On Mon, 27 Apr 2020, Patrick Palka wrote:
> On Mon, 27 Apr 2020, Jason Merrill wrote:
>
> > On 4/26/20 6:48 PM, Patrick Palka wrote:
> > > In the testcase below, the call to the target constructor foo{} from foo's
> > > delegating constructor is encoded as the INIT_EXPR
> > >
> > > *(struct foo *) this = AGGR_INIT_EXPR <4, __ct_comp, D.2140, ...>;
> > >
> > > During initialization of the variable 'bar', we prematurely set
> > > TREE_READONLY on
> > > bar's CONSTRUCTOR in two places before the outer delegating constructor
> > > has
> > > returned: first, at the end of cxx_eval_call_expression after evaluating
> > > the
> > > RHS
> > > of the above INIT_EXPR, and second, at the end of
> > > cxx_eval_store_expression
> > > after having finished evaluating the above INIT_EXPR. This then prevents
> > > the
> > > rest of the outer delegating constructor from mutating 'bar'.
> > >
> > > This (hopefully minimally risky) patch makes cxx_eval_call_expression
> > > refrain
> > > from setting TREE_READONLY when evaluating the target constructor of a
> > > delegating constructor. It also makes cxx_eval_store_expression refrain
> > > from
> > > setting TREE_READONLY when the object being initialized is "*this', on the
> > > basis
> > > that it should be the responsibility of the routine that set 'this' in the
> > > first
> > > place to set the object's TREE_READONLY appropriately.
> > >
> > > Passes 'make check-c++', does this look OK to commit after full
> > > bootstrap/regtest?
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > PR c++/94772
> > > * constexpr.c (cxx_eval_call_expression): Don't set new_obj if we're
> > > evaluating the target constructor of a delegating constructor.
> > > (cxx_eval_store_expression): Don't set TREE_READONLY if the LHS of the
> > > INIT_EXPR is '*this'.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > PR c++/94772
> > > * g++.dg/cpp1y/constexpr-tracking-const23.C: New test.
> > > ---
> > > gcc/cp/constexpr.c | 29 +++++++++++++++----
> > > .../g++.dg/cpp1y/constexpr-tracking-const23.C | 21 ++++++++++++++
> > > 2 files changed, 45 insertions(+), 5 deletions(-)
> > > create mode 100644
> > > gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
> > >
> > > diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> > > index 6b3e514398b..a9ddd861195 100644
> > > --- a/gcc/cp/constexpr.c
> > > +++ b/gcc/cp/constexpr.c
> > > @@ -2367,10 +2367,20 @@ cxx_eval_call_expression (const constexpr_ctx
> > > *ctx,
> > > tree t,
> > > /* In a constructor, it should be the first `this' argument.
> > > At this point it has already been evaluated in the call
> > > to cxx_bind_parameters_in_call. */
> > > - new_obj = TREE_VEC_ELT (new_call.bindings, 0);
> > > - STRIP_NOPS (new_obj);
> > > - if (TREE_CODE (new_obj) == ADDR_EXPR)
> > > - new_obj = TREE_OPERAND (new_obj, 0);
> > > +
> > > + if (ctx->call && ctx->call->fundef
> > > + && DECL_CONSTRUCTOR_P (ctx->call->fundef->decl)
> > > + && (TREE_VEC_ELT (ctx->call->bindings, 0)
> > > + == TREE_VEC_ELT (new_call.bindings, 0)))
> > > + /* We're calling the target constructor of a delegating constructor,
> > > so
> > > + there is no new object. */;
> > > + else
> > > + {
> > > + new_obj = TREE_VEC_ELT (new_call.bindings, 0);
> > > + STRIP_NOPS (new_obj);
> > > + if (TREE_CODE (new_obj) == ADDR_EXPR)
> > > + new_obj = TREE_OPERAND (new_obj, 0);
> > > + }
> > > }
> > > tree result = NULL_TREE;
> > > @@ -4950,7 +4960,16 @@ cxx_eval_store_expression (const constexpr_ctx
> > > *ctx,
> > > tree t,
> > > if (TREE_CODE (t) == INIT_EXPR
> > > && TREE_CODE (*valp) == CONSTRUCTOR
> > > && TYPE_READONLY (type))
> > > - TREE_READONLY (*valp) = true;
> > > + {
> > > + if (INDIRECT_REF_P (target)
> > > + && (is_this_parameter
> > > + (tree_strip_nop_conversions (TREE_OPERAND (target, 0)))))
> > > + /* We've just initialized '*this' (perhaps via the target constructor
> > > of
> > > + a delegating constructor). Leave it up to the caller that set
> > > 'this'
> > > + to set TREE_READONLY appropriately. */;
> >
> > Let's checking_assert that target and *this are
> > same_type_ignoring_top_level_qualifiers_p.
>
> Like this? Bootstrap and regtest in progress.
>
> -- >8 --
>
> gcc/cp/ChangeLog:
>
> PR c++/94772
> * constexpr.c (cxx_eval_call_expression): Don't set new_obj if we're
> evaluating the target constructor of a delegating constructor.
> (cxx_eval_store_expression): Don't set TREE_READONLY if the LHS of the
> INIT_EXPR is '*this'.
>
> gcc/testsuite/ChangeLog:
>
> PR c++/94772
> * g++.dg/cpp1y/constexpr-tracking-const23.C: New test.
