On Mon, Jul 29, 2024 at 06:34:40PM -0400, Jason Merrill wrote:
> On 7/29/24 4:18 PM, Marek Polacek wrote:
> > On Tue, Jul 23, 2024 at 05:18:52PM -0400, Jason Merrill wrote:
> > > On 7/17/24 5:33 PM, Marek Polacek wrote:
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > >
> > > Hmm, I thought I had replied to this already.
> > >
> > > > -- >8 --
> > > > Unfortunately, my r15-1946 fix broke the attached testcase. In it,
> > > > we no longer go into the
> > > > /* P1009: Array size deduction in new-expressions. */
> > > > block, and instead generate an operator new [] call along with a loop
> > > > in build_new_1, which we can't constexpr-evaluate. So this patch
> > > > reverts r15-1946 and uses CONSTRUCTOR_IS_PAREN_INIT to distinguish
> > > > between () and {} to fix the original testcase (anew7.C).
> > > >
> > > > PR c++/115645
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * call.cc (convert_like_internal) <case ck_user>: Don't report
> > > > errors
> > > > about calling an explicit constructor when the constructor was
> > > > marked
> > > > CONSTRUCTOR_IS_PAREN_INIT.
> > > > * init.cc (build_new): Revert r15-1946. Set
> > > > CONSTRUCTOR_IS_PAREN_INIT.
> > > > (build_vec_init): Maybe set CONSTRUCTOR_IS_PAREN_INIT.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/cpp2a/constexpr-new23.C: New test.
> > > > ---
> > > > gcc/cp/call.cc | 2 ++
> > > > gcc/cp/init.cc | 17 ++++-----
> > > > gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C | 38
> > > > ++++++++++++++++++++
> > > > 3 files changed, 49 insertions(+), 8 deletions(-)
> > > > create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C
> > > >
> > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > index a5d3426b70c..2d94d5e0d07 100644
> > > > --- a/gcc/cp/call.cc
> > > > +++ b/gcc/cp/call.cc
> > > > @@ -8592,6 +8592,8 @@ convert_like_internal (conversion *convs, tree
> > > > expr, tree fn, int argnum,
> > > > && BRACE_ENCLOSED_INITIALIZER_P (expr)
> > > > /* Unless this is for direct-list-initialization. */
> > > > && (!CONSTRUCTOR_IS_DIRECT_INIT (expr) ||
> > > > convs->need_temporary_p)
> > > > + /* And it wasn't a ()-init. */
> > > > + && !CONSTRUCTOR_IS_PAREN_INIT (expr)
> > > > /* And in C++98 a default constructor can't be explicit. */
> > > > && cxx_dialect >= cxx11)
> > > > {
> > > > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > > > index e9561c146d7..4138a6077dd 100644
> > > > --- a/gcc/cp/init.cc
> > > > +++ b/gcc/cp/init.cc
> > > > @@ -4005,20 +4005,17 @@ build_new (location_t loc, vec<tree, va_gc>
> > > > **placement, tree type,
> > > > /* P1009: Array size deduction in new-expressions. */
> > > > const bool array_p = TREE_CODE (type) == ARRAY_TYPE;
> > > > if (*init
> > > > - /* If the array didn't specify its bound, we have to deduce it.
