In this problem, we are failing to properly perform copy elision with
a conditional operator, so this:
constexpr A a = true ? A{} : A{};
fails with:
error: 'A{((const A*)(&<anonymous>))}' is not a constant expression
The whole initializer is
TARGET_EXPR <D.2395, 1 ? TARGET_EXPR <D.2393, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct
A>}> : TARGET_EXPR <D.2394, {.p=(const struct A *) &<PLACEHOLDER_EXPR struct A>}>>
where the outermost TARGET_EXPR is elided, but not the nested ones.
Then we end up replacing the PLACEHOLDER_EXPRs with the temporaries the
TARGET_EXPRs represent, which is precisely what should *not* happen with
copy elision.
I've tried the approach of tweaking ctx->object, but I ran into gazillion
problems with that. I thought that I would let cxx_eval_constant_expression
/TARGET_EXPR create a new object only when ctx->object was null, then
adjust setting of ctx->object in places like cxx_bind_parameters_in_call
and cxx_eval_component_reference but that failed completely. Sometimes
ctx->object has to be reset, sometimes it cannot be reset, 'this' needed
special handling, etc. I gave up.
> But now that we have potential_prvalue_result_of, a simple but less
elegant solution is the following. I thought about setting a flag on
a TARGET_EXPR to avoid adding ctx.full_expr, but a new flag would be
overkill and using TARGET_EXPR_DIRECT_INIT_P broke things.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/105550
gcc/cp/ChangeLog:
* constexpr.cc (struct constexpr_ctx): Add a tree member.
(init_subob_ctx): Set it.
(cxx_eval_constant_expression): Don't initialize a temporary object
if potential_prvalue_result_of says true.
(cxx_eval_outermost_constant_expr): Adjust the ctx initializer. Set
ctx.full_expr.
* cp-tree.h (potential_prvalue_result_of): Declare.
* typeck2.cc (potential_prvalue_result_of): No longer static. Return
if full_expr is null.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-elision1.C: New test.
---
gcc/cp/constexpr.cc | 33 +++++++++---
gcc/cp/cp-tree.h | 1 +
gcc/cp/typeck2.cc | 4 +-
.../g++.dg/cpp1y/constexpr-elision1.C | 53 +++++++++++++++++++
4 files changed, 82 insertions(+), 9 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 45208478c3f..73880fb089e 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -1129,6 +1129,9 @@ struct constexpr_ctx {
tree ctor;
/* The object we're building the CONSTRUCTOR for. */
tree object;
+ /* The whole initializer expression. Currently only used when the whole
+ expression is a TARGET_EXPR. */
+ tree full_expr;
/* If inside SWITCH_EXPR. */
constexpr_switch_state *css_state;
/* The aggregate initialization context inside which this one is nested.
This
@@ -4700,8 +4703,11 @@ init_subob_ctx (const constexpr_ctx *ctx, constexpr_ctx
&new_ctx,
new_ctx.ctor = elt;
if (TREE_CODE (value) == TARGET_EXPR)
- /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR. */
- value = TARGET_EXPR_INITIAL (value);
+ {
+ new_ctx.full_expr = value;
+ /* Avoid creating another CONSTRUCTOR when we expand the TARGET_EXPR. */
+ value = TARGET_EXPR_INITIAL (value);
+ }
}
/* We're about to process an initializer for a class or array TYPE. Make
@@ -6822,6 +6828,14 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx,
tree t,
break;
}
gcc_checking_assert (!TARGET_EXPR_DIRECT_INIT_P (t));
+ /* This TARGET_EXPR may be nested inside another TARGET_EXPR, but
+ still serve as the initializer for the same object as the outer
+ TARGET_EXPR, as in
+ A a = true ? A{} : A{};
+ so we can't materialize a temporary. IOW, don't set ctx->object
+ to the TARGET_EXPR's slot. */
+ const bool prvalue = potential_prvalue_result_of (t, ctx->full_expr);
+ gcc_checking_assert (!prvalue || lval == vc_prvalue);
/* Avoid evaluating a TARGET_EXPR more than once. */
