On 10/12/23 04:53, Nathaniel Shead wrote:
On Wed, Oct 11, 2023 at 12:48:12AM +1100, Nathaniel Shead wrote:
On Mon, Oct 09, 2023 at 04:46:46PM -0400, Jason Merrill wrote:
On 10/8/23 21:03, Nathaniel Shead wrote:
Ping for https://gcc.gnu.org/pipermail/gcc-patches/2023-September/631203.html
+ && (TREE_CODE (t) == MODIFY_EXPR
+ /* Also check if initializations have implicit change of active
+ member earlier up the access chain. */
+ || !refs->is_empty())
I'm not sure what the cumulative point of these two tests is. TREE_CODE (t)
will be either MODIFY_EXPR or INIT_EXPR, and either should be OK.
As I understand it, the problematic case is something like
constexpr-union2.C, where we're also looking at a MODIFY_EXPR. So what is
this check doing?
The reasoning was to correctly handle cases like the the following (in
constexpr-union6.C):
constexpr int test1() {
U u {};
std::construct_at(&u.s, S{ 1, 2 });
return u.s.b;
}
static_assert(test1() == 2);
The initialisation of &u.s here is not a member access expression within
the call to std::construct_at, since it's just a pointer, but this code
is still legal; in general, an INIT_EXPR to initialise a union member
should always be OK (I believe?), hence constraining to just
MODIFY_EXPR.
However, just that would then (incorrectly) allow all the following
cases in that test to compile, such as
constexpr int test2() {
U u {};
int* p = &u.s.b;
std::construct_at(p, 5);
return u.s.b;
}
constexpr int x2 = test2();
since the INIT_EXPR is really only initialising 'b', but the implicit
"modification" of active member to 'u.s' is illegal.
Maybe a better way of expressing this condition would be something like
this?
/* An INIT_EXPR of the last member in an access chain is always OK,
but still check implicit change of members earlier on; see
cpp2a/constexpr-union6.C. */
&& !(TREE_CODE (t) == INIT_EXPR && refs->is_empty ())
Otherwise I'll see if I can rework some of the other conditions instead.
Incidentally, I think constexpr-union6.C could use a test where we pass &u.s
to a function other than construct_at, and then try (and fail) to assign to
the b member from that function.
Jason
Sounds good; I've added the following test:
constexpr void foo(S* s) {
s->b = 10; // { dg-error "accessing .U::s. member instead of initialized
.U::k." }
}
constexpr int test3() {
U u {};
foo(&u.s); // { dg-message "in .constexpr. expansion" }
return u.s.b;
}
constexpr int x3 = test3(); // { dg-message "in .constexpr. expansion" }
Incidentally I found this particular example caused a very unhelpful
error + ICE due to reporting that S could not be value-initialized in
the current version of the patch. The updated version below fixes that
by using 'build_zero_init' instead -- is this an appropriate choice
here?
A similar (but unrelated) issue is with e.g.
struct S { const int a; int b; };
union U { int k; S s; };
constexpr int test() {
U u {};
return u.s.b;
}
constexpr int x = test();
giving me this pretty unhelpful error message:
/home/ns/main.cpp:8:23: in ‘constexpr’ expansion of ‘test()’
/home/ns/main.cpp:6:12: error: use of deleted function ‘S::S()’
6 | return u.s.b;
| ~~^
/home/ns/main.cpp:1:8: note: ‘S::S()’ is implicitly deleted because the default
definition would be ill-formed:
1 | struct S { const int a; int b; };
| ^
/home/ns/main.cpp:1:8: error: uninitialised const member in ‘struct S’
/home/ns/main.cpp:1:22: note: ‘const int S::a’ should be initialised
1 | struct S { const int a; int b; };
| ^
/home/ns/main.cpp:8:23: in ‘constexpr’ expansion of ‘test()’
/home/ns/main.cpp:6:12: error: use of deleted function ‘S::S()’
6 | return u.s.b;
| ~~^
/home/ns/main.cpp:8:23: in ‘constexpr’ expansion of ‘test()’
/home/ns/main.cpp:6:12: error: use of deleted function ‘S::S()’
but I'll try and fix this separately (it exists on current trunk without
this patch as well).
While attempting to fix this I found a much better way of handling
value-initialised unions. Here's a new version of the patch which also
includes the fix for accessing the wrong member of a value-initialised
union as well.
Additionally includes an `auto_diagnostic_group` for the `inform`
diagnostics as Marek helpfully informed me about in my other patch.
Bootstrapped and regtested on x86_64-pc-linux-gnu.
@@ -4496,21 +4491,36 @@ cxx_eval_component_reference (const constexpr_ctx *ctx,
tree t,
break;
}
}
- if (TREE_CODE (TREE_TYPE (whole)) == UNION_TYPE
- && CONSTRUCTOR_NELTS (whole) > 0)
+ if (TREE_CODE (TREE_TYPE (whole)) == UNION_TYPE)
{
- /* DR 1188 says we don't have to deal with this. */
- if (!ctx->quiet)
+ if (CONSTRUCTOR_NELTS (whole) > 0)
{
- 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);
+ /* DR 1188 says we don't have to deal with this. */
+ if (!ctx->quiet)
+ {
+ 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;
+ }
+ else if (!CONSTRUCTOR_NO_CLEARING (whole))
+ {
+ /* Value-initialized union, check if looking at the first member. */
+ tree first = next_aggregate_field (TYPE_FIELDS (TREE_TYPE (whole)));
+ if (pmf ? DECL_NAME (first) != DECL_NAME (part) : first != part)
You don't need to consider pmf here, since a PMF isn't UNION_TYPE.
@@ -6267,23 +6288,74 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
break;
}
+ /* Process value-initialization of a union. */
+ if (code == UNION_TYPE
+ && !CONSTRUCTOR_NO_CLEARING (*valp)
+ && CONSTRUCTOR_NELTS (*valp) == 0)
+ if (tree first = next_aggregate_field (TYPE_FIELDS (type)))
+ CONSTRUCTOR_APPEND_ELT (CONSTRUCTOR_ELTS (*valp), first, NULL_TREE);
Isn't this adding an uninitialized member instead of value-initialized?
Jason
