[PATCH] c++: ICE initing lifetime-extended constexpr var [PR107079]

[PATCH] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Marek Polacek]

(This may not be a complete fix but I got stuck so I'm posting what
I have, which at least fixes the ICE.)

We ICE on the simple:

  struct X { const X* x = this; };
  constexpr const X& x = X{};

where store_init_value initializes 'x' with

  &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>

but we must lifetime-extend via extend_ref_init_temps and we get

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;

Since 'x' was declared constexpr, we do cxx_constant_init and we hit
the preeval code added in r269003 while evaluating the INIT_EXPR:

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>

but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
find anything that could replace X and we ICE on
 7861       /* A placeholder without a referent.  We can get here when
 7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
 7863          just say it's non-constant for now.  */
 7864       gcc_assert (ctx->quiet);
because cxx_constant_init means !ctx->quiet.  It's not correct that
there isn't a referent.  I think the following patch is a pretty
straightforward fix: pass the _ZGR var down to maybe_constant_init so
that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.

What I wasn't able to make work is the commented assert in the test.
It doesn't pass because we complain that _ZGR1x_ isn't a constexpr
variable, but making it so would just result in "used in its own
initializer" (which is true).  I notice that while clang++ compiles
the assert fine, MSVC++/icc reject it as non-constant.  So maybe we
don't have to/shouldn't make it work.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR c++/107079

gcc/cp/ChangeLog:

	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.
---
 gcc/cp/call.cc                                | 2 +-
 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..a0afab9b26a 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
 
   /* If the initializer is constant, put it in DECL_INITIAL so we get
      static initialization and use in constant expressions.  */
-  init = maybe_constant_init (expr);
+  init = maybe_constant_init (expr, var);
   /* As in store_init_value.  */
   init = cp_fully_fold (init);
   if (TREE_CONSTANT (init))
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
new file mode 100644
index 00000000000..6dbb7eb739a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
@@ -0,0 +1,9 @@
+// PR c++/107079
+// { dg-do compile { target c++11 } }
+
+struct X {
+  const X* x = this;
+};
+constexpr const X& x = X{};
+// TODO: Should the assert pass?
+//static_assert(x.x == &x);

base-commit: 77bb54b1b07add45007c664724b726541d672ef3
-- 
2.39.1

Re: [PATCH] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Jason Merrill]

On 2/8/23 13:01, Marek Polacek wrote:
> (This may not be a complete fix but I got stuck so I'm posting what
> I have, which at least fixes the ICE.)
> 
> We ICE on the simple:
> 
>    struct X { const X* x = this; };
>    constexpr const X& x = X{};
> 
> where store_init_value initializes 'x' with
> 
>    &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>
> 
> but we must lifetime-extend via extend_ref_init_temps and we get
> 
>    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;
> 
> Since 'x' was declared constexpr, we do cxx_constant_init and we hit
> the preeval code added in r269003 while evaluating the INIT_EXPR:
> 
>    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>
> 
> but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
> find anything that could replace X and we ICE on
>   7861       /* A placeholder without a referent.  We can get here when
>   7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
>   7863          just say it's non-constant for now.  */
>   7864       gcc_assert (ctx->quiet);
> because cxx_constant_init means !ctx->quiet.  It's not correct that
> there isn't a referent.  I think the following patch is a pretty
> straightforward fix: pass the _ZGR var down to maybe_constant_init so
> that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.
> 
> What I wasn't able to make work is the commented assert in the test.
> It doesn't pass because we complain that _ZGR1x_ isn't a constexpr
> variable, 

That sounds like we aren't (correctly) implementing

  https://eel.is/c++draft/expr.const#4.7

> but making it so would just result in "used in its own
> initializer" (which is true).

True, but not in the sense it means; its initializer doesn't depend on 
its (uninitialized) value.

> I notice that while clang++ compiles
> the assert fine, MSVC++/icc reject it as non-constant.  So maybe we
> don't have to/shouldn't make it work.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/107079
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.
> ---
>   gcc/cp/call.cc                                | 2 +-
>   gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 9 +++++++++
>   2 files changed, 10 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index f7c5d9da94b..a0afab9b26a 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
>   
>     /* If the initializer is constant, put it in DECL_INITIAL so we get
>        static initialization and use in constant expressions.  */
> -  init = maybe_constant_init (expr);
> +  init = maybe_constant_init (expr, var);

We should also pass true for manifestly_const_eval as in store_init_value.

