On Sat, Jul 22, 2023 at 12:28:59AM -0400, Jason Merrill wrote:
> On 7/21/23 18:38, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13?
> >
> > -- >8 --
> >
> > This code in cxx_eval_array_reference has been hard to get right.
> > In r12-2304 I added some code; in r13-5693 I removed some of it.
> >
> > Here the problematic line is "S s = arr[0];" which causes a crash
> > on the assert in verify_ctor_sanity:
> >
> > gcc_assert (!ctx->object || !DECL_P (ctx->object)
> > || ctx->global->get_value (ctx->object) == ctx->ctor);
> >
> > ctx->object is the VAR_DECL 's', which is correct here. The second
> > line points to the problem: we replaced ctx->ctor in
> > cxx_eval_array_reference:
> >
> > new_ctx.ctor = build_constructor (elem_type, NULL); // #1
>
> ...and this code doesn't also clear(/set) new_ctx.object like everywhere
> else in constexpr.cc that sets new_ctx.ctor. Fixing that should make the
> testcase work.
Right, but then we'd be back pre-r12-2304 or r13-5693...
...except it should work to always clear the object, like below.
> > which I think we shouldn't have; the CONSTRUCTOR we created in
> > cxx_eval_constant_expression/DECL_EXPR
> >
> > new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);
> >
> > had the right type.
>
> Indeed, and using it rather than building a new one seems like a valid
> optimization for trunk.
Agreed, I kept it.
> I also notice that the DECL_EXPR code calls unshare_constructor, which
> should be unnecessary if init == ctx->ctor?
It looks like init == ctx->ctor only happens only with this new testcase.
I'm not sure it's worth it adding code for such a rare case?
> > We still need #1 though. E.g., in constexpr-96241.C, we never
> > set ctx.ctor/object before calling cxx_eval_array_reference, so
> > we have to build a CONSTRUCTOR there. And in constexpr-101371-2.C
> > we have a ctx.ctor, but it has the wrong type, so we need a new one.
> >
> > PR c++/110382
> >
> > gcc/cp/ChangeLog:
> >
> > * constexpr.cc (cxx_eval_array_reference): Create a new constructor
> > only when we don't already have a matching one.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp1y/constexpr-110382.C: New test.
> > ---
> > gcc/cp/constexpr.cc | 5 ++++-
> > gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++
> > 2 files changed, 21 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
> >
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index fb94f3cefcb..518b7c7a2d5 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -4291,7 +4291,10 @@ cxx_eval_array_reference (const constexpr_ctx *ctx,
> > tree t,
> > else
> > val = build_value_init (elem_type, tf_warning_or_error);
> > - if (!SCALAR_TYPE_P (elem_type))
> > + if (!SCALAR_TYPE_P (elem_type)
> > + /* Create a new constructor only if we don't already have one that
> > + is suitable. */
> > + && !(ctx->ctor && same_type_p (elem_type, TREE_TYPE (ctx->ctor))))
>
> We generally use same_type_ignoring_top_level_qualifiers_p in the constexpr
> code.
True, changed.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
For 13, I guess I should only clear the object and leave out the
same_type_ bit.
-- >8 --
This code in cxx_eval_array_reference has been hard to get right.
In r12-2304 I added some code; in r13-5693 I removed some of it.
Here the problematic line is "S s = arr[0];" which causes a crash
on the assert in verify_ctor_sanity:
gcc_assert (!ctx->object || !DECL_P (ctx->object)
|| ctx->global->get_value (ctx->object) == ctx->ctor);
ctx->object is the VAR_DECL 's', which is correct here. The second
line points to the problem: we replaced ctx->ctor in
cxx_eval_array_reference:
new_ctx.ctor = build_constructor (elem_type, NULL); // #1
which I think we shouldn't have; the CONSTRUCTOR we created in
cxx_eval_constant_expression/DECL_EXPR
new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);
had the right type.
We still need #1 though. E.g., in constexpr-96241.C, we never
set ctx.ctor/object before calling cxx_eval_array_reference, so
we have to build a CONSTRUCTOR there. And in constexpr-101371-2.C
we have a ctx.ctor, but it has the wrong type, so we need a new one.
We can fix the problem by always clearing the object, and, as an
optimization, only create/free a new ctor when actually needed.
PR c++/110382
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_array_reference): Create a new constructor
only when we don't already have a matching one. Clear the object
when the type is non-scalar.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-110382.C: New test.
---
gcc/cp/constexpr.cc | 17 +++++++++++++++--
gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++
2 files changed, 32 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index fb94f3cefcb..054812bcb72 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4291,15 +4291,28 @@ cxx_eval_array_reference (const constexpr_ctx *ctx,
tree t,
else
val = build_value_init (elem_type, tf_warning_or_error);
+ bool new_ctor;
if (!SCALAR_TYPE_P (elem_type))
{
new_ctx = *ctx;
- new_ctx.ctor = build_constructor (elem_type, NULL);
+ /* We clear the object here. We used to replace it with T, but that
+ caused problems (101371, 108158); and anyway, T is the initializer,
+ not the target object. */
+ new_ctx.object = NULL_TREE;
+ /* Create a new constructor only if we don't already have a suitable
+ one. */
+ new_ctor = (!ctx->ctor
+ || !same_type_ignoring_top_level_qualifiers_p
+ (elem_type, TREE_TYPE (ctx->ctor)));
+ if (new_ctor)
+ new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
}
+ else
+ new_ctor = false;
t = cxx_eval_constant_expression (ctx, val, lval, non_constant_p,
overflow_p);
- if (!SCALAR_TYPE_P (elem_type) && t != ctx->ctor)
+ if (new_ctor && t != ctx->ctor)
free_constructor (ctx->ctor);
return t;
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
new file mode 100644
index 00000000000..317c5ecfcd5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C
@@ -0,0 +1,17 @@
+// PR c++/110382
+// { dg-do compile { target c++14 } }
+
+struct S {
+ double a = 0;
+};
+
+constexpr double
+g ()
+{
+ S arr[1];
+ S s = arr[0];
+ (void) arr[0];
+ return s.a;
+}
+
+int main() { return g (); }
base-commit: 96482ffe60d9bdec802fcad705c69641b2a3e040
--
2.41.0
