c++: Fix up constexpr evaluation of new with zero sized types [PR104568]

The new expression constant expression evaluation right now tries to
deduce how many elts the array it uses for the heap or heap [] vars
should have (or how many elts should its trailing array have if it has
cookie at the start).  As new is lowered at that point to
(some_type *) ::operator new (size)
or so, it computes it by subtracting cookie size if any from size, then
divides the result by sizeof (some_type).
This works fine for most types, except when sizeof (some_type) is 0,
then we divide by zero; size is then equal to cookie_size (or if there
is no cookie, to 0).
The following patch special cases those cases so that we don't divide
by zero and also recover the original outer_nelts from the expression
by forcing the size not to be folded in that case but be explicit
0 * outer_nelts or cookie_size + 0 * outer_nelts.

Note, we have further issues, we accept-invalid various cases, for both
zero sized elt_type and even non-zero sized elts, we aren't able to
diagnose out of bounds POINTER_PLUS_EXPR like:
constexpr bool
foo ()
{
  auto p = new int[2];
  auto q1 = &p[0];
  auto q2 = &p[1];
  auto q3 = &p[2];
  auto q4 = &p[3];
  delete[] p;
  return true;
}
constexpr bool a = foo ();
That doesn't look like a regression so I think we should resolve that for
GCC 13, but there are 2 problems.  Figure out why
cxx_fold_pointer_plus_expression doesn't deal with the &heap []
etc. cases, and for the zero sized arrays, I think we really need to preserve
whether user wrote an array ref or pointer addition, because in the
&p[3] case if sizeof(p[0]) == 0 we know that if it has 2 elements it is
out of bounds, while if we see p p+ 0 the information if it was
p + 2 or p + 3 in the source is lost.
clang++ seems to handle it fine even in the zero sized cases or with
new expressions.

2022-03-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104568
	* init.cc (build_new_constexpr_heap_type): Remove FULL_SIZE
	argument and its handling, instead add ITYPE2 argument.  Only
	support COOKIE_SIZE != NULL.
	(build_new_1): If size is 0, change it to 0 * outer_nelts if
	outer_nelts is non-NULL.  Pass type rather than elt_type to
	maybe_wrap_new_for_constexpr.
	* constexpr.cc (build_new_constexpr_heap_type): New function.
	(cxx_eval_constant_expression) <case CONVERT_EXPR>:
	If elt_size is zero sized type, try to recover outer_nelts from
	the size argument to operator new/new[] and pass that as
	arg_size to build_new_constexpr_heap_type.  Pass ctx,
	non_constant_p and overflow_p to that call too.

	* g++.dg/cpp2a/constexpr-new22.C: New test.

1 files changed, 93 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index bbb6cd9..a3136ce 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -6422,6 +6422,84 @@ maybe_warn_about_constant_value (location_t loc, tree decl)
     }
 }
 
