On Thu, 19 Mar 2020, Jason Merrill wrote:
> On 3/19/20 2:06 PM, Patrick Palka via Gcc-patches wrote:
> > On Thu, 19 Mar 2020, Marek Polacek wrote:
> >
> > > On Thu, Mar 19, 2020 at 01:06:35PM -0400, Patrick Palka via Gcc-patches
> > > wrote:
> > > > On Thu, 19 Mar 2020, Patrick Palka wrote:
> > > >
> > > > > This patch adds a check to detect changing the active union member
> > > > > during
> > > > > initialization of the union. It uses the CONSTRUCTOR_NO_CLEARING flag
> > > > > as a
> > > > > proxy for whether the non-empty CONSTRUCTOR of UNION_TYPE we're
> > > > > assigning to in
> > > > > cxx_eval_store_expression is in the process of being initialized,
> > > > > which seems to
> > > > > work well.
> > > >
> > > > If we can't rely on CONSTRUCTOR_NO_CLEARING to be set iff a CONSTRUCTOR
> > > > is in the process of being initialized, then here's an alternative patch
> > > > for consideration, that detects this UB in an indirect way and after the
> > > > fact.
> > >
> > > Yeah, I'm not sure if that would work well, especially in C++20 where we
> > > sometimes don't clear it:
> > >
> > > /* The result of a constexpr function must be completely initialized.
> > >
> > > However, in C++20, a constexpr constructor doesn't necessarily have
> > > to initialize all the fields, so we don't clear
> > > CONSTRUCTOR_NO_CLEARING
> > > in order to detect reading an unitialized object in constexpr
> > > instead
> > > of value-initializing it. (reduced_constant_expression_p is
> > > expected to
> > > take care of clearing the flag.) */
> > > if (TREE_CODE (result) == CONSTRUCTOR
> > > && (cxx_dialect < cxx2a
> > > || !DECL_CONSTRUCTOR_P (fun)))
> > > clear_no_implicit_zero (result);
> > >
> > > and rely on reduced_constant_expression_p to clear it.
> >
> > I see, thanks. Here's a reproducer for the issue you pointed out, which
> > is a valid testcase but gets rejected with the proposed patch:
> >
> > union U
> > {
> > int x;
> > char y;
> > };
> >
> > constexpr bool
> > baz ()
> > {
> > U u;
> > u.x = 3;
> > u.y = 7;
> > return (u.y == 7);
> > }
> >
> > static_assert (baz ());
> >
> > CONSTRUCTOR_NO_CLEARING is set for 'u' and is not cleared after its
> > constructor returns, and so the check yields a false positive for the
> > assignment to u.y. That's unfortunate...
>
> We should probably clear the flag when we assign to u.x because once we give a
> value to one union member, the union has a value.
That works well. Further testing revealed a couple of more issues
caused by the new hunk
@@ -3751,6 +3751,11 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree
t,
/* If we built a new CONSTRUCTOR, attach it now so that other
initializers can refer to it. */
CONSTRUCTOR_APPEND_ELT (*p, index, new_ctx.ctor);
+ else if (TREE_CODE (type) == UNION_TYPE)
+ /* If we're constructing a union, set the active union member now so
+ that we can later detect if the initializer attempts to activate
+ another member. */
+ CONSTRUCTOR_APPEND_ELT (*p, index, NULL_TREE);
tree elt = cxx_eval_constant_expression (&new_ctx, value,
lval,
non_constant_p, overflow_p);
where other routines aren't prepared to handle a NULL constructor union
element. They are cxx_eval_component_reference (fixed by emitting a
different error message given a NULL constructor elt) and
cxx_eval_bare_aggregate itself (fixed by not re-reducing a CONSTRUCTOR
returned by lookup_placeholder).
As a drive-by this patch adds a check in reduced_constant_expression_p
to correctly return false for an empty CONSTRUCTOR of UNION_TYPE. This
allows us to correctly diagnose the uninitialized use in
constexpr-union4.C where we weren't before.
-- >8 --
gcc/cp/ChangeLog:
PR c++/94066
* constexpr.c (reduced_constant_expression_p) [CONSTRUCTOR]: Properly
handle unions without an initializer.
(cxx_eval_component_reference): Emit a different diagnostic when the
constructor element corresponding to a union member is NULL.
(cxx_eval_bare_aggregate): When constructing a union, always set the
active union member before evaluating the initializer. Relax assertion
that verifies the index of the constructor element we're initializing
hasn't been changed.
(cxx_eval_store_expression): Diagnose changing the active union member
while the union is in the process of being initialized. After setting
an active union member, clear CONSTRUCTOR_NO_CLEARING on the underlying
CONSTRUCTOR.
(cxx_eval_constant_expression) [PLACEHOLDER_EXPR]: Don't re-reduce a
CONSTRUCTOR returned by lookup_placeholder.
gcc/testsuite/ChangeLog:
PR c++/94066
* g++.dg/cpp1y/constexpr-union2.C: New test.
