c: Handle MEM_REF in c_fully_fold* [PR94179]

The recent match.pd changes can generate a MEM_REF which can be seen by the
C FE folding routines.  Unlike the C++ FE, they weren't expected in the C FE
yet.  MEM_REF should be handled like INDIRECT_REF, except that it has two
operands rather than just one and that we should preserve the type of the
second operand.  Given that it already has to be an INTEGER_CST with pointer
type, I think we are fine, the recursive call should return the INTEGER_CST
unmodified and STRIP_TYPE_NOPS will not strip anything.

2020-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/94179
	* c-fold.c (c_fully_fold_internal): Handle MEM_REF.

	* gcc.c-torture/compile/pr94179.c: New test.

gcc/c/ChangeLog

+2020-03-16  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/94179
+	* c-fold.c (c_fully_fold_internal): Handle MEM_REF.
+
 2020-03-13  Martin Sebor  <msebor@redhat.com>
 
 	PR c/94040

gcc/c/c-fold.c

@@ -346,6 +346,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
     case UNGT_EXPR:
     case UNGE_EXPR:
     case UNEQ_EXPR:
+    case MEM_REF:
       /* Binary operations evaluating both arguments (increment and
 	 decrement are binary internally in GCC).  */
       orig_op0 = op0 = TREE_OPERAND (expr, 0);
@@ -435,6 +436,14 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
 	      || TREE_CODE (TREE_TYPE (orig_op0)) == FIXED_POINT_TYPE)
 	  && TREE_CODE (TREE_TYPE (orig_op1)) == INTEGER_TYPE)
 	warn_for_div_by_zero (loc, op1);
+      if (code == MEM_REF
+	  && ret != expr
+	  && TREE_CODE (ret) == MEM_REF)
+	{
+	  TREE_READONLY (ret) = TREE_READONLY (expr);
+	  TREE_SIDE_EFFECTS (ret) = TREE_SIDE_EFFECTS (expr);
+	  TREE_THIS_VOLATILE (ret) = TREE_THIS_VOLATILE (expr);
+	}
       goto out;
 
     case ADDR_EXPR:

gcc/testsuite/ChangeLog

+2020-03-16  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/94179
+	* gcc.c-torture/compile/pr94179.c: New test.
+
 2020-03-16  Vladimir Makarov  <vmakarov@redhat.com>
 
 	PR target/94185

gcc/testsuite/gcc.c-torture/compile/pr94179.c

+/* PR c/94179 */
+
+struct S { char c, d, e[64]; } a;
+
+unsigned char *
+foo (int b)
+{
+  return (unsigned char *)((char *)&a.e[b != 0] - (char *)&((struct S *)0)->d);
+}