[PATCH v2] c++: fix ICE with constexpr ARRAY_REF [PR110382]

[Marek Polacek <polacek@redhat.com>]

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