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

[Jason Merrill <jason@redhat.com>]

On 2/21/23 15:18, Patrick Palka wrote:
> 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?

That does look better.

OK (perhaps with a local variable to hold the mce_value).

> -- >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
>>
>>
>