On Fri, 28 Apr 2023, Patrick Palka wrote:
> After mechanically replacing RESULT_DECL within a constexpr call result
> (for sake of RVO), we can in some cases simplify the call result
> further.
>
> In the below testcase the result of get() during evaluation of a's
> initializer is the self-referential CONSTRUCTOR:
>
> {._M_p=(char *) &<retval>._M_local_buf}
>
> which after replacing RESULT_DECL with ctx->object (aka *D.2603, where
> the D.2603 temporary points to the current element of _M_elems under
> construction) becomes:
>
> {._M_p=(char *) &D.2603->_M_local_buf}
>
> but what we really want is:
>
> {._M_p=(char *) &a._M_elems[0]._M_local_buf}.
>
> so that the value of _M_p is independent of the value of the mutable
> D.2603 temporary.
>
> So to that end, it seems we should constexpr evaluate the result again
> after RESULT_DECL replacement, which is what this patch implements.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
>
> PR libstdc++/105440
>
> gcc/cp/ChangeLog:
>
> * constexpr.cc (cxx_eval_call_expression): If any RESULT_DECLs get
> replaced in the call result, try further evaluating the result.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/constexpr-dtor16.C: New test.
> ---
> gcc/cp/constexpr.cc | 12 +++++-
> gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C | 39 +++++++++++++++++++
> 2 files changed, 49 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
>
> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> index d1097764b10..22a1609e664 100644
> --- a/gcc/cp/constexpr.cc
> +++ b/gcc/cp/constexpr.cc
> @@ -3213,7 +3213,12 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> tree t,
> && CLASS_TYPE_P (TREE_TYPE (res))
> && !is_empty_class (TREE_TYPE (res)))
> if (replace_decl (&result, res, ctx->object))
> - cacheable = false;
> + {
> + cacheable = false;
> + result = cxx_eval_constant_expression (ctx, result, lval,
> + non_constant_p,
> + overflow_p);
> + }
> }
> else
> /* Couldn't get a function copy to evaluate. */
> @@ -5988,9 +5993,12 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
> object = probe;
> else
> {
> + tree orig_probe = probe;
> probe = cxx_eval_constant_expression (ctx, probe, vc_glvalue,
> non_constant_p, overflow_p);
> evaluated = true;
> + if (orig_probe == target)
> + target = probe;
Whoops, thanks to an accidental git commit --amend this patch contains
an alternative approach that I considered: in cxx_eval_store_expression,
ensure that we always set ctx->object to a fully reduced result (so
&a._M_elems[0] instead of of *D.2603 in this case), which means later
RESULT_DECL replacement with ctx->object should yield an already reduced
result as well. But with this approach I ran into a bogus "modifying
const object" error on cpp1y/constexpr-tracking-const23.C so I gave up
on it :(
Here's the correct patch:
PR libstdc++/105440
gcc/cp/ChangeLog:
* constexpr.cc (cxx_eval_call_expression): If any RESULT_DECLs get
replaced in the call result, try further evaluating the result.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/constexpr-dtor16.C: New test.
---
gcc/cp/constexpr.cc | 7 +++-
gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C | 39 +++++++++++++++++++
2 files changed, 45 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index d1097764b10..9dbbf6eec03 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -3213,7 +3213,12 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
&& CLASS_TYPE_P (TREE_TYPE (res))
&& !is_empty_class (TREE_TYPE (res)))
if (replace_decl (&result, res, ctx->object))
- cacheable = false;
+ {
+ cacheable = false;
+ result = cxx_eval_constant_expression (ctx, result, lval,
+ non_constant_p,
+ overflow_p);
+ }
}
else
/* Couldn't get a function copy to evaluate. */
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
new file mode 100644
index 00000000000..707a3e025b1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
@@ -0,0 +1,39 @@
+// PR c++/105440
+// { dg-do compile { target c++20 } }
+
+struct basic_string {
+ char _M_local_buf[32];
+ char* _M_p;
+ constexpr basic_string() : _M_p{_M_local_buf} { }
+ constexpr void f() { if (_M_p) { } }
+ constexpr ~basic_string() { if (_M_p) { } }
+};
+
+template<int N>
+struct array {
+ basic_string _M_elems[N];
+};
+
+constexpr basic_string get() { return {}; }
+
+constexpr bool f1() {
+ array<1> a{get()};
+ a._M_elems[0].f();
+
+ return true;
+}
+
+constexpr bool f2() {
+ array<2> a2{get(), get()};
+ array<3> a3{get(), get(), get()};
+
+ for (basic_string& e : a2._M_elems)
+ e.f();
+ for (basic_string& e : a3._M_elems)
+ e.f();
+
+ return true;
+}
+
+static_assert(f1());
+static_assert(f2());
--
2.40.1.445.gf85cd430b1
> if (*non_constant_p)
> return t;
> }
> @@ -6154,7 +6162,7 @@ cxx_eval_store_expression (const constexpr_ctx *ctx,
> tree t,
> if (!empty_base && !(same_type_ignoring_top_level_qualifiers_p
> (initialized_type (init), type)))
> {
> - gcc_assert (is_empty_class (TREE_TYPE (target)));
> + gcc_assert (is_empty_class (TREE_TYPE (TREE_OPERAND (t, 0))));
> empty_base = true;
> }
>
> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
> b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
> new file mode 100644
> index 00000000000..707a3e025b1
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor16.C
> @@ -0,0 +1,39 @@
> +// PR c++/105440
> +// { dg-do compile { target c++20 } }
> +
> +struct basic_string {
> + char _M_local_buf[32];
> + char* _M_p;
> + constexpr basic_string() : _M_p{_M_local_buf} { }
> + constexpr void f() { if (_M_p) { } }
> + constexpr ~basic_string() { if (_M_p) { } }
> +};
> +
> +template<int N>
> +struct array {
> + basic_string _M_elems[N];
> +};
> +
> +constexpr basic_string get() { return {}; }
> +
> +constexpr bool f1() {
> + array<1> a{get()};
> + a._M_elems[0].f();
> +
> + return true;
> +}
> +
> +constexpr bool f2() {
> + array<2> a2{get(), get()};
> + array<3> a3{get(), get(), get()};
> +
> + for (basic_string& e : a2._M_elems)
> + e.f();
> + for (basic_string& e : a3._M_elems)
> + e.f();
> +
> + return true;
> +}
> +
> +static_assert(f1());
> +static_assert(f2());
> --
> 2.40.1.445.gf85cd430b1
>
>