> ---
> gcc/cp/constexpr.c | 31 ++++++++++++++++---
> .../g++.dg/cpp1y/constexpr-tracking-const23.C | 21 +++++++++++++
> 2 files changed, 47 insertions(+), 5 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
>
> diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
> index 6b3e514398b..c7923897e23 100644
> --- a/gcc/cp/constexpr.c
> +++ b/gcc/cp/constexpr.c
> @@ -2367,10 +2367,20 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> /* In a constructor, it should be the first `this' argument.
> At this point it has already been evaluated in the call
> to cxx_bind_parameters_in_call. */
> - new_obj = TREE_VEC_ELT (new_call.bindings, 0);
> - STRIP_NOPS (new_obj);
> - if (TREE_CODE (new_obj) == ADDR_EXPR)
> - new_obj = TREE_OPERAND (new_obj, 0);
> +
> + if (ctx->call && ctx->call->fundef
> + && DECL_CONSTRUCTOR_P (ctx->call->fundef->decl)
> + && (TREE_VEC_ELT (ctx->call->bindings, 0)
> + == TREE_VEC_ELT (new_call.bindings, 0)))
> + /* We're calling the target constructor of a delegating constructor, so
> + there is no new object. */;
Further experimentation revealed that testing the 'this' arguments for
pointer equality here is too strict because the target constructor could
belong to a base class, in which case its 'this' argument would be
(base *)&bar instead of (foo *)&bar, as in the new testcase below.
Fixed by comparing the objects pointed to by the 'this' arguments more
directly. Bootstrap and regtest is in progress..
-- >8 --
gcc/cp/ChangeLog:
PR c++/94772
* constexpr.c (cxx_eval_call_expression): Don't set new_obj if we're
evaluating the target constructor of a delegating constructor.
(cxx_eval_store_expression): Don't set TREE_READONLY if the LHS of the
INIT_EXPR is '*this'.
gcc/testsuite/ChangeLog:
PR c++/94772
* g++.dg/cpp1y/constexpr-tracking-const23.C: New test.
* g++.dg/cpp1y/constexpr-tracking-const24.C: New test.
---
gcc/cp/constexpr.c | 26 ++++++++++++++++++-
.../g++.dg/cpp1y/constexpr-tracking-const23.C | 21 +++++++++++++++
.../g++.dg/cpp1y/constexpr-tracking-const24.C | 26 +++++++++++++++++++
3 files changed, 72 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const24.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 6b3e514398b..5d9b10c63d4 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2371,6 +2371,19 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
STRIP_NOPS (new_obj);
if (TREE_CODE (new_obj) == ADDR_EXPR)
new_obj = TREE_OPERAND (new_obj, 0);
+
+ if (ctx->call && ctx->call->fundef
+ && DECL_CONSTRUCTOR_P (ctx->call->fundef->decl))
+ {
+ tree cur_obj = TREE_VEC_ELT (ctx->call->bindings, 0);
+ STRIP_NOPS (cur_obj);
+ if (TREE_CODE (cur_obj) == ADDR_EXPR)
+ cur_obj = TREE_OPERAND (cur_obj, 0);
+ if (new_obj == cur_obj)
+ /* We're calling the target constructor of a delegating constructor,
+ so there is no new object. */
+ new_obj = NULL_TREE;
+ }
}
tree result = NULL_TREE;
@@ -4950,7 +4963,18 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
tree t,
if (TREE_CODE (t) == INIT_EXPR
&& TREE_CODE (*valp) == CONSTRUCTOR
&& TYPE_READONLY (type))
- TREE_READONLY (*valp) = true;
+ {
+ if (INDIRECT_REF_P (target)
+ && (is_this_parameter
+ (tree_strip_nop_conversions (TREE_OPERAND (target, 0)))))
+ /* We've just initialized '*this' (perhaps via the target constructor of
+ a delegating constructor). Leave it up to the caller that set 'this'
+ to set TREE_READONLY appropriately. */
+ gcc_checking_assert (same_type_ignoring_top_level_qualifiers_p
+ (TREE_TYPE (target), type));
+ else
+ TREE_READONLY (*valp) = true;
+ }
/* Update TREE_CONSTANT and TREE_SIDE_EFFECTS on enclosing
CONSTRUCTORs, if any. */
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
new file mode 100644
index 00000000000..c6643c78a6f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const23.C
@@ -0,0 +1,21 @@
+// PR c++/94772
+// { dg-do compile { target c++14 } }
+
+struct foo
+{
+ int x{};
+
+ constexpr foo() noexcept = default;
+
+ constexpr foo(int a) : foo{}
+ { x = -a; }
+
+ constexpr foo(int a, int b) : foo{a}
+ { x += a + b; }
+};
+
+int main()
+{
+ constexpr foo bar{1, 2};
+ static_assert(bar.x == 2, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const24.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const24.C
new file mode 100644
index 00000000000..2c923f69cf4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-tracking-const24.C
@@ -0,0 +1,26 @@
+// PR c++/94772
+// { dg-do compile { target c++14 } }
+
+struct base
+{
+ base() = default;
+
+ constexpr base(int) : base{} { }
+};
+
+struct foo : base
+{
+ int x{};
+
+ constexpr foo(int a) : base{a}
+ { x = -a; }
+
+ constexpr foo(int a, int b) : foo{a}
+ { x += a + b; }
+};
+
+int main()
+{
+ constexpr foo bar{1, 2};
+ static_assert(bar.x == 2, "");
+}
--
2.26.2.266.ge870325ee8
