On 7/25/23 12:59, Marek Polacek wrote:
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?

OK.

-- >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

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