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

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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.
This commit is contained in:
Jakub Jelinek 2022-03-18 18:49:23 +01:00
parent a9131a329a
commit 0a0c2c3f06
3 changed files with 149 additions and 26 deletions
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95
gcc/cp/constexpr.cc
View File

@ -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));
}

38
gcc/cp/init.cc
View File

@ -2931,33 +2931,17 @@ std_placement_new_fn_p (tree alloc_fn)
}
/* 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. FULL_SIZE is NULL if it is unknown how big the heap allocation
will be, otherwise size of the heap object. If COOKIE_SIZE is NULL,
return array type ELT_TYPE[FULL_SIZE / sizeof(ELT_TYPE)], otherwise return
containing such element(s). COOKIE_SIZE is the size of cookie in bytes.
Return
struct { size_t[COOKIE_SIZE/sizeof(size_t)]; ELT_TYPE[N]; }
where N is nothing (flexible array member) if FULL_SIZE is NULL, otherwise
it is computed such that the size of the struct fits into FULL_SIZE. */
where N is nothing (flexible array member) if ITYPE2 is NULL, otherwise
the array has ITYPE2 as its TYPE_DOMAIN. */
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 itype2)
{
gcc_assert (cookie_size == NULL_TREE || tree_fits_uhwi_p (cookie_size));
gcc_assert (full_size == NULL_TREE || tree_fits_uhwi_p (full_size));
unsigned HOST_WIDE_INT csz = cookie_size ? tree_to_uhwi (cookie_size) : 0;
tree itype2 = NULL_TREE;
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);
itype2 = build_index_type (size_int (fsz - 1));
if (!cookie_size)
return build_cplus_array_type (elt_type, itype2);
}
else
gcc_assert (cookie_size);
gcc_assert (tree_fits_uhwi_p (cookie_size));
unsigned HOST_WIDE_INT csz = tree_to_uhwi (cookie_size);
csz /= int_size_in_bytes (sizetype);
tree itype1 = build_index_type (size_int (csz - 1));
tree atype1 = build_cplus_array_type (sizetype, itype1);
@ -3408,6 +3392,12 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
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,
@ -3484,7 +3474,7 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
}
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. */

42
gcc/testsuite/g++.dg/cpp2a/constexpr-new22.C Normal file
View File

@ -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 ();