fold: Unwrap MEM_REF after get_inner_reference in split_address_to_core_and_offset [PR121355]

Inside split_address_to_core_and_offset, this calls get_inner_reference.

Take:
```
  _6 = t_3(D) + 12;
  _8 = &MEM[(struct s1 *)t_3(D) + 4B].t;
  _1 = _6 - _8;
```

On the assignement of _8, get_inner_reference will return `MEM[(struct s1 *)t_3(D) + 4B]`
and an offset but that does not match up with `t_3(D)` which is how split_address_to_core_and_offset
handles pointer plus.
So this patch adds the unwrapping of the MEM_REF after the call to get_inner_reference
and have it act like a pointer plus.

Changes since v1:
* v2: Remove check on operand 1 for poly_int_tree_p, it is always.
      Add before the check to see if it fits in shwi instead of after.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/121355

gcc/ChangeLog:

	* fold-const.cc (split_address_to_core_and_offset): Handle an MEM_REF after the call
	to get_inner_reference.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ptrdiff-1.c: New test.

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

2 files changed, 56 insertions, 0 deletions

diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
index 8867540..fe7a5fe 100644
--- a/gcc/fold-const.cc
+++ b/gcc/fold-const.cc
@@ -16514,6 +16514,17 @@ split_address_to_core_and_offset (tree exp,
       core = get_inner_reference (TREE_OPERAND (exp, 0), &bitsize, pbitpos,
 				  poffset, &mode, &unsignedp, &reversep,
 				  &volatilep);
+      /* If we are left with MEM[a + CST] strip that and add it to the
+	 pbitpos and return a. */
+      if (TREE_CODE (core) == MEM_REF)
+	{
+	  poly_offset_int tem;
+	  tem = wi::to_poly_offset (TREE_OPERAND (core, 1));
+	  tem <<= LOG2_BITS_PER_UNIT;
+	  tem += *pbitpos;
+	  if (tem.to_shwi (pbitpos))
+	    return TREE_OPERAND (core, 0);
+	}
       core = build_fold_addr_expr_loc (loc, core);
     }
   else if (TREE_CODE (exp) == POINTER_PLUS_EXPR)
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ptrdiff-1.c b/gcc/testsuite/gcc.dg/tree-ssa/ptrdiff-1.c
new file mode 100644
index 0000000..af9291c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ptrdiff-1.c
@@ -0,0 +1,45 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+/* PR tree-optimization/121355 */
+
+#define array_size 2
+struct s1
+{
+  int t[array_size];
+};
+
+struct s2
+{
+  int p;
+  struct s1 t;
+};
+static inline int *b(struct s1 *t)
+{
+  return t->t;
+}
+static inline int *e(struct s1 *t)
+{
+  return b(t) + array_size;
+}
+void g(struct s2 *t)
+{
+  struct s1 *t2 = &t->t;
+  int *te = e(t2);
+  int *ts = b(t2);
+  int tt = te - ts;
+/*
+  _6 = t_3(D) + 12;
+  _8 = &MEM[(struct s1 *)t_3(D) + 4B].t;
+  _1 = _6 - _8;
+
+  _1 should be optimized to 2*sizeof(int) == 8.
+ */
+
+  if (tt != array_size)
+    __builtin_abort();
+}
+
+/* the call to abort should be removed. */
+
+/* { dg-final { scan-tree-dump-not "abort " "optimized" } } */