> > > > */
> > > > - && ((array_p && !TYPE_DOMAIN (type))
> > > > - /* For C++20 array with parenthesized-init, we have to process
> > > > - the parenthesized-list. But don't do it for (), which is
> > > > - value-initialization, and INIT should stay empty. */
> > > > - || (cxx_dialect >= cxx20
> > > > - && (array_p || nelts)
> > > > - && !(*init)->is_empty ())))
> > > > + /* If ARRAY_P, we have to deduce the array bound. For C++20
> > > > paren-init,
> > > > + we have to process the parenthesized-list. But don't do it
> > > > for (),
> > > > + which is value-initialization, and INIT should stay empty. */
> > > > + && (array_p || (cxx_dialect >= cxx20 && nelts &&
> > > > !(*init)->is_empty ())))
> > > > {
> > > > /* This means we have 'new T[]()'. */
> > > > if ((*init)->is_empty ())
> > > > {
> > > > tree ctor = build_constructor (init_list_type_node, NULL);
> > > > CONSTRUCTOR_IS_DIRECT_INIT (ctor) = true;
> > > > + CONSTRUCTOR_IS_PAREN_INIT (ctor) = true;
> > > > vec_safe_push (*init, ctor);
> > > > }
> > > > tree &elt = (**init)[0];
> > > > @@ -4735,6 +4732,9 @@ build_vec_init (tree base, tree maxindex, tree
> > > > init,
> > > > bool do_static_init = (DECL_P (obase) && TREE_STATIC (obase));
> > > > bool empty_list = false;
> > > > + const bool paren_init_p = (init
> > > > + && TREE_CODE (init) == CONSTRUCTOR
> > > > + && CONSTRUCTOR_IS_PAREN_INIT (init));
> > >
> > > I think rather than recognizing paren-init in general, we want to
> > > recognize
> > > () specifically, and set explicit_value_init_p...
> > >
> > > > if (init && BRACE_ENCLOSED_INITIALIZER_P (init)
> > > > && CONSTRUCTOR_NELTS (init) == 0)
> > > > /* Skip over the handling of non-empty init lists. */
> > > > @@ -4927,6 +4927,7 @@ build_vec_init (tree base, tree maxindex, tree
> > > > init,
> > > > || TREE_CODE (type) == ARRAY_TYPE))
> > > > {
> > > > init = build_constructor (init_list_type_node, NULL);
> > > > + CONSTRUCTOR_IS_PAREN_INIT (init) = paren_init_p;
> > > > }
> > > > else
> > > > {
> > >
> > > ...by taking the else branch here. Then we shouldn't need the
> > > convert_like
> > > change.
> >
> > Unfortunately that then breaks Jon's test (constexpr-new23.C which this
> > patch is adding). The problem is that if we do *not* create a new {}, and
> > do explicit_value_init_p, we end up with
> >
> > int[1] * D.2643;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2643 = (int[1] *) D.2642) >>>;
> > int[1] * D.2644;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2644 = D.2643) >>>;
> > TARGET_EXPR <D.2645, 0>;
> > <<< Unknown tree: for_stmt
> > D.2645 > -1
> > <<cleanup_point <<< Unknown tree: expr_stmt
> > *(int[1] &) int * D.2646;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2646 = (int *) D.2644) >>>;
> > int * D.2647;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2647 = D.2646) >>>;
> > TARGET_EXPR <D.2648, 0>;
> > <<< Unknown tree: for_stmt
> >
> > D.2648 > -1
> >
> > <<cleanup_point <<< Unknown tree: expr_stmt
> > *D.2647 = 0, --D.2648 >>>>>;
> > <<< Unknown tree: expr_stmt
> > (void) ++D.2647 >>>;
> > >>>;
> > D.2646, --D.2645 >>>>>;
> > <<< Unknown tree: expr_stmt
> > (void) ++D.2644 >>>;
> > >>>;
> > D.2643
> >
> > rather than:
> >
> > int[1] * D.2643;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2643 = (int[1] *) D.2642) >>>;
> > int[1] * D.2644;
> > <<< Unknown tree: expr_stmt
> > (void) (D.2644 = D.2643) >>>;
> > TARGET_EXPR <D.2645, 0>;
> > <<< Unknown tree: for_stmt
> > D.2645 > -1
> > <<cleanup_point <<< Unknown tree: expr_stmt
> > *D.2644 = {}, --D.2645 >>>>>;
> > <<< Unknown tree: expr_stmt
> > (void) ++D.2644 >>>;
> > >>>;
> > D.2643
> >
> > In the former, the "*D.2647 = 0" assignment is what breaks constexpr,
> > which then complains:
> >
> > constexpr-new23.C:16:16: error: accessing 'test_array()::U::arr' member
> > instead of initialized 'test_array()::U::x' member in constant expression
> > 16 | return ::new((void*)p) T[1]();
> > | ^~~~~~~~~~~~~~~~~~~~~~
> > constexpr-new23.C:31:9: note: initializing 'test_array()::U::arr' requires
> > a member access expression as the left operand of the assignment
> > 31 | int arr[4];
> >
> >
> > If there is no bug in constexpr, then it looks like we need to
> > initialize using a {} rather than a loop that assigns 0 to each
> > element.
