forwprop: Improve the reject case for copy prop [PR107051]

Currently the code rejects:
```
tmp = *a;
*b = tmp;
```
(unless *a == *b). This can be improved such that if a and b are known to
share the same base, then only reject it if they overlap; that is the
difference of the offsets (from the base) is maybe less than the size.

This fixes the testcase in comment #0 of PR 107051.

Changes since v1:
* v2: Use ranges_maybe_overlap_p instead of manually checking the overlap.
      Allow for the case where the alignment is known to be greater than
      the size.

	PR tree-optimization/107051

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (optimize_agr_copyprop_1): Allow for
	memory sharing the same base if they known not to overlap over
	the size.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c: New test.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>

2 files changed, 60 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c b/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
new file mode 100644
index 0000000..206f6e1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-forwprop1-details" } */
+/* PR tree-optimization/107051 */
+
+
+union U2 {
+   unsigned  f0;
+   char * f1;
+};
+
+/* Since g_284[0] and g_284[1] are known not overlap,
+   copy prop can happen.  */
+union U2 g_284[2] = {{0UL},{0xC2488F72L}};
+
+int e;
+void func_1() {
+	union U2 c = {7};
+	int *d[2];
+	for (; e;)
+		*d[1] = 0;
+	g_284[0] = c = g_284[1];
+}
+
+/* { dg-final { scan-tree-dump-times "after previous" 1 "forwprop1" } } */
diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
index df876f7..59b1878 100644
--- a/gcc/tree-ssa-forwprop.cc
+++ b/gcc/tree-ssa-forwprop.cc
@@ -1455,7 +1455,42 @@ optimize_agr_copyprop_1 (gimple *stmt, gimple *use_stmt,
      */
   if (!operand_equal_p (dest2, src, 0)
       && !DECL_P (dest2) && !DECL_P (src))
-    return false;
+    {
+      /* If *a and *b have the same base see if
+	 the offset between the two is greater than
+	 or equal to the size of the type. */
+      poly_int64 offset1, offset2;
+      tree len = TYPE_SIZE_UNIT (TREE_TYPE (src));
+      if (len == NULL_TREE
+	  || !tree_fits_poly_int64_p (len))
+	return false;
+      tree base1 = get_addr_base_and_unit_offset (dest2, &offset1);
+      tree base2 = get_addr_base_and_unit_offset (src, &offset2);
+      poly_int64 size = tree_to_poly_int64 (len);
+      /* If we can't figure out the base or the bases are
+	 not equal then fall back to an alignment check.  */
+      if (!base1
+	  || !base2
+	  || !operand_equal_p (base1, base2))
+	{
+	  unsigned int align1 = get_object_alignment (src);
+	  unsigned int align2 = get_object_alignment (dest2);
+	  align1 /= BITS_PER_UNIT;
+	  align2 /= BITS_PER_UNIT;
+	  /* If the alignment of either object is less
+	     than the size then there is a possibility
+	     of overlapping.  */
+	  if (maybe_lt (align1, size)
+	      || maybe_lt (align2, size))
+	    return false;
+	}
+      /* Make sure [offset1, offset1 + len - 1] does
+	 not overlap with [offset2, offset2 + len - 1],
+	 it is ok if they are at the same location though.  */
+      else if (ranges_maybe_overlap_p (offset1, size, offset2, size)
+	  && !known_eq (offset2, offset1))
+	return false;
+    }
 
   if (dump_file && (dump_flags & TDF_DETAILS))
     {