Full disable of C++20 time zone support

Full disable of C++20 time zone support

[Alexey Lapshin]

Hi!

I faced an issue with compiling libstdc++-v3/src/c++20/tzdb.cc:


[ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
[ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'


Any plans to create a configure option to disable tzdb from build? Or at least make it compilable?

Tried --with-libstdcxx-zoneinfo=no but it does not take effect, because it does not cover line with the bug.

For now using workaround with changes in libstdc++-v3/src/c++20/Makefile*

Thanks

Re: Full disable of C++20 time zone support

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 880 bytes --]

On Wed, 28 Jun 2023 at 19:42, Alexey Lapshin wrote:

> Hi!
>
> I faced an issue with compiling libstdc++-v3/src/c++20/tzdb.cc:
>
>
> [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: 'constinit'
> variable 'std::chrono::{anonymous}::list_mutex' does not have a constant
> initializer
> [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: call to
> non-'constexpr' function 'std::mutex::mutex()'
>
>
Which target are you building for?



>
> Any plans to create a configure option to disable tzdb from build? Or at
> least make it compilable?
>
> Tried --with-libstdcxx-zoneinfo=no but it does not take effect, because it
> does not cover line with the bug.
>

Well then we should fix that. We don't want *another* configure option when
we already have two for tzdb.



>
> For now using workaround with changes in libstdc++-v3/src/c++20/Makefile*
>
> Thanks
>

Re: Full disable of C++20 time zone support

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 1376 bytes --]

On Wed, 28 Jun 2023 at 20:33, Jonathan Wakely <jwakely@redhat.com> wrote:

> On Wed, 28 Jun 2023 at 19:42, Alexey Lapshin wrote:
>
>> Hi!
>>
>> I faced an issue with compiling libstdc++-v3/src/c++20/tzdb.cc:
>>
>>
>> [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: 'constinit'
>> variable 'std::chrono::{anonymous}::list_mutex' does not have a constant
>> initializer
>> [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: call to
>> non-'constexpr' function 'std::mutex::mutex()'
>>
>>
> Which target are you building for?
>
>
>
>>
>> Any plans to create a configure option to disable tzdb from build? Or at
>> least make it compilable?
>>
>> Tried --with-libstdcxx-zoneinfo=no but it does not take effect, because
>> it does not cover line with the bug.
>>
>
> Well then we should fix that. We don't want *another* configure option
> when we already have two for tzdb.
>

I think this should fix it:

--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -43,8 +43,8 @@

 #ifndef __GTHREADS
 # define USE_ATOMIC_SHARED_PTR 0
-#elif _WIN32
-// std::mutex cannot be constinit, so Windows must use
atomic<shared_ptr<>>.
+#elif ! defined __GTHREAD_MUTEX_INIT
+// std::mutex cannot be constinit, so must use atomic<shared_ptr<>>.
 # define USE_ATOMIC_SHARED_PTR 1
 #elif ATOMIC_POINTER_LOCK_FREE < 2
 # define USE_ATOMIC_SHARED_PTR 0

Re: Full disable of C++20 time zone support

[Alexey Lapshin]

> Which target are you building for?

riscv32-esp-elf

build with gcc v4.9.2

> I think this should fix it:

This brings next errors:


gcc/libstdc++-v3/src/c++20/tzdb.cc:157:27: error: 'list_mutex' was not declared in this scope
gcc/libstdc++-v3/src/c++20/tzdb.cc:158:28: error: 'using std::chrono::tzdb_list::_Node::head_ptr = class std::atomic<std::shared_ptr<std::chrono::tzdb_list::_Node> >' {aka 'class std::atomic<std::shared_ptr<std::chrono::tzdb_list::_Node> >'} has no member named 'get'



Why it could not be disabled at all? like this:


--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -22,6 +22,8 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#ifndef TZDB_DISABLED
+
 // The -Wabi warnings in this file are all for non-exported symbols.
 #pragma GCC diagnostic ignored "-Wabi"
 
@@ -2037,3 +2039,5 @@ namespace std::chrono
   } // namespace
 #endif // TZDB_DISABLED
 } // namespace std::chrono
+
+#endif



