On Tue, Jul 25, 2023 at 11:15:07AM -0400, Jason Merrill wrote:
> On 7/24/23 18:37, Marek Polacek wrote:
> > 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);
>
> I think clearing .object should (only) happen along with changing .ctor; if
> we leave .ctor alone, we should also leave the matching .object alone.
Ah, true. Thus:
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >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 | 13 +++++++++++--
gcc/testsuite/g++.dg/cpp1y/constexpr-110382.C | 17 +++++++++++++++++
2 files changed, 28 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..5e0fe93ebc0 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4291,15 +4291,24 @@ 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))
+ /* Create a new constructor only if we don't already have a suitable one. */
+ const bool new_ctor = (!SCALAR_TYPE_P (elem_type)
+ && (!ctx->ctor
+ || !same_type_ignoring_top_level_qualifiers_p
+ (elem_type, TREE_TYPE (ctx->ctor))));
+ if (new_ctor)
{
new_ctx = *ctx;
+ /* 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;
new_ctx.ctor = build_constructor (elem_type, NULL);
ctx = &new_ctx;
}
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: 09dda270380fe13e7b4722305cb1122df1f779a0
--
2.41.0