>     /* As in store_init_value.  */
>     init = cp_fully_fold (init);
>     if (TREE_CONSTANT (init))
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> new file mode 100644
> index 00000000000..6dbb7eb739a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> @@ -0,0 +1,9 @@
> +// PR c++/107079
> +// { dg-do compile { target c++11 } }
> +
> +struct X {
> +  const X* x = this;
> +};
> +constexpr const X& x = X{};
> +// TODO: Should the assert pass?
> +//static_assert(x.x == &x);
> 
> base-commit: 77bb54b1b07add45007c664724b726541d672ef3

[PATCH v2] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Marek Polacek]

On Wed, Feb 08, 2023 at 04:00:25PM -0800, Jason Merrill wrote:
> On 2/8/23 13:01, Marek Polacek wrote:
> > (This may not be a complete fix but I got stuck so I'm posting what
> > I have, which at least fixes the ICE.)
> > 
> > We ICE on the simple:
> > 
> >    struct X { const X* x = this; };
> >    constexpr const X& x = X{};
> > 
> > where store_init_value initializes 'x' with
> > 
> >    &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>
> > 
> > but we must lifetime-extend via extend_ref_init_temps and we get
> > 
> >    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;
> > 
> > Since 'x' was declared constexpr, we do cxx_constant_init and we hit
> > the preeval code added in r269003 while evaluating the INIT_EXPR:
> > 
> >    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>
> > 
> > but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
> > find anything that could replace X and we ICE on
> >   7861       /* A placeholder without a referent.  We can get here when
> >   7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
> >   7863          just say it's non-constant for now.  */
> >   7864       gcc_assert (ctx->quiet);
> > because cxx_constant_init means !ctx->quiet.  It's not correct that
> > there isn't a referent.  I think the following patch is a pretty
> > straightforward fix: pass the _ZGR var down to maybe_constant_init so
> > that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.
> > 
> > What I wasn't able to make work is the commented assert in the test.
> > It doesn't pass because we complain that _ZGR1x_ isn't a constexpr
> > variable,
> 
> That sounds like we aren't (correctly) implementing
> 
>  https://eel.is/c++draft/expr.const#4.7

Ah yes, this is DR2126 = c++/101588.  I wonder if the fix will include
checking startswith (IDENTIFIER_POINTER (DECL_NAME (variable)), "_ZGR")
to see if its lifetime is extended.
 
> > but making it so would just result in "used in its own
> > initializer" (which is true).
> 
> True, but not in the sense it means; its initializer doesn't depend on its
> (uninitialized) value.

...because we're only interested in its address.

> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index f7c5d9da94b..a0afab9b26a 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
> >     /* If the initializer is constant, put it in DECL_INITIAL so we get
> >        static initialization and use in constant expressions.  */
> > -  init = maybe_constant_init (expr);
> > +  init = maybe_constant_init (expr, var);
> 
> We should also pass true for manifestly_const_eval as in store_init_value.

Done.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
We ICE on the simple:

  struct X { const X* x = this; };
  constexpr const X& x = X{};

where store_init_value initializes 'x' with

  &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>

but we must lifetime-extend via extend_ref_init_temps and we get

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;

Since 'x' was declared constexpr, we do cxx_constant_init and we hit
the preeval code added in r269003 while evaluating the INIT_EXPR:

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>

but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
find anything that could replace X and we ICE on
 7861       /* A placeholder without a referent.  We can get here when
 7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
 7863          just say it's non-constant for now.  */
 7864       gcc_assert (ctx->quiet);
because cxx_constant_init means !ctx->quiet.  It's not correct that
there isn't a referent.  I think the following patch is a pretty
straightforward fix: pass the _ZGR var down to maybe_constant_init so
that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.

The commented assert in the test doesn't pass: we complain that _ZGR1x_
isn't a constexpr variable because we don't implement DR2126 (PR101588).

	PR c++/107079

gcc/cp/ChangeLog:

	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.
---
 gcc/cp/call.cc                                | 2 +-
 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..a349d8e79db 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
 
   /* If the initializer is constant, put it in DECL_INITIAL so we get
      static initialization and use in constant expressions.  */
-  init = maybe_constant_init (expr);
+  init = maybe_constant_init (expr, var, /*manifestly_const_eval=*/true);
   /* As in store_init_value.  */
   init = cp_fully_fold (init);
   if (TREE_CONSTANT (init))
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
new file mode 100644
index 00000000000..d711b8051c0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
@@ -0,0 +1,9 @@
+// PR c++/107079
+// { dg-do compile { target c++11 } }
+
+struct X {
+  const X* x = this;
+};
+constexpr const X& x = X{};
+// TODO: The assert should pass once we implement DR2126 (c++/101588).
+//static_assert(x.x == &x);

base-commit: f6fc79d0c90ff3a924d272eff74b32656bdf5481
-- 
2.39.1

Re: [PATCH v2] c++: ICE initing lifetime-extended constexpr var [PR107079]

[Jason Merrill]

