Re: [PATCH] Fix unaligned access when predictive commoning (PR 71083)

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

On 08/09/16 09:29, Richard Biener wrote:
> On Mon, 8 Aug 2016, Bernd Edlinger wrote:
>
>> Hi!
>>
>>
>> As described in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71083
>> we generate unaligned accesses because tree-predcom rewrites
>> bitfield and packed accesses in a way that looses the proper
>> alignment information.
>>
>> The attached patch fixes this by re-using the gimple memory
>> expression from the original access.
>>
>> I was not sure, if any non-constant array references would be
>> valid at where the ref_at_iteration is expanded, I set these
>> to the array-lower-bound, as the DR_OFFSET contains the folded
>> value of all non-constant array references.
>>
>> The patch compensates for the constant offset from the outer
>> reference to the inner reference, and replaces the inner
>> reference instead of the outer reference.
>>
>>
>> Boot-strapped and reg-tested on x86_64-linux-gnu.
>> OK for trunk?
>
> I don't think what you do is correct.  Consider
>
>   for (i)
>    for (j)
>     {
>       ... = a[i][j];
>     }
>
> predcom considers innermost loops only and thus the DRs will
> be analyzed with respect to that which means DR_BASE_ADDRESS
> is &a[i][0] but you end up generating (*(&a) + i * ..)[0][0]
> which is at best suspicious with respect to further analyses
> by data-ref and TBAA.  Also note that this may get alignment
> wrong as well as changing [i] to [0] may lead to false alignment
> (consider a [2][2] char array aligned to 4 bytes).
>
> Your patch goes halfway back to code we had in the past
> (looking at the 4.3 branch right now) which had numerous
> issues (don't remember exactly).
>

Hmm.  Yes.  These ARRAY_REFs ruin the whole idea :)

> I believe that if we'd handle bitfields by
>
>    if (TREE_CODE (DR_REF (dr)) == COMPONENT_REF
>        && DECL_BIT_FIELD_TYPE (TREE_OPERAND (DR_REF (dr), 1)))
>      ref = TREE_OPERAND (DR_REF (dr), 0);
>    else
>      ref = DR_REF (dr);
>    unsigned align = get_object_alignment (ref);
>
> and use the type / alignment of that ref for the built MEM_REF
> (with coff adjusted by the split bitfield component-ref offset)
> and build a COMPONENT_REF around the MEM_REF to handle the
> bitfield part this should work ok.
>

Ooookay.  I think your idea works in principle.

Attached a new version of the patch, I hope it did not become
too ugly.

I think from Eric's comment in get_inner_ref it can be possible
in Ada that the outer object is accidentally byte-aligned
but the inner reference is not.  In that case I think it is
better to generate a BIT_FIELD_REF instead of a COMPONENT_REF.

Eric do you agree?  Are there any Ada test cases where the
pcom optimization jumps in?

We prefer the COMPONENT_REF just because it is likely more
aligned than the bit field itself.  The mem_ref should be
correctly aligned in both cases.

I was not sure if I should pass the TREE_OPERAND(ref, 2) to
the COMPONENT_REF constructor. Is that used at all? All other
places where COMPONENT_REF are built, seen to pass NULL there.


Bootstrap on x86_64-linux-gnu, reg-test is still running.

Is it OK it no regressions?


Thanks
Bernd.

Attachment: changelog-pr71083.txt
Description: changelog-pr71083.txt

Index: gcc/tree-predcom.c
===================================================================
--- gcc/tree-predcom.c	(revision 239193)
+++ gcc/tree-predcom.c	(working copy)
@@ -213,6 +213,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-scalar-evolution.h"
 #include "params.h"
 #include "tree-affine.h"
+#include "builtins.h"
 
 /* The maximum number of iterations between the considered memory
    references.  */
@@ -1364,11 +1365,16 @@ replace_ref_with (gimple *stmt, tree new_tree, boo
 /* Returns a memory reference to DR in the ITER-th iteration of
    the loop it was analyzed in.  Append init stmts to STMTS.  */
 
