[PATCH, PR 49094] Refrain from creating misaligned accesses in SRA

Martin Jambor mjambor@suse.cz
Tue Jun 28 12:16:00 GMT 2011


Hi,

On Mon, Jun 27, 2011 at 03:18:01PM +0200, Richard Guenther wrote:
> On Sun, 26 Jun 2011, Martin Jambor wrote:
> 
> > Hi,
> > 
> > under some circumstances involving user specified alignment and/or
> > packed attributes, SRA can create a misaligned MEM_REF.  As the
> > testcase demonstrates, it is not enough to not consider variables with
> > these type attributes, mainly because we might attempt to load/store
> > the scalar replacements from/to right/left sides of original aggregate
> > assignments which might be misaligned.
> > 

...

> 
> I think you want something like
> 
> static bool
> tree_non_mode_aligned_mem_p (tree exp)
> {
>   enum machine_mode mode = TYPE_MODE (TREE_TYPE (exp));
>   unsigned int align;
> 
>   if (mode == BLKmode
>       || !STRICT_ALIGNMENT)
>     return false;
> 
>   align = get_object_alignment (exp, BIGGEST_ALIGNMENT);
>   if (GET_MODE_ALIGNMENT (mode) > align)
>     return true;
> 
>   return false;
> }
> 
> as for STRICT_ALIGNMENT targets we assume that the loads/stores SRA
> inserts have the alignment of the mode.
> 

I admit to be surprised this works, I did not know aggregates could
have non-BLK modes.  Anyway, it does, and so I intend to commit the
following  this evening, after a testsuite run on sparc64.  Please
stop me if the previous message was not a pre-approval of sorts.

Thanks a lot,

Martin


2011-06-28  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/49094
	* tree-sra.c (tree_non_mode_aligned_mem_p): New function.
	(build_accesses_from_assign): Use it.

	* testsuite/gcc.dg/tree-ssa/pr49094.c: New test.


Index: src/gcc/tree-sra.c
===================================================================
--- src.orig/gcc/tree-sra.c
+++ src/gcc/tree-sra.c
@@ -1050,6 +1050,25 @@ disqualify_ops_if_throwing_stmt (gimple
   return false;
 }
 
+/* Return true iff type of EXP is not sufficiently aligned.  */
+
+static bool
+tree_non_mode_aligned_mem_p (tree exp)
+{
+  enum machine_mode mode = TYPE_MODE (TREE_TYPE (exp));
+  unsigned int align;
+
+  if (mode == BLKmode
+      || !STRICT_ALIGNMENT)
+    return false;
+
+  align = get_object_alignment (exp, BIGGEST_ALIGNMENT);
+  if (GET_MODE_ALIGNMENT (mode) > align)
+    return true;
+
+  return false;
+}
+
 /* Scan expressions occuring in STMT, create access structures for all accesses
    to candidates for scalarization and remove those candidates which occur in
    statements or expressions that prevent them from being split apart.  Return
@@ -1074,7 +1093,10 @@ build_accesses_from_assign (gimple stmt)
   lacc = build_access_from_expr_1 (lhs, stmt, true);
 
   if (lacc)
-    lacc->grp_assignment_write = 1;
+    {
+      lacc->grp_assignment_write = 1;
+      lacc->grp_unscalarizable_region |= tree_non_mode_aligned_mem_p (rhs);
+    }
 
   if (racc)
     {
@@ -1082,6 +1104,7 @@ build_accesses_from_assign (gimple stmt)
       if (should_scalarize_away_bitmap && !gimple_has_volatile_ops (stmt)
 	  && !is_gimple_reg_type (racc->type))
 	bitmap_set_bit (should_scalarize_away_bitmap, DECL_UID (racc->base));
+      racc->grp_unscalarizable_region |= tree_non_mode_aligned_mem_p (lhs);
     }
 
   if (lacc && racc
Index: src/gcc/testsuite/gcc.dg/tree-ssa/pr49094.c
===================================================================
--- /dev/null
+++ src/gcc/testsuite/gcc.dg/tree-ssa/pr49094.c
@@ -0,0 +1,38 @@
+/* { dg-do run } */
+/* { dg-options "-O" } */
+
+struct in_addr {
+	unsigned int s_addr;
+};
+
+struct ip {
+	unsigned char ip_p;
+	unsigned short ip_sum;
+	struct	in_addr ip_src,ip_dst;
+} __attribute__ ((aligned(1), packed));
+
+struct ip ip_fw_fwd_addr;
+
+int test_alignment( char *m )
+{
+  struct ip *ip = (struct ip *) m;
+  struct in_addr pkt_dst;
+  pkt_dst = ip->ip_dst ;
+  if( pkt_dst.s_addr == 0 )
+    return 1;
+  else
+    return 0;
+}
+
+int __attribute__ ((noinline, noclone))
+intermediary (char *p)
+{
+  return test_alignment (p);
+}
+
+int
+main (int argc, char *argv[])
+{
+  ip_fw_fwd_addr.ip_dst.s_addr = 1;
+  return intermediary ((void *) &ip_fw_fwd_addr);
+}



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