>
> Ah, thanks.
>
> It looks like the first bug is that build_vec_init wrongly leaves try_const
> false for this case (without your patch) because int doesn't have a
> constexpr default constructor, failing to consider that value-initialization
> of scalars is constexpr.
Oh wow, I should have noticed that.
> Then, once we're into the looping initialization, we aren't expressing it in
> a way that will satisfy the strict checking in constexpr evaluation; it
> needs to initialize the array, not just its elements.
>
> I expect we could fix that with something like
>
> > /* Start array lifetime before accessing elements. */
> > if (TREE_CODE (atype) == ARRAY_TYPE)
> > {
> > elt_init = build_constructor (atype, nullptr);
> > CONSTRUCTOR_NO_CLEARING (elt_init) = true;
> > for_stmt = build2 (INIT_EXPR, atype, obase, elt_init);
> > finish_expr_stmt (for_stmt);
> > }
>
> but if we're only concerned about constexpr, fixing the first bug ought to
> be enough; in constant evaluation if we don't get a constant initializer
> we're failing anyway.
This patch fixes the first bug. Thanks!
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Unfortunately, my r15-1946 fix broke the attached testcase; the
constexpr evaluation reported an error about not being able to
evaluate the code emitted by build_vec_init. Jason figured out
it's because we were wrongly setting try_const to false, where
in fact it should have been true. Value-initialization of scalars
is constexpr, so we should check that alongside of
type_has_constexpr_default_constructor.
PR c++/115645
gcc/cp/ChangeLog:
* init.cc (build_vec_init): When initializing a scalar type, try to
create a constant initializer.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-new23.C: New test.
---
gcc/cp/init.cc | 4 ++-
gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C | 38 ++++++++++++++++++++
2 files changed, 41 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C
diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
index e9561c146d7..a3a97e2c128 100644
--- a/gcc/cp/init.cc
+++ b/gcc/cp/init.cc
@@ -4724,7 +4724,9 @@ build_vec_init (tree base, tree maxindex, tree init,
&& TREE_CONSTANT (maxindex)
&& (init ? TREE_CODE (init) == CONSTRUCTOR
: (type_has_constexpr_default_constructor
- (inner_elt_type)))
+ (inner_elt_type)
+ /* Value-initialization of scalars is constexpr. */
+ || SCALAR_TYPE_P (inner_elt_type)))
&& (literal_type_p (inner_elt_type)
|| TYPE_HAS_CONSTEXPR_CTOR (inner_elt_type)));
vec<constructor_elt, va_gc> *const_vec = NULL;
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C
new file mode 100644
index 00000000000..1abbef18fae
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-new23.C
@@ -0,0 +1,38 @@
+// PR c++/115645
+// { dg-do compile { target c++20 } }
+
+using size_t = decltype(sizeof(0));
+
+void* operator new(size_t, void* p) { return p; }
+void* operator new[](size_t, void* p) { return p; }
+
+#define VERIFY(C) if (!(C)) throw
+
+namespace std {
+ template<typename T>
+ constexpr T* construct_at(T* p)
+ {
+ if constexpr (__is_array(T))
+ return ::new((void*)p) T[1]();
+ else
+ return ::new((void*)p) T();
+ }
+}
+
+constexpr void
+test_array()
+{
+ int arr[1] { 99 };
+ std::construct_at(&arr);
+ VERIFY( arr[0] == 0 );
+
+ union U {
+ long long x = -1;
+ int arr[4];
+ } u;
+
+ auto p = std::construct_at(&u.arr);
+ VERIFY( (*p)[0] == 0 );
+}
+
+static_assert( [] { test_array(); return true; }() );
base-commit: acc70606c59e3f14072cc8a164362e728d8df5d6
--
2.45.2
