On 2/9/23 09:39, Marek Polacek wrote:
In constexpr-nsdmi3.C, with -fno-elide-constructors, we don't elide
the Y::Y(const Y&) call used to initialize o.c. So store_init_value
-> cxx_constant_init must constexpr-evaluate the call to Y::Y(const Y&)
in cxx_eval_call_expression. It's a trivial function, so we do the
"Shortcut trivial constructor/op=" code and rather than evaluating
the function, we just create an assignment
o.c = *(const struct Y &) (const struct Y *) &(&<PLACEHOLDER_EXPR struct
X>)->b
which is a MODIFY_EXPR, so the preeval code in cxx_eval_store_expression
clears .ctor and .object, therefore we can't replace the PLACEHOLDER_EXPR
whereupon we crash at
/* A placeholder without a referent. We can get here when
checking whether NSDMIs are noexcept, or in massage_init_elt;
just say it's non-constant for now. */
gcc_assert (ctx->quiet);
The PLACEHOLDER_EXPR can also be on the LHS as in constexpr-nsdmi10.C.
I don't think we can do much here, but I noticed that the whole
trivial_fn_p (fun) block is only entered when -fno-elide-constructors.
This is true since GCC 9; it wasn't easy to bisect what changes made it
so, but r240845 is probably one of them. -fno-elide-constructors is an
option for experiments only so it's not clear to me why we'd still want
to shortcut trivial constructor/op=. I propose to remove the code and
add a checking assert to make sure we're not getting a trivial_fn_p
unless -fno-elide-constructors.
Hmm, trivial op= doesn't ever hit this code?
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? I don't
think I want to backport this.
PR c++/101073
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): Replace shortcutting trivial
constructor/op= with a checking assert.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-nsdmi3.C: New test.
* g++.dg/cpp1y/constexpr-nsdmi10.C: New test.
---
gcc/cp/constexpr.cc | 25 +++----------------
gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C | 17 +++++++++++++
.../g++.dg/cpp1y/constexpr-nsdmi10.C | 18 +++++++++++++
3 files changed, 38 insertions(+), 22 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 564766c8a00..1d53dcf0f20 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -2865,28 +2865,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
ctx = &new_ctx;
}
- /* Shortcut trivial constructor/op=. */
- if (trivial_fn_p (fun))
- {
- tree init = NULL_TREE;
- if (call_expr_nargs (t) == 2)
- init = convert_from_reference (get_nth_callarg (t, 1));
- else if (TREE_CODE (t) == AGGR_INIT_EXPR
- && AGGR_INIT_ZERO_FIRST (t))
- init = build_zero_init (DECL_CONTEXT (fun), NULL_TREE, false);
- if (init)
- {
- tree op = get_nth_callarg (t, 0);
- if (is_dummy_object (op))
- op = ctx->object;
- else
- op = build1 (INDIRECT_REF, TREE_TYPE (TREE_TYPE (op)), op);
- tree set = build2 (MODIFY_EXPR, TREE_TYPE (op), op, init);
I think the problem is using MODIFY_EXPR instead of INIT_EXPR to
represent a constructor; that's why cxx_eval_store_expression thinks
it's OK to preevaluate. This should properly use those two tree codes
for op= and ctor, respectively.
- new_ctx.call = &new_call;
- return cxx_eval_constant_expression (&new_ctx, set, lval,
- non_constant_p, overflow_p);
- }
- }
+ /* We used to shortcut trivial constructor/op= here, but nowadays
+ we can only get a trivial function here with -fno-elide-constructors. */
+ gcc_checking_assert (!trivial_fn_p (fun) || !flag_elide_constructors);
...but if this optimization is so rarely triggered, this simplification
is OK too.
bool non_constant_args = false;
new_call.bindings
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
new file mode 100644
index 00000000000..ec1c4e53387
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi3.C
@@ -0,0 +1,17 @@
+// PR c++/101073
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fno-elide-constructors" }
+
+struct Y
+{
+ int a;
+};
+
+struct X
+{
+ Y b = Y{1};
+ Y c = this->b;
+};
+
+constexpr X o = { };
+static_assert(o.b.a == 1 && o.c.a == 1, "");
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
new file mode 100644
index 00000000000..35cb8acc15b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-nsdmi10.C
@@ -0,0 +1,18 @@
+// PR c++/101073
+// { dg-do compile { target c++14 } }
+// { dg-additional-options "-fno-elide-constructors" }
+// A copy of constexpr-nsdmi9.C.
+
+struct Y
+{
+ int a;
+};
+
+struct X
+{
+ Y b = (c={5});
+ Y c = (b={1});
+};
+
+constexpr X o = { };
+static_assert(o.b.a == 1 && o.c.a == 1, "");
base-commit: b24e9c083093a9e1b1007933a184c02f7ff058db