* g++.dg/cpp1y/constexpr-union3.C: New test.
* g++.dg/cpp1y/constexpr-union4.C: New test.
* g++.dg/cpp1y/constexpr-union5.C: New test.
* g++.dg/cpp1y/pr94066.C: New test.
* g++.dg/cpp1y/pr94066-2.C: New test.
* g++.dg/cpp1y/pr94066-3.C: New test.
* g++.dg/cpp2a/constexpr-union1.C: New test.
---
gcc/cp/constexpr.c | 69 ++++++++++++++++---
gcc/testsuite/g++.dg/cpp1y/constexpr-union2.C | 9 +++
gcc/testsuite/g++.dg/cpp1y/constexpr-union3.C | 9 +++
gcc/testsuite/g++.dg/cpp1y/constexpr-union4.C | 9 +++
gcc/testsuite/g++.dg/cpp1y/constexpr-union5.C | 15 ++++
gcc/testsuite/g++.dg/cpp1y/pr94066-2.C | 19 +++++
gcc/testsuite/g++.dg/cpp1y/pr94066-3.C | 16 +++++
gcc/testsuite/g++.dg/cpp1y/pr94066.C | 18 +++++
gcc/testsuite/g++.dg/cpp2a/constexpr-union1.C | 18 +++++
9 files changed, 173 insertions(+), 9 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union2.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union3.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union4.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-union5.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr94066.C
create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union1.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 192face9a3a..6284ab25b7d 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2591,10 +2591,17 @@ reduced_constant_expression_p (tree t)
return false;
else if (cxx_dialect >= cxx2a
/* An ARRAY_TYPE doesn't have any TYPE_FIELDS. */
- && (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE
- /* A union only initializes one member. */
- || TREE_CODE (TREE_TYPE (t)) == UNION_TYPE))
+ && TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
field = NULL_TREE;
+ else if (cxx_dialect >= cxx2a
+ && TREE_CODE (TREE_TYPE (t)) == UNION_TYPE)
+ {
+ if (CONSTRUCTOR_NELTS (t) == 0)
+ /* An initialized union has a constructor element. */
+ return false;
+ /* And it only initializes one member. */
+ field = NULL_TREE;
+ }
else
field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
}
@@ -3446,8 +3453,14 @@ cxx_eval_component_reference (const constexpr_ctx *ctx,
tree t,
{
/* DR 1188 says we don't have to deal with this. */
if (!ctx->quiet)
- error ("accessing %qD member instead of initialized %qD member in "
- "constant expression", part, CONSTRUCTOR_ELT (whole, 0)->index);
+ {
+ constructor_elt *cep = CONSTRUCTOR_ELT (whole, 0);
+ if (cep->value == NULL_TREE)
+ error ("accessing uninitialized member %qD", part);
+ else
+ error ("accessing %qD member instead of initialized %qD member in "
+ "constant expression", part, cep->index);
+ }
*non_constant_p = true;
return t;
}
@@ -3751,6 +3764,11 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree
t,
/* If we built a new CONSTRUCTOR, attach it now so that other
initializers can refer to it. */
CONSTRUCTOR_APPEND_ELT (*p, index, new_ctx.ctor);
+ else if (TREE_CODE (type) == UNION_TYPE)
+ /* If we're constructing a union, set the active union member now so
+ that we can later detect if the initializer attempts to activate
+ another member. */
+ CONSTRUCTOR_APPEND_ELT (*p, index, NULL_TREE);
tree elt = cxx_eval_constant_expression (&new_ctx, value,
lval,
non_constant_p, overflow_p);
@@ -3784,7 +3802,13 @@ cxx_eval_bare_aggregate (const constexpr_ctx *ctx, tree
t,
}
else
{
- if (new_ctx.ctor != ctx->ctor)
+ if (TREE_CODE (type) == UNION_TYPE
+ && (*p)->last().index != index)
+ /* The initializer may have erroneously changed the active union
+ member that we're initializing. */
+ gcc_assert (*non_constant_p);
+ else if (new_ctx.ctor != ctx->ctor
+ || TREE_CODE (type) == UNION_TYPE)
{
/* We appended this element above; update the value. */
gcc_assert ((*p)->last().index == index);
@@ -4567,6 +4591,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree
t,
bool no_zero_init = true;
releasing_vec ctors;
+ bool changed_active_union_member_p = false;
while (!refs->is_empty ())
{
if (*valp == NULL_TREE)
@@ -4647,6 +4672,19 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
index);
*non_constant_p = true;
}
+ else if (TREE_CODE (t) == MODIFY_EXPR
+ && CONSTRUCTOR_NO_CLEARING (*valp))
+ {
+ /* Diagnose changing the active union member while the union
+ is in the process of being initialized. */
+ if (!ctx->quiet)
+ error_at (cp_expr_loc_or_input_loc (t),
+ "change of the active member of a union "
+ "from %qD to %qD during initialization",
+ CONSTRUCTOR_ELT (*valp, 0)->index,
+ index);
+ *non_constant_p = true;
+ }
/* Changing active member. */
vec_safe_truncate (CONSTRUCTOR_ELTS (*valp), 0);
no_zero_init = true;
@@ -4675,6 +4713,10 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
vec_safe_insert (CONSTRUCTOR_ELTS (*valp), idx, ce);
cep = CONSTRUCTOR_ELT (*valp, idx);
