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

[Jason Merrill <jason@redhat.com>]

On 2/21/22 04:25, Jakub Jelinek wrote:
> Hi!
> 
> 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.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 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.

But array ref is defined to be equivalent to pointer addition, and we 
also want to handle p+2 properly.  It seems to me that the problem is 
lowering to POINTER_PLUS_EXPR too soon, but that's definitely a stage 1 
project.

> clang++ seems to handle it fine even in the zero sized cases or with
> new expressions.
> 
> 2022-02-21  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/104568
> 	* cp-tree.h (build_new_constexpr_heap_type): Add FULL_SIZE_ADJUSTED
> 	argument.
> 	* init.cc (build_new_constexpr_heap_type): Add FULL_SIZE_ADJUSTED
> 	argument.  If true, don't subtract csz from it nor divide by
> 	int_size_in_bytes (elt_type).  Don't do that division if
> 	int_size_in_bytes is zero either.
> 	(maybe_wrap_new_for_constexpr): Pass false to
> 	build_new_constexpr_heap_type.
> 	(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 (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
> 	var_size to build_new_constexpr_heap_type together with true
> 	for the last argument.
> 
> 	* g++.dg/cpp2a/constexpr-new22.C: New test.
> 
> --- gcc/cp/cp-tree.h.jj	2022-02-09 20:13:51.541304861 +0100
> +++ gcc/cp/cp-tree.h	2022-02-17 15:34:30.804453673 +0100
> @@ -7038,7 +7038,7 @@ extern tree build_offset_ref			(tree, tr
>   extern tree throw_bad_array_new_length		(void);
>   extern bool type_has_new_extended_alignment	(tree);
>   extern unsigned malloc_alignment		(void);
> -extern tree build_new_constexpr_heap_type	(tree, tree, tree);
> +extern tree build_new_constexpr_heap_type	(tree, tree, tree, bool);
>   extern tree build_new				(location_t,
>   						 vec<tree, va_gc> **, tree,
>   						 tree, vec<tree, va_gc> **,
> --- gcc/cp/init.cc.jj	2022-02-05 10:50:05.000000000 +0100
> +++ gcc/cp/init.cc	2022-02-17 15:56:30.010056499 +0100
> @@ -2930,7 +2930,8 @@ std_placement_new_fn_p (tree alloc_fn)
>      it is computed such that the size of the struct fits into FULL_SIZE.  */
>   
>   tree
> -build_new_constexpr_heap_type (tree elt_type, tree cookie_size, tree full_size)
> +build_new_constexpr_heap_type (tree elt_type, tree cookie_size, tree full_size,
> +			       bool full_size_adjusted)
>   {
>     gcc_assert (cookie_size == NULL_TREE || tree_fits_uhwi_p (cookie_size));
>     gcc_assert (full_size == NULL_TREE || tree_fits_uhwi_p (full_size));
> @@ -2939,9 +2940,14 @@ build_new_constexpr_heap_type (tree elt_
>     if (full_size)
>       {
>         unsigned HOST_WIDE_INT fsz = tree_to_uhwi (full_size);
> -      gcc_assert (fsz >= csz);
> -      fsz -= csz;
> -      fsz /= int_size_in_bytes (elt_type);
> +      unsigned HOST_WIDE_INT esz = int_size_in_bytes (elt_type);
> +      if (!full_size_adjusted)
> +	{
> +	  gcc_assert (fsz >= csz);
> +	  fsz -= csz;
> +	  if (esz)
> +	    fsz /= esz;
> +	}
>         itype2 = build_index_type (size_int (fsz - 1));
>         if (!cookie_size)
>   	return build_cplus_array_type (elt_type, itype2);
> @@ -2992,7 +2998,7 @@ maybe_wrap_new_for_constexpr (tree alloc
>       return alloc_call;
>   
>     tree rtype = build_new_constexpr_heap_type (elt_type, cookie_size,
> -					      NULL_TREE);
> +					      NULL_TREE, false);
>     return build_nop (build_pointer_type (rtype), alloc_call);
>   }
>   
> @@ -3398,6 +3404,12 @@ build_new_1 (vec<tree, va_gc> **placemen
>   	    outer_nelts_check = NULL_TREE;
>   	}
>   
> +      /* If size is zero e.g. due to type having zero size, try to
> +	 preserve outer_nelts for constant expression evaluation
> +	 purposes.  */
> +      if (integer_zerop (size) && outer_nelts)
> +	size = build2 (MULT_EXPR, TREE_TYPE (size), size, outer_nelts);
> +
>         alloc_call = build_operator_new_call (fnname, placement,
>   					    &size, &cookie_size,
>   					    align_arg, outer_nelts_check,
> @@ -3474,7 +3486,7 @@ build_new_1 (vec<tree, va_gc> **placemen
>       }
>   
>     if (cookie_size)
> -    alloc_call = maybe_wrap_new_for_constexpr (alloc_call, elt_type,
> +    alloc_call = maybe_wrap_new_for_constexpr (alloc_call, type,
>   					       cookie_size);
>   
>     /* In the simple case, we can stop now.  */
> --- gcc/cp/constexpr.cc.jj	2022-02-17 10:24:16.779112954 +0100
> +++ gcc/cp/constexpr.cc	2022-02-17 15:43:23.399026306 +0100
> @@ -7253,6 +7253,7 @@ cxx_eval_constant_expression (const cons
>   	    tree var_size = TYPE_SIZE_UNIT (TREE_TYPE (var));
>   	    tree elt_type = TREE_TYPE (type);
>   	    tree cookie_size = NULL_TREE;
> +	    bool var_size_adjusted = false;
>   	    if (TREE_CODE (elt_type) == RECORD_TYPE
>   		&& TYPE_NAME (elt_type) == heap_identifier)
>   	      {
> @@ -7264,9 +7265,66 @@ cxx_eval_constant_expression (const cons
>   	    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)))
> +		  {
> +		    tree arg0 = CALL_EXPR_ARG (oldop, 0);

