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[RFC PATCH 1/3] Misaligned top level MEM_REFs on LHS of assignments


Hi,

the first patch in the series deals with plain MEM_REFs on LHS of
assignments to handle situations such as the testcase which currently
fails on strict alignment platforms (the array and the loop are there
to cross a cache line on ia64 so that it fails there too).

This patch piggy-backs on already existing code for generating
movmisalign operation if available, and deals with the store by
calling store_bit_field if it is not but it would be a
SLOW_UNALIGNED_ACCESS.

I have added !mem_ref_refers_to_non_mem_p (to) to the condition
guarding both of these paths because without it I encountered
testsuite failures within expand_expr when the MEM_REF address
argument was expanded into something else than memory.  This call is
common for both movmisalign_optab and store_bit_field paths so I
believe it is necessary on both but so far I did not come up with a
testcase in which it would fail with movmisalign.

Thanks,

Martin



2012-02-28  Martin Jambor  <mjambor@suse.cz>

	* expr.c (expand_assignment): Handle misaligned scalar writes to
	memory through top-level MEM_REFs by calling store_bit_field.

	* testsuite/gcc.dg/misaligned-expand-2.c: New test.


Index: src/gcc/expr.c
===================================================================
--- src.orig/gcc/expr.c
+++ src/gcc/expr.c
@@ -4593,10 +4593,12 @@ expand_assignment (tree to, tree from, b
   if ((TREE_CODE (to) == MEM_REF
        || TREE_CODE (to) == TARGET_MEM_REF)
       && mode != BLKmode
+      && !mem_ref_refers_to_non_mem_p (to)
       && ((align = get_object_or_type_alignment (to))
 	  < GET_MODE_ALIGNMENT (mode))
-      && ((icode = optab_handler (movmisalign_optab, mode))
-	  != CODE_FOR_nothing))
+      && (((icode = optab_handler (movmisalign_optab, mode))
+	   != CODE_FOR_nothing)
+	  || SLOW_UNALIGNED_ACCESS (mode, align)))
     {
       addr_space_t as
 	= TYPE_ADDR_SPACE (TREE_TYPE (TREE_TYPE (TREE_OPERAND (to, 0))));
@@ -4639,11 +4641,17 @@ expand_assignment (tree to, tree from, b
       if (TREE_THIS_VOLATILE (to))
 	MEM_VOLATILE_P (mem) = 1;
 
-      create_fixed_operand (&ops[0], mem);
-      create_input_operand (&ops[1], reg, mode);
-      /* The movmisalign<mode> pattern cannot fail, else the assignment would
-	 silently be omitted.  */
-      expand_insn (icode, 2, ops);
+      if (icode != CODE_FOR_nothing)
+	{
+	  create_fixed_operand (&ops[0], mem);
+	  create_input_operand (&ops[1], reg, mode);
+	  /* The movmisalign<mode> pattern cannot fail, else the assignment would
+	     silently be omitted.  */
+	  expand_insn (icode, 2, ops);
+	}
+      else
+	store_bit_field (mem, GET_MODE_BITSIZE (mode),
+			 0, 0, 0, mode, reg);
       return;
     }
 
Index: src/gcc/testsuite/gcc.dg/misaligned-expand-2.c
===================================================================
--- /dev/null
+++ src/gcc/testsuite/gcc.dg/misaligned-expand-2.c
@@ -0,0 +1,42 @@
+/* Test that expand can generate correct stores to misaligned data even on
+   strict alignment platforms.  */
+
+/* { dg-do run } */
+/* { dg-options "-O0" } */
+
+extern void abort ();
+
+typedef unsigned int myint __attribute__((aligned(1)));
+
+void
+foo (myint *p, unsigned int i)
+{
+  *p = i;
+}
+
+#define cst 0xdeadbeef
+#define NUM 8
+
+struct blah
+{
+  char c;
+  myint i[NUM];
+};
+
+struct blah g;
+
+#define cst 0xdeadbeef
+
+int
+main (int argc, char **argv)
+{
+  int k;
+
+  for (k = 0; k < NUM; k++)
+    {
+      foo (&g.i[k], cst);
+      if (g.i[k] != cst)
+	abort ();
+    }
+  return 0;
+}


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