On 4/9/20 4:23 PM, Martin Sebor wrote:
On 4/9/20 1:32 PM, Jason Merrill wrote:
On 4/9/20 3:24 PM, Martin Sebor wrote:
On 4/9/20 1:03 PM, Jason Merrill wrote:
On 4/8/20 1:23 PM, Martin Sebor wrote:
On 4/7/20 3:36 PM, Marek Polacek wrote:
On Tue, Apr 07, 2020 at 02:46:52PM -0600, Martin Sebor wrote:
On 4/7/20 1:50 PM, Marek Polacek wrote:
On Tue, Apr 07, 2020 at 12:50:48PM -0600, Martin Sebor via
Gcc-patches wrote:
Among the numerous regressions introduced by the change committed
to GCC 9 to allow string literals as template arguments is a
failure
to recognize the C++ nullptr and GCC's __null constants as
pointers.
For one, I didn't realize that nullptr, being a null pointer
constant,
doesn't have a pointer type, and two, I didn't think of __null
(which
is a special integer constant that NULL sometimes expands to).
The attached patch adjusts the special handling of trailing zero
initializers in reshape_init_array_1 to recognize both kinds of
constants and avoid treating them as zeros of the array integer
element type. This restores the expected diagnostics when either
constant is used in the initializer list.
Martin
PR c++/94510 - nullptr_t implicitly cast to zero twice in
std::array
gcc/cp/ChangeLog:
PR c++/94510
* decl.c (reshape_init_array_1): Exclude mismatches with
all kinds
of pointers.
gcc/testsuite/ChangeLog:
PR c++/94510
* g++.dg/init/array57.C: New test.
* g++.dg/init/array58.C: New test.
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index a127734af69..692c8ed73f4 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -6041,9 +6041,14 @@ reshape_init_array_1 (tree elt_type,
tree max_index, reshape_iter *d,
TREE_CONSTANT (new_init) = false;
/* Pointers initialized to strings must be treated as
non-zero
- even if the string is empty. */
+ even if the string is empty. Handle all kinds of pointers,
+ including std::nullptr and GCC's __nullptr, neither of which
+ has a pointer type. */
tree init_type = TREE_TYPE (elt_init);
- if (POINTER_TYPE_P (elt_type) != POINTER_TYPE_P (init_type)
+ bool init_is_ptr = (POINTER_TYPE_P (init_type)
+ || NULLPTR_TYPE_P (init_type)
+ || null_node_p (elt_init));
+ if (POINTER_TYPE_P (elt_type) != init_is_ptr
|| !type_initializer_zero_p (elt_type, elt_init))
last_nonzero = index;
It looks like this still won't handle e.g. pointers to member
functions,
e.g.
struct S { };
int arr[3] = { (void (S::*) ()) 0, 0, 0 };
would still be accepted. You could use TYPE_PTR_OR_PTRMEM_P
instead of
POINTER_TYPE_P to catch this case.
Good catch! That doesn't fail because unlike null data member
pointers
which are represented as -1, member function pointers are
represented
as a zero.
I had looked for an API that would answer the question: "is this
expression a pointer?" without having to think of all the different
kinds of them but all I could find was null_node_p(). Is this a
rare,
isolated case that having an API like that wouldn't be worth having
or should I add one like in the attached update?
Martin
PR c++/94510 - nullptr_t implicitly cast to zero twice in std::array
gcc/cp/ChangeLog:
PR c++/94510
* decl.c (reshape_init_array_1): Exclude mismatches with all
kinds
of pointers.
* gcc/cp/cp-tree.h (null_pointer_constant_p): New function.
(Drop the gcc/cp/.)
