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