On Wed, 2023-06-28 at 20:40 +0100, Jonathan Wakely wrote:
> [External: This email originated outside Espressif]
> 
> 
> On Wed, 28 Jun 2023 at 20:33, Jonathan Wakely <jwakely@redhat.com> wrote:
> > On Wed, 28 Jun 2023 at 19:42, Alexey Lapshin wrote:
> > > Hi!
> > > 
> > > I faced an issue with compiling libstdc++-v3/src/c++20/tzdb.cc:
> > > 
> > > 
> > > [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
> > > [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'
> > > 
> > > 
> > 
> > 
> > Which target are you building for?
> > 
> >  
> > > 
> > > Any plans to create a configure option to disable tzdb from build? Or at least make it compilable?
> > > 
> > > Tried --with-libstdcxx-zoneinfo=no but it does not take effect, because it does not cover line with the bug.
> > > 
> > 
> > 
> > Well then we should fix that. We don't want *another* configure option when we already have two for tzdb.
> > 
> 
> 
> I think this should fix it:
> 
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -43,8 +43,8 @@
>  
>  #ifndef __GTHREADS
>  # define USE_ATOMIC_SHARED_PTR 0
> -#elif _WIN32
> -// std::mutex cannot be constinit, so Windows must use atomic<shared_ptr<>>.
> +#elif ! defined __GTHREAD_MUTEX_INIT
> +// std::mutex cannot be constinit, so must use atomic<shared_ptr<>>.
>  # define USE_ATOMIC_SHARED_PTR 1
>  #elif ATOMIC_POINTER_LOCK_FREE < 2
>  # define USE_ATOMIC_SHARED_PTR 0
> 
>  

Re: Full disable of C++20 time zone support

[Jonathan Wakely]

On Thu, 29 Jun 2023 at 10:16, Alexey Lapshin via Gcc-help
<gcc-help@gcc.gnu.org> wrote:
>
> > Which target are you building for?
>
> riscv32-esp-elf
>
> build with gcc v4.9.2
>
> > I think this should fix it:
>
> This brings next errors:
>
>
> gcc/libstdc++-v3/src/c++20/tzdb.cc:157:27: error: 'list_mutex' was not declared in this scope
> gcc/libstdc++-v3/src/c++20/tzdb.cc:158:28: error: 'using std::chrono::tzdb_list::_Node::head_ptr = class std::atomic<std::shared_ptr<std::chrono::tzdb_list::_Node> >' {aka 'class std::atomic<std::shared_ptr<std::chrono::tzdb_list::_Node> >'} has no member named 'get'
>
>
>
> Why it could not be disabled at all? like this:
>
>
> --- a/libstdc++-v3/src/c++20/tzdb.cc
> +++ b/libstdc++-v3/src/c++20/tzdb.cc
> @@ -22,6 +22,8 @@
>  // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>  // <http://www.gnu.org/licenses/>.
>
> +#ifndef TZDB_DISABLED

That macro isn't even defined until later in the file, so would never
be defined at this point.

But more importantly, that would give linker errors when including
<chrono>. The point of that macro is to disable trying to process
tzdata, not to create a broken libstdc++ with missing parts of the
C++20 library. It should always be possible to call
std::chrono::get_tzdb() even if the tzdb only contains the UTC time
zone.

> +
>  // The -Wabi warnings in this file are all for non-exported symbols.
>  #pragma GCC diagnostic ignored "-Wabi"
>
> @@ -2037,3 +2039,5 @@ namespace std::chrono
>    } // namespace
>  #endif // TZDB_DISABLED
>  } // namespace std::chrono
> +
> +#endif
>
>
>
> On Wed, 2023-06-28 at 20:40 +0100, Jonathan Wakely wrote:
> > [External: This email originated outside Espressif]
> >
> >
> > On Wed, 28 Jun 2023 at 20:33, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > On Wed, 28 Jun 2023 at 19:42, Alexey Lapshin wrote:
> > > > Hi!
> > > >
> > > > I faced an issue with compiling libstdc++-v3/src/c++20/tzdb.cc:
> > > >
> > > >
> > > > [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
> > > > [ERROR]    gcc/libstdc++-v3/src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'
> > > >
> > > >
> > >
> > >
> > > Which target are you building for?
> > >
> > >
> > > >
> > > > Any plans to create a configure option to disable tzdb from build? Or at least make it compilable?
> > > >
> > > > Tried --with-libstdcxx-zoneinfo=no but it does not take effect, because it does not cover line with the bug.
> > > >
> > >
> > >
> > > Well then we should fix that. We don't want *another* configure option when we already have two for tzdb.
> > >
> >
> >
> > I think this should fix it:
> >
> > --- a/libstdc++-v3/src/c++20/tzdb.cc
> > +++ b/libstdc++-v3/src/c++20/tzdb.cc
> > @@ -43,8 +43,8 @@
> >
> >  #ifndef __GTHREADS
> >  # define USE_ATOMIC_SHARED_PTR 0
> > -#elif _WIN32
> > -// std::mutex cannot be constinit, so Windows must use atomic<shared_ptr<>>.
> > +#elif ! defined __GTHREAD_MUTEX_INIT
> > +// std::mutex cannot be constinit, so must use atomic<shared_ptr<>>.
> >  # define USE_ATOMIC_SHARED_PTR 1
> >  #elif ATOMIC_POINTER_LOCK_FREE < 2
> >  # define USE_ATOMIC_SHARED_PTR 0
> >
> >
>

