Re: [PATCH] c++: constant non-copy-init is manifestly constant [PR108243]

[Patrick Palka <ppalka@redhat.com>]

On Mon, 20 Feb 2023, Patrick Palka wrote:

> According to [basic.start.static]/2 and [expr.const]/2, a variable
> with static storage duration initialized with a constant initializer
> has constant initialization, and such an initializer is manifestly
> constant-evaluated.
> 
> We're already getting this right with copy initialization because in
> that case check_initializer would consistently call store_init_value
> (which for TREE_STATIC variables calls fold_non_dependent_init with
> m_c_e=true).
> 
> But for direct (or default) initialization, we don't always call
> store_init_value.  We instead however always call maybe_constant_init
> from expand_default_init[1], albeit with m_c_e=false which means we
> don't always get the "manifestly constant-evaluated" part right for
> copy-init.
> 
> This patch fixes this by simply passing m_c_e=true to this call to
> maybe_constant_init for static storage duration variables, mirroring
> what store_init_value basically does.
> 
> [1]: this maybe_constant_init call isn't reached in the copy-init
> case because there init is a CONSTRUCTOR rather than a TREE_LIST so
> expand_default_init exits early returning an INIT_EXPR.  This INIT_EXPR
> is ultimately what causes us to consistently hit the store_init_value
> code path from check_initializer in the copy-init case.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?  Would it be suitable to backport this to the 12 branch since
> it should only affect C++20 code?
> 
> 	PR c++/108243
> 
> gcc/cp/ChangeLog:
> 
> 	* init.cc (expand_default_init): Pass m_c_e=true instead of
> 	=false to maybe_constant_init when initializing a variable
> 	with static storage duration.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/is-constant-evaluated14.C: New test.
> ---
>  gcc/cp/init.cc                                |   5 +-
>  .../g++.dg/cpp2a/is-constant-evaluated14.C    | 140 ++++++++++++++++++
>  2 files changed, 144 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C
> 
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index 52e96fbe590..705a5b3bdb6 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -2203,7 +2203,10 @@ expand_default_init (tree binfo, tree true_exp, tree exp, tree init, int flags,
>        tree fn = get_callee_fndecl (rval);
>        if (fn && DECL_DECLARED_CONSTEXPR_P (fn))
>  	{
> -	  tree e = maybe_constant_init (rval, exp);
> +	  bool manifestly_const_eval = false;
> +	  if (VAR_P (exp) && TREE_STATIC (exp))
> +	    manifestly_const_eval = true;
> +	  tree e = maybe_constant_init (rval, exp, manifestly_const_eval);
>  	  if (TREE_CONSTANT (e))
>  	    rval = cp_build_init_expr (exp, e);
>  	}

Hmm, alternatively we could just override manifestly_const_eval to true
from maybe_constant_init for static storage duration variables, like so.
I guess this approach much be preferable since it potentially benefits
all maybe_constant_init callers?

-- >8 --

gcc/cp/ChangeLog:

	* constexpr.cc (maybe_constant_init_1): Override
	manifestly_const_eval to true if is_static.

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index aa2c14355f8..8ae83a6eadf 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8760,7 +8760,8 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
       bool is_static = (decl && DECL_P (decl)
 			&& (TREE_STATIC (decl) || DECL_EXTERNAL (decl)));
       t = cxx_eval_outermost_constant_expr (t, allow_non_constant, !is_static,
-					    mce_value (manifestly_const_eval),
+					    (is_static ? mce_true
+					     : mce_value (manifestly_const_eval)),
 					    false, decl);
     }
   if (TREE_CODE (t) == TARGET_EXPR)

