[PATCH] c++: Fix up constexpr evaluation of new with zero sized types [PR104568]
Jakub Jelinek
jakub@redhat.com
Mon Feb 21 08:25:03 GMT 2022
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.
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);
+ 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
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