On 2/8/23 18:15, Marek Polacek wrote:
> On Wed, Feb 08, 2023 at 04:00:25PM -0800, Jason Merrill wrote:
>> On 2/8/23 13:01, Marek Polacek wrote:
>>> (This may not be a complete fix but I got stuck so I'm posting what
>>> I have, which at least fixes the ICE.)
>>>
>>> We ICE on the simple:
>>>
>>>     struct X { const X* x = this; };
>>>     constexpr const X& x = X{};
>>>
>>> where store_init_value initializes 'x' with
>>>
>>>     &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>
>>>
>>> but we must lifetime-extend via extend_ref_init_temps and we get
>>>
>>>     _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;
>>>
>>> Since 'x' was declared constexpr, we do cxx_constant_init and we hit
>>> the preeval code added in r269003 while evaluating the INIT_EXPR:
>>>
>>>     _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>
>>>
>>> but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
>>> find anything that could replace X and we ICE on
>>>    7861       /* A placeholder without a referent.  We can get here when
>>>    7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
>>>    7863          just say it's non-constant for now.  */
>>>    7864       gcc_assert (ctx->quiet);
>>> because cxx_constant_init means !ctx->quiet.  It's not correct that
>>> there isn't a referent.  I think the following patch is a pretty
>>> straightforward fix: pass the _ZGR var down to maybe_constant_init so
>>> that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.
>>>
>>> What I wasn't able to make work is the commented assert in the test.
>>> It doesn't pass because we complain that _ZGR1x_ isn't a constexpr
>>> variable,
>>
>> That sounds like we aren't (correctly) implementing
>>
>>   https://eel.is/c++draft/expr.const#4.7
> 
> Ah yes, this is DR2126 = c++/101588.  I wonder if the fix will include
> checking startswith (IDENTIFIER_POINTER (DECL_NAME (variable)), "_ZGR")
> to see if its lifetime is extended.
>   
>>> but making it so would just result in "used in its own
>>> initializer" (which is true).
>>
>> True, but not in the sense it means; its initializer doesn't depend on its
>> (uninitialized) value.
> 
> ...because we're only interested in its address.
> 
>>> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
>>> index f7c5d9da94b..a0afab9b26a 100644
>>> --- a/gcc/cp/call.cc
>>> +++ b/gcc/cp/call.cc
>>> @@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
>>>      /* If the initializer is constant, put it in DECL_INITIAL so we get
>>>         static initialization and use in constant expressions.  */
>>> -  init = maybe_constant_init (expr);
>>> +  init = maybe_constant_init (expr, var);
>>
>> We should also pass true for manifestly_const_eval as in store_init_value.
> 
> Done.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> We ICE on the simple:
> 
>    struct X { const X* x = this; };
>    constexpr const X& x = X{};
> 
> where store_init_value initializes 'x' with
> 
>    &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>
> 
> but we must lifetime-extend via extend_ref_init_temps and we get
> 
>    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;
> 
> Since 'x' was declared constexpr, we do cxx_constant_init and we hit
> the preeval code added in r269003 while evaluating the INIT_EXPR:
> 
>    _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>
> 
> but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
> find anything that could replace X and we ICE on
>   7861       /* A placeholder without a referent.  We can get here when
>   7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
>   7863          just say it's non-constant for now.  */
>   7864       gcc_assert (ctx->quiet);
> because cxx_constant_init means !ctx->quiet.  It's not correct that
> there isn't a referent.  I think the following patch is a pretty
> straightforward fix: pass the _ZGR var down to maybe_constant_init so
> that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.
> 
> The commented assert in the test doesn't pass: we complain that _ZGR1x_
> isn't a constexpr variable because we don't implement DR2126 (PR101588).
> 
> 	PR c++/107079
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.
> ---
>   gcc/cp/call.cc                                | 2 +-
>   gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C | 9 +++++++++
>   2 files changed, 10 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index f7c5d9da94b..a349d8e79db 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
>   
>     /* If the initializer is constant, put it in DECL_INITIAL so we get
>        static initialization and use in constant expressions.  */
> -  init = maybe_constant_init (expr);
> +  init = maybe_constant_init (expr, var, /*manifestly_const_eval=*/true);
>     /* As in store_init_value.  */
>     init = cp_fully_fold (init);
>     if (TREE_CONSTANT (init))
> diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> new file mode 100644
> index 00000000000..d711b8051c0
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C
> @@ -0,0 +1,9 @@
> +// PR c++/107079
> +// { dg-do compile { target c++11 } }
> +
> +struct X {
> +  const X* x = this;
> +};
> +constexpr const X& x = X{};
> +// TODO: The assert should pass once we implement DR2126 (c++/101588).
> +//static_assert(x.x == &x);
> 
> base-commit: f6fc79d0c90ff3a924d272eff74b32656bdf5481