+/* Returns true if EXPR is a null pointer constant of any type. */
+
+inline bool
+null_pointer_constant_p (tree expr)
+{
+ STRIP_ANY_LOCATION_WRAPPER (expr);
+ if (expr == null_node)
+ return true;
+ tree type = TREE_TYPE (expr);
+ if (NULLPTR_TYPE_P (type))
+ return true;
+ if (POINTER_TYPE_P (type))
+ return integer_zerop (expr);
+ return null_member_pointer_value_p (expr);
+}
+
We already have a null_ptr_cst_p so it would be sort of confusing
to have
this as well. But are you really interested in whether it's a
null pointer,
not just a pointer?
The goal of the code is to detect a mismatch in "pointerness" between
an initializer expression and the type of the initialized element, so
it needs to know if the expression is a pointer (non-nulls pointers
are detected in type_initializer_zero_p). That means testing a number
of IMO unintuitive conditions:
TYPE_PTR_OR_PTRMEM_P (TREE_TYPE (expr))
|| NULLPTR_TYPE_P (TREE_TYPE (expr))
|| null_node_p (expr)
I don't know if this type of a query is common in the C++ FE but
unless
this is an isolated use case then besides fixing the bug I thought it
would be nice to make it easier to get the test above right, or at
least
come close to it.
Since null_pointer_constant_p already exists (but isn't suitable here
because it returns true for plain literal zeros)
Why is that unsuitable? A literal zero is a perfectly good
zero-initializer for a pointer.
Right, that's why it's not suitable here. Because a literal zero
is also not a pointer.
The question the code asks is: "is the initializer expression
a pointer (of any kind)?"
Why is that a question we want to ask? What we need here is to know
whether the initializer expression is equivalent to implicit
zero-initialization. For initializing a pointer, a literal 0 is
equivalent, so we don't want to update last_nonzero.
Yes, but that's not the bug we're fixing. The problem occurs with
an integer array and a pointer initializer:
int a[2] = { nullptr, 0 };
Aha, you're fixing a different bug than the one I was seeing.
and with elt_type = TREE_TYPE (a) and init_type TREE_TYPE (nullptr)
the test
POINTER_TYPE_P (elt_type) != POINTER_TYPE_P (init_type)
evaluates to false because neither type is a pointer type and
type_initializer_zero_p (elt_type, elt_init)
returns true because nullptr is zero, and so last_nonzero doesn't
get set, the element gets trimmed, and the invalid initialization
of int with nullptr isn't diagnosed.
But I'm not sure if you're questioning the current code, the simple
fix quoted above, or my assertion that null_pointer_constant_p would
not be a suitable function to call to tell if an initializer is
nullptr vs plain zero.
Also, why is the pointer check here rather than part of the
POINTER_TYPE_P handling in type_initializer_zero_p?
type_initializer_zero_p is implemented in terms of initializer_zerop
with the only difference that empty strings are considered to be zero
only for char arrays and not char pointers.
Yeah, but that's the fundamental problem: We're assuming that any zero
is suitable for initializing any type except for a few exceptions, and
adding more exceptions when we find a new testcase that breaks.
Handling this in process_init_constructor_array avoids all these
problems by looking at the initializers after they've been converted to
the desired type, at which point it's much clearer whether they are zero
or not; then we don't need type_initializer_zero_p because the
initializer already has the proper type and for zero_init_p types we can
just use initializer_zero_p.
We do probably want some function that tests whether a particular
initializer is equivalent to zero-initialization, which is either
initializer_zero_p for zero_init_p types, !expr for pointers to members,
and recursing for aggregates. Maybe cp_initializer_zero_p or
zero_init_expr_p?
It could be changed to return false for incompatible initializers
like pointers (or even __null) for non-pointer types, even if they
are zero, but that's not what it's designed to do.
But that's exactly what we did for 90938. Now you're proposing another
small exception, only putting it in the caller instead. I think we'll
keep running into these problems until we fix the design issue.
It would also be possible to improve things by doing the conversion in
type_initializer_zero_p before considering its zeroness, but that would
again be duplicating work that we're already doing elsewhere.
Jason