> diff --git a/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C
> new file mode 100644
> index 00000000000..365bca3fd9a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/is-constant-evaluated14.C
> @@ -0,0 +1,140 @@
> +// PR c++/108243
> +// Verify a variable with static storage duration initialized with a
> +// constant initializer has constant initialization, and the initializer
> +// is manifestly constant-evaluated.
> +// { dg-do run { target c++11 } }
> +// { dg-additional-options "-fdump-tree-original" }
> +
> +#include <initializer_list>
> +
> +struct A {
> +  constexpr A(int n) : n(n), m(__builtin_is_constant_evaluated()) { }
> +  constexpr A() : A(42) { }
> +  void verify_mce() const {
> +    if (m != 1) __builtin_abort();
> +  }
> +  int n;
> +  int m;
> +};
> +
> +A a1 = {42};
> +A a2{42};
> +A a3(42);
> +A a4;
> +A a5{};
> +
> +void f() {
> +  static A a1 = {42};
> +  static A a2{42};
> +  static A a3(42);
> +  static A a4;
> +  static A a5{};
> +  for (auto& a : {a1, a2, a3, a4, a5})
> +    a.verify_mce();
> +}
> +
> +template<int... N>
> +void g() {
> +  static A a1 = {42};
> +  static A a2{42};
> +  static A a3(42);
> +  static A a4;
> +  static A a5{};
> +  static A a6 = {N...};
> +  static A a7{N...};
> +  static A a8(N...);
> +  for (auto& a : {a1, a2, a3, a4, a5, a6, a7, a8})
> +    a.verify_mce();
> +}
> +
> +struct B {
> +  static A a1;
> +  static A a2;
> +  static A a3;
> +  static A a4;
> +  static A a5;
> +  static void verify_mce() {
> +    for (auto& a : {a1, a2, a3, a4, a5})
> +      a.verify_mce();
> +  }
> +};
> +
> +A B::a1 = {42};
> +A B::a2{42};
> +A B::a3(42);
> +A B::a4;
> +A B::a5{};
> +
> +template<int... N>
> +struct BT {
> +  static A a1;
> +  static A a2;
> +  static A a3;
> +  static A a4;
> +  static A a5;
> +  static A a6;
> +  static A a7;
> +  static A a8;
> +  static void verify_mce() {
> +    for (auto& a : {a1, a2, a3, a4, a5})
> +      a.verify_mce();
> +  }
> +};
> +
> +template<int... N> A BT<N...>::a1 = {42};
> +template<int... N> A BT<N...>::a2{42};
> +template<int... N> A BT<N...>::a3(42);
> +template<int... N> A BT<N...>::a4;
> +template<int... N> A BT<N...>::a5{};
> +template<int... N> A BT<N...>::a6 = {N...};
> +template<int... N> A BT<N...>::a7{N...};
> +template<int... N> A BT<N...>::a8(N...);
> +
> +#if __cpp_inline_variables
> +struct BI {
> +  static inline A a1 = {42};
> +  static inline A a2{42};
> +  static inline A a3;
> +  static inline A a4{};
> +  static void verify_mce() {
> +    for (auto& a : {a1, a2, a3, a4})
> +      a.verify_mce();
> +  }
> +};
> +
> +template<int... N>
> +struct BIT {
> +  static inline A a1 = {42};
> +  static inline A a2{42};
> +  static inline A a3;
> +  static inline A a4{};
> +  static inline A a5 = {N...};
> +  static inline A a6{N...};
> +  static void verify_mce() {
> +    for (auto& a : {a1, a2, a3, a4, a5, a6})
> +      a.verify_mce();
> +  }
> +};
> +#endif
> +
> +int main() {
> +  for (auto& a : {a1, a2, a3, a4, a5})
> +    a.verify_mce();
> +
> +  f();
> +  g<42>();
> +  g<>();
> +
> +  B::verify_mce();
> +  BT<42>::verify_mce();
> +  BT<>::verify_mce();
> +
> +#if __cpp_inline_variables
> +  BI::verify_mce();
> +  BIT<42>::verify_mce();
> +  BIT<>::verify_mce();
> +#endif
> +}
> +
> +// { dg-final { scan-tree-dump-not "static initializers for" "original" } }
> +// { dg-final { scan-tree-dump-not "cxa_guard_acquire" "original" } }
> -- 
> 2.39.2.501.gd9d677b2d8
> 
>