+
+ if (code == UNION_TYPE)
+ /* Record that we've changed an active union member. */
+ changed_active_union_member_p = true;
}
found:;
}
@@ -4805,13 +4847,17 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
unsigned i;
bool c = TREE_CONSTANT (init);
bool s = TREE_SIDE_EFFECTS (init);
- if (!c || s)
+ if (!c || s || changed_active_union_member_p)
FOR_EACH_VEC_ELT (*ctors, i, elt)
{
if (!c)
TREE_CONSTANT (elt) = false;
if (s)
TREE_SIDE_EFFECTS (elt) = true;
+ /* Clear CONSTRUCTOR_NO_CLEARING since we've activated a member of
+ this union. */
+ if (TREE_CODE (TREE_TYPE (elt)) == UNION_TYPE)
+ CONSTRUCTOR_NO_CLEARING (elt) = false;
}
if (*non_constant_p)
@@ -6133,8 +6179,13 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx,
tree t,
case PLACEHOLDER_EXPR:
/* Use of the value or address of the current object. */
if (tree ctor = lookup_placeholder (ctx, lval, TREE_TYPE (t)))
- return cxx_eval_constant_expression (ctx, ctor, lval,
- non_constant_p, overflow_p);
+ {
+ if (TREE_CODE (ctor) == CONSTRUCTOR)
+ return ctor;
+ else
+ return cxx_eval_constant_expression (ctx, ctor, lval,
+ non_constant_p, overflow_p);
+ }
/* A placeholder without a referent. We can get here when
checking whether NSDMIs are noexcept, or in massage_init_elt;
just say it's non-constant for now. */
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union2.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-union2.C
new file mode 100644
index 00000000000..7a6a818742b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union2.C
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++14 } }
+
+union U
+{
+ char *x = &y;
+ char y;
+};
+
+constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union3.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-union3.C
new file mode 100644
index 00000000000..5cf62e46cb5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union3.C
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++14 } }
+
+union U
+{
+ int x = (x = x + 1);
+ char y;
+};
+
+constexpr U u = {}; // { dg-error "accessing uninitialized member" }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union4.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-union4.C
new file mode 100644
index 00000000000..3e44a1378f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union4.C
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++14 } }
+
+union U
+{
+ int x = y;
+ char y;
+};
+
+constexpr U u = {}; // { dg-error "accessing uninitialized member" }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-union5.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-union5.C
new file mode 100644
index 00000000000..55fe9fa2f0b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-union5.C
@@ -0,0 +1,15 @@
+// { dg-do compile { target c++14 } }
+
+union U;
+constexpr int foo(U *up);
+
+union U {
+ int a = foo(this); int y;
+};
+
+constexpr int foo(U *up) {
+ up->a++;
+ return {42};
+}
+
+extern constexpr U u = {}; // { dg-error "accessing uninitialized member" }
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
new file mode 100644
index 00000000000..1c00b650961
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066-2.C
@@ -0,0 +1,19 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+struct A { long x; };
+
+union U;
+constexpr A foo(U *up);
+
+union U {
+ U() = default;
+ A a = foo(this); int y;
+};
+
+constexpr A foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
new file mode 100644
index 00000000000..175018acf86
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066-3.C
@@ -0,0 +1,16 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+union U;
+constexpr int foo(U *up);
+
+union U {
+ int a = foo(this); int y;
+};
+
+constexpr int foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr94066.C
b/gcc/testsuite/g++.dg/cpp1y/pr94066.C
new file mode 100644
index 00000000000..6725c8c737f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr94066.C
@@ -0,0 +1,18 @@
+// PR c++/94066
+// { dg-do compile { target c++14 } }
+
+struct A { long x; };
+
+union U;
+constexpr A foo(U *up);
+
+union U {
+ A a = foo(this); int y;
+};
+
+constexpr A foo(U *up) {
+ up->y = 11; // { dg-error "'U::a' to 'U::y'" }
+ return {42};
+}
+
+extern constexpr U u = {};
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union1.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-union1.C
new file mode 100644
index 00000000000..c38167ad798
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union1.C
@@ -0,0 +1,18 @@
+// { dg-do compile { target c++2a } }
+
+union U
+{
+ int x;
+ char y;
+};
+
+constexpr bool
+baz ()
+{
+ U u;
+ u.x = 3;
+ u.y = 7;
+ return (u.y == 7);
+}
+
+static_assert (baz ());
--
2.26.0.rc1.11.g30e9940356