Re: Full disable of C++20 time zone support

[Alexey Lapshin]

On Thu, 2023-06-29 at 10:23 +0100, Jonathan Wakely wrote:
> That macro isn't even defined until later in the file, so would never
> be defined at this point.

Was a simplified example.


Why do you still need to have some mutexes or whatever in case TZDB_DISABLED?

> It should always be possible to call
std::chrono::get_tzdb() even if the tzdb only contains the UTC time
zone.

Just return NULL in case TZDB_DISABLED?

Thanks

Re: Full disable of C++20 time zone support

[Jonathan Wakely]

On Thu, 29 Jun 2023 at 10:27, Alexey Lapshin
<alexey.lapshin@espressif.com> wrote:
>
> On Thu, 2023-06-29 at 10:23 +0100, Jonathan Wakely wrote:
> > That macro isn't even defined until later in the file, so would never
> > be defined at this point.
>
> Was a simplified example.
>
>
> Why do you still need to have some mutexes or whatever in case TZDB_DISABLED?

Because there is still a global object that needs to be lazily
constructed, and multiple threads can try to access it concurrently.

> > It should always be possible to call
> std::chrono::get_tzdb() even if the tzdb only contains the UTC time
> zone.
>
> Just return NULL in case TZDB_DISABLED?

It returns a reference.

As I said (and you quoted above), the intention is to return a valid
object that contains the UTC time zone. That is a graceful failure for
missing tzdata, not a crash.



>
> Thanks

Re: Full disable of C++20 time zone support

[Jonathan Wakely]

[-- Attachment #1: Type: text/plain, Size: 627 bytes --]

On Thu, 29 Jun 2023 at 10:39, Jonathan Wakely wrote:
>
> On Thu, 29 Jun 2023 at 10:27, Alexey Lapshin
> <alexey.lapshin@espressif.com> wrote:
> >
> > On Thu, 2023-06-29 at 10:23 +0100, Jonathan Wakely wrote:
> > > That macro isn't even defined until later in the file, so would never
> > > be defined at this point.
> >
> > Was a simplified example.
> >
> >
> > Why do you still need to have some mutexes or whatever in case TZDB_DISABLED?
>
> Because there is still a global object that needs to be lazily
> constructed, and multiple threads can try to access it concurrently.

The attached patch should work for your target.

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 5030 bytes --]

commit 881299967d8778e29f803d8801af9a0b1a5bd1bd
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jun 29 11:40:32 2023

    libstdc++: Fix src/c++20/tzdb.cc for non-constexpr std::mutex
    
    Building libstdc++ reportedly fails for targets without lock-free
    std::atomic<T*> which don't define __GTHREAD_MUTEX_INIT:
    
    src/c++20/tzdb.cc:110:21: error: 'constinit' variable 'std::chrono::{anonymous}::list_mutex' does not have a constant initializer
    src/c++20/tzdb.cc:110:21: error: call to non-'constexpr' function 'std::mutex::mutex()'
    
    The solution implemented by this commit is to use a local static mutex
    when it can't be constinit, so that it's constructed on first use.
    
    With this change, we can also simplify the preprocessor logic for
    defining USE_ATOMIC_SHARED_PTR. It now depends on the same conditions as
    USE_ATOMIC_LIST_HEAD, so in theory we could have a single macro. Keeping
    them separate would allow us to replace the use of atomic<shared_ptr<T>>
    with a mutex if that performs better, without having to give up on the
    lock-free cache for fast access to the list head.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++20/tzdb.cc (USE_ATOMIC_SHARED_PTR): Define consistently
            with USE_ATOMIC_LIST_HEAD.
            (list_mutex): Replace global object with function. Use local
            static object when std::mutex constructor isn't constexpr.

