they are still needed).
>
>> nonoverlapping_component_refs_of_decl_p on RTL should go - in fact
>> we do call the tree oracle from all its callers so we only ever do redundant
>> work (after your proposed patch even more so).
>
> Not clear if the tree oracle can catch the above case with *p and a, but, yes,
> nonoverlapping_component_refs_p should go in the long term.
>
>
>         * alias.c (nonoverlapping_component_refs_p): Protect again LTO quirk.
>         * tree-ssa-alias.c (nonoverlapping_component_refs_of_decl_p): New.
>         (decl_refs_may_alias_p): Add REF1 and REF2 parameters.
>         Use nonoverlapping_component_refs_of_decl_p to disambiguate component
>         references.
>         (refs_may_alias_p_1): Adjust call to decl_refs_may_alias_p.
>         * tree-streamer.c (record_common_node): Adjust reference in comment.

Index: alias.c
===================================================================
--- alias.c     (revision 197926)
+++ alias.c     (working copy)
@@ -2232,8 +2232,11 @@ nonoverlapping_component_refs_p (const_r

     found:
       /* If we're left with accessing different fields of a structure, then no
-        possible overlap, unless they are both bitfields.  */
-      if (TREE_CODE (typex) == RECORD_TYPE && fieldx != fieldy)
+        possible overlap, unless they are both bitfields.
+        ??? Pointer inequality is too fragile in the LTO compiler.  */
+      if (TREE_CODE (typex) == RECORD_TYPE
+         && fieldx != fieldy
+         && DECL_NAME (fieldx) != DECL_NAME (fieldy))

this, if at all, should go in with a separate patch and a testcase.
And I think it should _not_ go in.  Instead, as the case passes

              if (typex == typey)
                goto found;

earlier you should assert that DECL_CONTEXT (fieldx) == DECL_CONTEXT
(fieldy) == typex == typey here.  Note that fails of this test are
expected even in the
non-LTO case because I cannot find any IL verification that would verify
that for a COMPONENT_REF TREE_TYPE (TREE_OPERAND (cr, 0))
== DECL_CONTEXT (TREE_OPERAND (cr, 1)) (due to sharing of the FIELD_DECL
chain between different type variants the check will fail for all
non-main-variants
I think, so refining it to look at the main variant is probably advised).

Otoh...

+      /* ??? We cannot simply use the type of operand #0 of the refs here
+        as the Fortran compiler smuggles type punning into COMPONENT_REFs
+        for common blocks instead of using unions like everyone else.  */
+      tree type1 = TYPE_MAIN_VARIANT (DECL_CONTEXT (field1));
+      tree type2 = TYPE_MAIN_VARIANT (DECL_CONTEXT (field2));
+
+      if (type1 != type2 || TREE_CODE (type1) != RECORD_TYPE)
+        goto may_overlap;
+
+      /* ??? Pointer inequality is too fragile in the LTO compiler.  */
+      if (field1 != field2 && DECL_NAME (field1) != DECL_NAME (field2))

this suggests you are seeing multiple FIELD_DECLs for the same field
in the _same_ FIELD_DECL chain ...?!  Are you sure this happens with
GCC 4.8?  There were some fixes in that area in the LTO type merging code.

Index: tree-streamer.c
===================================================================
--- tree-streamer.c     (revision 197926)
+++ tree-streamer.c     (working copy)
@@ -267,10 +267,9 @@ record_common_node (struct streamer_tree
       /* The FIELD_DECLs of structures should be shared, so that every
         COMPONENT_REF uses the same tree node when referencing a field.
         Pointer equality between FIELD_DECLs is used by the alias
-        machinery to compute overlapping memory references (See
-        nonoverlapping_component_refs_p).  */
-      tree f;
-      for (f = TYPE_FIELDS (node); f; f = TREE_CHAIN (f))
+        machinery to compute overlapping component references (see
+        nonoverlapping_component_refs_of_decl_p).  */
+      for (tree f = TYPE_FIELDS (node); f; f = TREE_CHAIN (f))
        record_common_node (cache, f);
     }
 }

without actually removing nonoverlapping_component_refs_p it still applies
to both...


Can you port the non-quadratic algorithm to the RTL
nonoverlapping_component_refs_p first?  The core code should exactly
be the same ... (and shared, and only called once from the RTL oracle).

+  /* Pop the stacks in parallel and examine the COMPONENT_REFs of the same
+     rank.  This is sufficient because you cannot reference several fields
+     at a time (unlike for arrays with slices), unless you're in a union,
+     in which case the return value will be false in any case.  */
+  while (true)

I don't understand the "reference several fields" comment.  Because I
can clearly have aggregate copies of RECORD_TYPE.  Can you try
do elaborate more on why the algorithm should be sufficient to catch
all cases?

You could enhance it to not require

+  /* We must have the same base DECL.  */
+  gcc_assert (ref1 == ref2);

for MEM_REF bases under the same conditions like aliasing_component_refs_p,
that is if the MEM_REF isn't view-converting.

That said, if the bases are the same DECLs then indeed you do not
rely on TBAA.  The RTL nonoverlapping_component_refs_p does not
disable itself properly for pointer based accesses that might be
view-converted / aliased accesses (a simple testcase with ref-all
pointers properly offsetted to alias two adjacent fields may be enough to
show that).

Also with your patch enhanced like I suggest we should be able to
remove aliasing_component_refs_p as well, no?

Thanks,
Richard.

>
> --
> Eric Botcazou