tree slot = TARGET_EXPR_SLOT (t);
if (tree *p = ctx->global->values.get (slot))
@@ -6831,7 +6845,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx,
tree t,
r = *p;
break;
}
- if ((AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
+ if (!prvalue && (AGGREGATE_TYPE_P (type) || VECTOR_TYPE_P (type)))
{
/* We're being expanded without an explicit target, so start
initializing a new object; expansion with an explicit target
@@ -7748,8 +7762,8 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
}
constexpr_global_ctx global_ctx;
- constexpr_ctx ctx = { &global_ctx, NULL, NULL, NULL, NULL, NULL, NULL,
- allow_non_constant, strict,
+ constexpr_ctx ctx = { &global_ctx, nullptr, nullptr, NULL_TREE, NULL_TREE,
+ NULL_TREE, nullptr, nullptr, allow_non_constant, strict,
manifestly_const_eval || !allow_non_constant };
/* Turn off -frounding-math for manifestly constant evaluation. */
@@ -7834,9 +7848,12 @@ cxx_eval_outermost_constant_expr (tree t, bool
allow_non_constant,
if (object && DECL_P (object))
global_ctx.values.put (object, ctx.ctor);
if (TREE_CODE (r) == TARGET_EXPR)
- /* Avoid creating another CONSTRUCTOR when we expand the
- TARGET_EXPR. */
- r = TARGET_EXPR_INITIAL (r);
+ {
+ ctx.full_expr = r;
+ /* Avoid creating another CONSTRUCTOR when we expand the
+ TARGET_EXPR. */
+ r = TARGET_EXPR_INITIAL (r);
+ }
}
auto_vec<tree, 16> cleanups;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index d77fd1eb8a9..c3c1f5889a3 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8160,6 +8160,7 @@ extern tree build_functional_cast (location_t,
tree, tree,
tsubst_flags_t);
extern tree add_exception_specifier (tree, tree, tsubst_flags_t);
extern tree merge_exception_specifiers (tree, tree);
+extern bool potential_prvalue_result_of (tree, tree);
/* in mangle.cc */
extern void init_mangle (void);
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1a96be3d412..6578d5ed6d2 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -1383,9 +1383,11 @@ digest_init_flags (tree type, tree init, int flags,
tsubst_flags_t complain)
FULL_EXPR is the whole expression, SUBOB is its TARGET_EXPR subobject. */
-static bool
+bool
potential_prvalue_result_of (tree subob, tree full_expr)
{
+ if (!full_expr)
+ return false;
if (subob == full_expr)
return true;
else if (TREE_CODE (full_expr) == TARGET_EXPR)
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
new file mode 100644
index 00000000000..b225ae29cde
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-elision1.C
@@ -0,0 +1,53 @@
+// PR c++/105550
+// { dg-do compile { target c++14 } }
+
+struct A {
+ const A *p = this;
+};
+
+struct B {
+ const B *p = this;
+ constexpr operator A() const { return {}; }
+};
+
+constexpr A
+bar (A)
+{
+ return {};
+}
+
+constexpr A baz() { return {}; }
+
+struct E {
+ A a1 = true ? A{} : A{};
+ A a2 = true ? A{} : B{};
+ A a3 = false ? A{} : B{};
+ A a4 = false ? B{} : B{};
+ A a5 = A{};
+ A a6 = B{};
+ A a7 = false ? B{} : (true ? A{} : A{});
+ A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+ A a9 = (A{});
+ A a10 = (true, A{});
+ A a11 = bar (A{});
+ A a12 = baz ();
+ A a13 = (A{}, A{});
+};
+
+constexpr E e{};
+
+constexpr A a1 = true ? A{} : A{};
+constexpr A a2 = true ? A{} : B{};
+constexpr A a3 = false ? A{} : B{};
+constexpr A a4 = false ? B{} : B{};
+constexpr A a5 = A{};
+constexpr A a6 = B{};
+constexpr A a7 = false ? B{} : (true ? A{} : A{});
+constexpr A a8 = false ? (true ? A{} : B{}) : (true ? A{} : A{});
+constexpr A a9 = (A{});
+constexpr A a10 = (true, A{});
+constexpr A a11 = bar (A{});
+//static_assert(a10.p == &a10, ""); // bug, 105619
+constexpr A a12 = baz ();
+//static_assert(a11.p == &a11, ""); // bug, 105619
+constexpr A a13 = (A{}, A{});
base-commit: 97dc78d705a90c1ae83c78a7f2e24942cc3a6257