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index a43b4f33eba..8d27726016e 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -41,22 +41,17 @@
 # include <cstdlib>   // getenv
 #endif
 
-#ifndef __GTHREADS
-# define USE_ATOMIC_SHARED_PTR 0
-#elif _WIN32
-// std::mutex cannot be constinit, so Windows must use atomic<shared_ptr<>>.
-# define USE_ATOMIC_SHARED_PTR 1
-#elif ATOMIC_POINTER_LOCK_FREE < 2
-# define USE_ATOMIC_SHARED_PTR 0
-#else
-// TODO benchmark atomic<shared_ptr<>> vs mutex.
-# define USE_ATOMIC_SHARED_PTR 1
-#endif
-
 #if defined __GTHREADS && ATOMIC_POINTER_LOCK_FREE == 2
 # define USE_ATOMIC_LIST_HEAD 1
+// TODO benchmark atomic<shared_ptr<>> vs mutex.
+# define USE_ATOMIC_SHARED_PTR 1
 #else
 # define USE_ATOMIC_LIST_HEAD 0
+# define USE_ATOMIC_SHARED_PTR 0
+#endif
+
+#if USE_ATOMIC_SHARED_PTR && ! USE_ATOMIC_LIST_HEAD
+# error Unsupported combination
 #endif
 
 #if ! __cpp_constinit
@@ -106,9 +101,18 @@ namespace std::chrono
     // Dummy no-op mutex type for single-threaded targets.
     struct mutex { void lock() { } void unlock() { } };
 #endif
-    /// XXX std::mutex::mutex() not constexpr on Windows, so can't be constinit
-    constinit mutex list_mutex;
+    inline mutex& list_mutex()
+    {
+#ifdef __GTHREAD_MUTEX_INIT
+      constinit static mutex m;
+#else
+      // Cannot use a constinit mutex, so use a local static.
+      alignas(mutex) constinit static char buf[sizeof(mutex)];
+      static mutex& m = *::new(buf) mutex();
 #endif
+      return m;
+    }
+#endif // ! USE_ATOMIC_SHARED_PTR
 
     struct Rule;
   }
@@ -154,7 +158,7 @@ namespace std::chrono
     static _Node*
     _S_list_head(memory_order)
     {
-      lock_guard<mutex> l(list_mutex);
+      lock_guard<mutex> l(list_mutex());
       return _S_head_owner.get();
     }
 
@@ -1279,7 +1283,7 @@ namespace std::chrono
       }
     // XXX small window here where _S_head_cache still points to previous tzdb.
 #else
-    lock_guard<mutex> l(list_mutex);
+    lock_guard<mutex> l(list_mutex());
     if (const _Node* h = _S_head_owner.get())
       {
 	if (h->db.version == new_head_ptr->db.version)
@@ -1406,11 +1410,12 @@ namespace std::chrono
 #else
     if (Node::_S_list_head(memory_order::relaxed) != nullptr) [[likely]]
     {
-      lock_guard<mutex> l(list_mutex);
+      lock_guard<mutex> l(list_mutex());
       const tzdb& current = Node::_S_head_owner->db;
       if (current.version == version)
 	return current;
     }
+    shared_ptr<Node> head; // Passed as unused arg to _S_replace_head.
 #endif
 
     auto [leaps, leaps_ok] = Node::_S_read_leap_seconds();
@@ -1499,9 +1504,6 @@ namespace std::chrono
     ranges::sort(node->db.links, {}, &time_zone_link::name);
     ranges::stable_sort(node->rules, {}, &Rule::name);
 
-#if ! USE_ATOMIC_SHARED_PTR
-    shared_ptr<Node> head;
-#endif
     return Node::_S_replace_head(std::move(head), std::move(node));
 #else
     __throw_disabled();
@@ -1526,7 +1528,7 @@ namespace std::chrono
 #if USE_ATOMIC_SHARED_PTR
     return const_iterator{_Node::_S_head_owner.load()};
 #else
-    lock_guard<mutex> l(list_mutex);
+    lock_guard<mutex> l(list_mutex());
     return const_iterator{_Node::_S_head_owner};
 #endif
   }
@@ -1539,7 +1541,7 @@ namespace std::chrono
     if (p._M_node) [[likely]]
     {
 #if ! USE_ATOMIC_SHARED_PTR
-      lock_guard<mutex> l(list_mutex);
+      lock_guard<mutex> l(list_mutex());
 #endif
       if (auto next = p._M_node->next) [[likely]]
 	return const_iterator{p._M_node->next = std::move(next->next)};

Re: Full disable of C++20 time zone support

[Alexey Lapshin]

The patch does work. Thank you for a quick fix!