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Re: [PATCH] Alias analysis of zero sized arrays


On Wed, Apr 26, 2017 at 12:27 AM, Steve Ellcey <sellcey@cavium.com> wrote:
> This patch changes get_ref_base_and_extent to treat zero sized arrays
> like C99 flexible arrays and assume that references to zero sized
> arrays can also be made beyond the 'end' of the array.  C99 flexible
> arrays are recognized by not having an INTEGER_CST limit/size and the
> routine then sets maxsize to -1 for special handling.  This patch
> checks for a value of 0 when we do have an INTEGER_CST limit/size and
> handles it like a flexible array.
>
> Tested with a bootstrap on aarch64 and a GCC test run with no
> regressions.
>
> I did not create a testcase because the test I have (attached) is not
> runnable.  If you compile this test case for aarch64 with
> '-UFLEX -O2 -fno-strict-aliasing' you will get two loads, then two
> stores in the main loop.  In the original large program that this came
> from, that generated bad code.  If compiled with -DFLEX instead of
> -UFLEX, you get load, store, load, store and that was working.  With
> this patch, both versions (-UFLEX and -DFLEX) generate the load, store,
> load, store sequence.
>
> OK for checkin?

Huh.  This doesn't look like the correct fix for anything.

Ah, so the issue is that we prune MEM_EXPR of MEMs off ARRAY_REFs
and end up with a MEM_EXPR of a.0_1->o which has zero size.  Then
we run into

  /* If MEM_OFFSET or MEM_SIZE are unknown what we got from MEM_EXPR
     is conservative, so trust it.  */
  if (!MEM_OFFSET_KNOWN_P (mem)
      || !MEM_SIZE_KNOWN_P (mem))
    return true;

but MEM_SIZE_KNOWN_P is true so we miss to do

  /* Refine size and offset we got from analyzing MEM_EXPR by using
     MEM_SIZE and MEM_OFFSET.  */

  ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
  ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;

thus a nearly minimal fix would be (looks like some other sanity checks
might better be moved before the early "trust it" out):

Index: gcc/alias.c
===================================================================
--- gcc/alias.c (revision 247273)
+++ gcc/alias.c (working copy)
@@ -322,6 +322,13 @@ ao_ref_from_mem (ao_ref *ref, const_rtx

   ref->ref_alias_set = MEM_ALIAS_SET (mem);

+  /* Refine size and offset we got from analyzing MEM_EXPR by using
+     MEM_SIZE and MEM_OFFSET.  */
+  if (MEM_OFFSET_KNOWN_P (mem))
+    ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
+  if (MEM_SIZE_KNOWN_P (mem))
+    ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;
+
   /* If MEM_OFFSET or MEM_SIZE are unknown what we got from MEM_EXPR
      is conservative, so trust it.  */
   if (!MEM_OFFSET_KNOWN_P (mem)
@@ -336,12 +343,6 @@ ao_ref_from_mem (ao_ref *ref, const_rtx
              > ref->max_size)))
     ref->ref = NULL_TREE;

-  /* Refine size and offset we got from analyzing MEM_EXPR by using
-     MEM_SIZE and MEM_OFFSET.  */
-
-  ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
-  ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;
-
   /* The MEM may extend into adjacent fields, so adjust max_size if
      necessary.  */
   if (ref->max_size != -1

note that in the end we might want to stop pruning MEM_EXPR so much...
(set_mem_attributes_minus_bitpos).  At least dropping component/array
refs in this case isn't conservative as the size zero access doesn't cover
the array ref stripped.  So arguably the bug is in
set_mem_attributes_minus_bitpos
itself.  Thus

Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c      (revision 247273)
+++ gcc/emit-rtl.c      (working copy)
@@ -1954,7 +1954,9 @@ set_mem_attributes_minus_bitpos (rtx ref
          while (TREE_CODE (t2) == ARRAY_REF);

          if (DECL_P (t2)
-             || TREE_CODE (t2) == COMPONENT_REF)
+             || (TREE_CODE (t2) == COMPONENT_REF
+                 && (! DECL_SIZE (TREE_OPERAND (t2, 1))
+                     || ! integer_zerop (DECL_SIZE (TREE_OPERAND (t2, 1))))))
            {
              attrs.expr = t2;
              attrs.offset_known_p = false;

is an alternative / additional fix (OTOH for all trailing arrays this
isn't really a
conservative MEM_EXPR, not just for size zero ones).  Thus

Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c      (revision 247273)
+++ gcc/emit-rtl.c      (working copy)
@@ -1954,7 +1954,10 @@ set_mem_attributes_minus_bitpos (rtx ref
          while (TREE_CODE (t2) == ARRAY_REF);

          if (DECL_P (t2)
-             || TREE_CODE (t2) == COMPONENT_REF)
+             || (TREE_CODE (t2) == COMPONENT_REF
+                 /* For trailing arrays t2 doesn't have a size that
+                    covers all valid accesses.  */
+                 && ! array_at_struct_end_p (t, false)))
            {
              attrs.expr = t2;
              attrs.offset_known_p = false;

is probably better.

Richard.

> Steve Ellcey
> sellcey@cavium.com
>
>
> 2017-04-25  Steve Ellcey  <sellcey@cavium.com>
>
>         * tree-dfa.c (get_ref_base_and_extent): Treat zero size array like
>         a C99 flexible array.
>


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