+/* For element type ELT_TYPE, return the appropriate type of the heap object
+   containing such element(s).  COOKIE_SIZE is NULL or the size of cookie
+   in bytes.  If COOKIE_SIZE is NULL, return array type
+   ELT_TYPE[FULL_SIZE / sizeof(ELT_TYPE)], otherwise return
+   struct { size_t[COOKIE_SIZE/sizeof(size_t)]; ELT_TYPE[N]; }
+   where N is is computed such that the size of the struct fits into FULL_SIZE.
+   If ARG_SIZE is non-NULL, it is the first argument to the new operator.
+   It should be passed if ELT_TYPE is zero sized type in which case FULL_SIZE
+   will be also 0 and so it is not possible to determine the actual array
+   size.  CTX, NON_CONSTANT_P and OVERFLOW_P are used during constant
+   expression evaluation of subexpressions of ARG_SIZE.  */
+
+static tree
+build_new_constexpr_heap_type (const constexpr_ctx *ctx, tree elt_type,
+			       tree cookie_size, tree full_size, tree arg_size,
+			       bool *non_constant_p, bool *overflow_p)
+{
+  gcc_assert (cookie_size == NULL_TREE || tree_fits_uhwi_p (cookie_size));
+  gcc_assert (tree_fits_uhwi_p (full_size));
+  unsigned HOST_WIDE_INT csz = cookie_size ? tree_to_uhwi (cookie_size) : 0;
+  if (arg_size)
+    {
+      STRIP_NOPS (arg_size);
+      if (cookie_size)
+	{
+	  if (TREE_CODE (arg_size) != PLUS_EXPR)
+	    arg_size = NULL_TREE;
+	  else if (TREE_CODE (TREE_OPERAND (arg_size, 0)) == INTEGER_CST
+		   && tree_int_cst_equal (cookie_size,
+					  TREE_OPERAND (arg_size, 0)))
+	    {
+	      arg_size = TREE_OPERAND (arg_size, 1);
+	      STRIP_NOPS (arg_size);
+	    }
+	  else if (TREE_CODE (TREE_OPERAND (arg_size, 1)) == INTEGER_CST
+		   && tree_int_cst_equal (cookie_size,
+					  TREE_OPERAND (arg_size, 1)))
+	    {
+	      arg_size = TREE_OPERAND (arg_size, 0);
+	      STRIP_NOPS (arg_size);
+	    }
+	  else
+	    arg_size = NULL_TREE;
+	}
+      if (arg_size && TREE_CODE (arg_size) == MULT_EXPR)
+	{
+	  tree op0 = TREE_OPERAND (arg_size, 0);
+	  tree op1 = TREE_OPERAND (arg_size, 1);
+	  if (integer_zerop (op0))
+	    arg_size
+	      = cxx_eval_constant_expression (ctx, op1, false, non_constant_p,
+					      overflow_p);
+	  else if (integer_zerop (op1))
+	    arg_size
+	      = cxx_eval_constant_expression (ctx, op0, false, non_constant_p,
+					      overflow_p);
+	  else
+	    arg_size = NULL_TREE;
+	}
+      else
+	arg_size = NULL_TREE;
+    }
+
+  unsigned HOST_WIDE_INT fsz = tree_to_uhwi (arg_size ? arg_size : full_size);
+  if (!arg_size)
+    {
+      unsigned HOST_WIDE_INT esz = int_size_in_bytes (elt_type);
+      gcc_assert (fsz >= csz);
+      fsz -= csz;
+      if (esz)
+	fsz /= esz;
+    }
+  tree itype2 = build_index_type (size_int (fsz - 1));
+  if (!cookie_size)
+    return build_cplus_array_type (elt_type, itype2);
+  return build_new_constexpr_heap_type (elt_type, cookie_size, itype2);
+}
+
 /* Attempt to reduce the expression T to a constant value.
    On failure, issue diagnostic and return error_mark_node.  */
 /* FIXME unify with c_fully_fold */
@@ -7253,6 +7331,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    tree var_size = TYPE_SIZE_UNIT (TREE_TYPE (var));
 	    tree elt_type = TREE_TYPE (type);
 	    tree cookie_size = NULL_TREE;
+	    tree arg_size = NULL_TREE;
 	    if (TREE_CODE (elt_type) == RECORD_TYPE
 		&& TYPE_NAME (elt_type) == heap_identifier)
 	      {
@@ -7264,9 +7343,21 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	    DECL_NAME (var)
 	      = (DECL_NAME (var) == heap_uninit_identifier
 		 ? heap_identifier : heap_vec_identifier);
+	    /* For zero sized elt_type, try to recover how many outer_nelts
+	       it should have.  */
+	    if ((cookie_size ? tree_int_cst_equal (var_size, cookie_size)
+			     : integer_zerop (var_size))
+		&& !int_size_in_bytes (elt_type)
+		&& TREE_CODE (oldop) == CALL_EXPR
+		&& call_expr_nargs (oldop) >= 1)
+	      if (tree fun = get_function_named_in_call (oldop))
+		if (cxx_replaceable_global_alloc_fn (fun)
+		    && IDENTIFIER_NEW_OP_P (DECL_NAME (fun)))
+		  arg_size = CALL_EXPR_ARG (oldop, 0);
 	    TREE_TYPE (var)
-	      = build_new_constexpr_heap_type (elt_type, cookie_size,
-					       var_size);
+	      = build_new_constexpr_heap_type (ctx, elt_type, cookie_size,
+					       var_size, arg_size,
+					       non_constant_p, overflow_p);
 	    TREE_TYPE (TREE_OPERAND (op, 0))
 	      = build_pointer_type (TREE_TYPE (var));
 	  }