-static tree 
+static tree
 ref_at_iteration (data_reference_p dr, int iter, gimple_seq *stmts)
 {
   tree off = DR_OFFSET (dr);
   tree coff = DR_INIT (dr);
+  tree ref = DR_REF (dr);
+  enum tree_code ref_code = ERROR_MARK;
+  tree ref_type = NULL_TREE;
+  tree ref_op1 = NULL_TREE;
+  tree ref_op2 = NULL_TREE;
   if (iter == 0)
     ;
   else if (TREE_CODE (DR_STEP (dr)) == INTEGER_CST)
@@ -1377,27 +1383,43 @@ ref_at_iteration (data_reference_p dr, int iter, g
   else
     off = size_binop (PLUS_EXPR, off,
 		      size_binop (MULT_EXPR, DR_STEP (dr), ssize_int (iter)));
+  if (TREE_CODE (ref) == COMPONENT_REF
+      && DECL_BIT_FIELD (TREE_OPERAND (ref, 1)))
+    {
+      unsigned HOST_WIDE_INT boff;
+      tree field = TREE_OPERAND (ref, 1);
+      ref_type = TREE_TYPE (ref);
+      boff = tree_to_uhwi (DECL_FIELD_BIT_OFFSET (field));
+      /* This can occur in Ada.  See Eric's comment in get_bit_range.  */
+      if ((boff % BITS_PER_UNIT) != 0)
+	{
+	  ref_code = BIT_FIELD_REF;
+	  ref_op1 = DECL_SIZE (field);
+	  ref_op2 = bitsize_zero_node;
+	}
+      else
+	{
+	  boff >>= LOG2_BITS_PER_UNIT;
+	  boff += tree_to_uhwi (component_ref_field_offset (ref));
+	  coff = size_binop (MINUS_EXPR, coff, ssize_int (boff));
+	  ref_code = COMPONENT_REF;
+	  ref_op1 = field;
+	  ref_op2 = TREE_OPERAND (ref, 2);
+	  ref = TREE_OPERAND (ref, 0);
+	}
+    }
   tree addr = fold_build_pointer_plus (DR_BASE_ADDRESS (dr), off);
   addr = force_gimple_operand_1 (unshare_expr (addr), stmts,
 				 is_gimple_mem_ref_addr, NULL_TREE);
-  tree alias_ptr = fold_convert (reference_alias_ptr_type (DR_REF (dr)), coff);
+  tree alias_ptr = fold_convert (reference_alias_ptr_type (ref), coff);
+  tree type = build_aligned_type (TREE_TYPE (ref),
+				  get_object_alignment (ref));
+  ref = build2 (MEM_REF, type, addr, alias_ptr);
   /* While data-ref analysis punts on bit offsets it still handles
-     bitfield accesses at byte boundaries.  Cope with that.  Note that
-     we cannot simply re-apply the outer COMPONENT_REF because the
-     byte-granular portion of it is already applied via DR_INIT and
-     DR_OFFSET, so simply build a BIT_FIELD_REF knowing that the bits
-     start at offset zero.  */
-  if (TREE_CODE (DR_REF (dr)) == COMPONENT_REF
-      && DECL_BIT_FIELD (TREE_OPERAND (DR_REF (dr), 1)))
-    {
-      tree field = TREE_OPERAND (DR_REF (dr), 1);
-      return build3 (BIT_FIELD_REF, TREE_TYPE (DR_REF (dr)),
-		     build2 (MEM_REF, DECL_BIT_FIELD_TYPE (field),
-			     addr, alias_ptr),
-		     DECL_SIZE (field), bitsize_zero_node);
-    }
-  else
-    return fold_build2 (MEM_REF, TREE_TYPE (DR_REF (dr)), addr, alias_ptr);
+     bitfield accesses at byte boundaries.  Cope with that.  */
+  if (ref_type)
+    ref = build3 (ref_code, ref_type, ref, ref_op1, ref_op2);
+  return ref;
 }
 
 /* Get the initialization expression for the INDEX-th temporary variable
Index: gcc/testsuite/gcc.c-torture/execute/pr71083.c
===================================================================
--- gcc/testsuite/gcc.c-torture/execute/pr71083.c	(revision 0)
+++ gcc/testsuite/gcc.c-torture/execute/pr71083.c	(working copy)
@@ -0,0 +1,43 @@
+struct lock_chain {
+  unsigned int irq_context: 2,
+    depth: 6,
+    base: 24;
+};
+
+__attribute__((noinline, noclone))
+struct lock_chain * foo (struct lock_chain *chain)
+{
+  int i;
+  for (i = 0; i < 100; i++)
+    {
+      chain[i+1].base = chain[i].base;
+    }
+  return chain;
+}
+
+struct lock_chain1 {
+  char x;
+  unsigned short base;
+} __attribute__((packed));
+
+__attribute__((noinline, noclone))
+struct lock_chain1 * bar (struct lock_chain1 *chain)
+{
+  int i;
+  for (i = 0; i < 100; i++)
+    {
+      chain[i+1].base = chain[i].base;
+    }
+  return chain;
+}
+
+struct lock_chain test [101];
+struct lock_chain1 test1 [101];
+
+int
+main ()
+{
+  foo (test);
+  bar (test1);
+  return 0;
+}