How about setting var_size to arg0 at this point, and moving the 
decomposition of the size expression into build_new_constexpr_heap_type?

> +		    STRIP_NOPS (arg0);
> +		    if (cookie_size)
> +		      {
> +			if (TREE_CODE (arg0) != PLUS_EXPR)
> +			  arg0 = NULL_TREE;
> +			else if (TREE_CODE (TREE_OPERAND (arg0, 0))
> +				 == INTEGER_CST
> +				 && tree_int_cst_equal (cookie_size,
> +							TREE_OPERAND (arg0,
> +								      0)))
> +			  {
> +			    arg0 = TREE_OPERAND (arg0, 1);
> +			    STRIP_NOPS (arg0);
> +			  }
> +			else if (TREE_CODE (TREE_OPERAND (arg0, 1))
> +				 == INTEGER_CST
> +				 && tree_int_cst_equal (cookie_size,
> +							TREE_OPERAND (arg0,
> +								      1)))
> +			  {
> +			    arg0 = TREE_OPERAND (arg0, 0);
> +			    STRIP_NOPS (arg0);
> +			  }
> +			else
> +			  arg0 = NULL_TREE;
> +		      }
> +		    if (arg0 && TREE_CODE (arg0) == MULT_EXPR)
> +		      {
> +			tree op0 = TREE_OPERAND (arg0, 0);
> +			tree op1 = TREE_OPERAND (arg0, 1);
> +			var_size_adjusted = true;
> +			if (integer_zerop (op0))
> +			  var_size
> +			    = cxx_eval_constant_expression (ctx, op1, false,
> +							    non_constant_p,
> +							    overflow_p);
> +			else if (integer_zerop (op1))
> +			  var_size
> +			    = cxx_eval_constant_expression (ctx, op0, false,
> +							    non_constant_p,
> +							    overflow_p);
> +			else
> +			  var_size_adjusted = false;
> +		      }
> +		  }
>   	    TREE_TYPE (var)
>   	      = build_new_constexpr_heap_type (elt_type, cookie_size,
> -					       var_size);
> +					       var_size, var_size_adjusted);
>   	    TREE_TYPE (TREE_OPERAND (op, 0))
>   	      = build_pointer_type (TREE_TYPE (var));
>   	  }
> --- gcc/testsuite/g++.dg/cpp2a/constexpr-new22.C.jj	2022-02-17 16:01:24.804945381 +0100
> +++ gcc/testsuite/g++.dg/cpp2a/constexpr-new22.C	2022-02-17 16:00:55.720350985 +0100
> @@ -0,0 +1,42 @@
> +// PR c++/104568
> +// { dg-do compile { target c++20 } }
> +// { dg-options "" }
> +
> +struct S { int s; constexpr S () : s (0) {} constexpr ~S () {} };
> +typedef int T[0];
> +typedef int U[0];
> +
> +constexpr bool
> +foo ()
> +{
> +  auto p = new T[2];
> +  auto q1 = &p[0];
> +  auto q2 = &p[1];
> +  auto q3 = &p[2];
> +  delete[] p;
> +  return true;
> +}
> +
> +constexpr bool
> +bar ()
> +{
> +  auto p = new U[2];
> +  auto q1 = &p[0];
> +  auto q2 = &p[1];
> +  auto q3 = &p[2];
> +  delete[] p;
> +  return true;
> +}
> +
> +constexpr bool
> +baz ()
> +{
> +  auto p = new T[0];
> +  auto q1 = &p[0];
> +  delete[] p;
> +  return true;
> +}
> +
> +constexpr bool a = foo ();
> +constexpr bool b = bar ();
> +constexpr bool c = baz ();
> 
> 	Jakub
>