[gcc r13-3707] libstdc++: Move stream initialization into compiled library [PR44952]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:4e4e3ffd10f53ef71696bc728ab40258751a2df4

commit r13-3707-g4e4e3ffd10f53ef71696bc728ab40258751a2df4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sun Nov 6 11:16:00 2022 -0500

    libstdc++: Move stream initialization into compiled library [PR44952]
    
    This patch moves the static object for constructing the standard streams
    out from <iostream> and into the compiled library on systems that support
    init priorities.  This'll mean <iostream> no longer introduces a separate
    global constructor in each TU that includes it.
    
    We can do this only if the init_priority attribute is supported because
    we need a way to ensure the stream initialization runs first before any
    user global initializer, particularly when linking with a static
    libstdc++.a.
    
            PR libstdc++/44952
            PR libstdc++/39796
            PR libstdc++/98108
    
    libstdc++-v3/ChangeLog:
    
            * include/std/iostream (__ioinit): No longer define here if
            the init_priority attribute is usable.
            * src/c++98/ios_init.cc (__ioinit): Define here instead if
            init_priority is usable, via ...
            * src/c++98/ios_base_init.h: ... this new file.

Diff:
---
 libstdc++-v3/include/std/iostream      |  4 ++++
 libstdc++-v3/src/c++98/ios_base_init.h | 12 ++++++++++++
 libstdc++-v3/src/c++98/ios_init.cc     |  2 ++
 3 files changed, 18 insertions(+)

diff --git a/libstdc++-v3/include/std/iostream b/libstdc++-v3/include/std/iostream
index 70318a45891..ff78e1cfb87 100644
--- a/libstdc++-v3/include/std/iostream
+++ b/libstdc++-v3/include/std/iostream
@@ -73,7 +73,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   ///@}
 
   // For construction of filebuffers for cout, cin, cerr, clog et. al.
+  // When the init_priority attribute is usable, we do this initialization
+  // in the compiled library instead (src/c++98/ios_init.cc).
+#if !__has_attribute(__init_priority__)
   static ios_base::Init __ioinit;
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
diff --git a/libstdc++-v3/src/c++98/ios_base_init.h b/libstdc++-v3/src/c++98/ios_base_init.h
new file mode 100644
index 00000000000..1c71038f4a1
--- /dev/null
+++ b/libstdc++-v3/src/c++98/ios_base_init.h
@@ -0,0 +1,12 @@
+// This is only in a header so we can use the system_header pragma,
+// to suppress the warning caused by using a reserved init_priority.
+#pragma GCC system_header
+
+// If the target supports init priorities, set up a static object in the
+// compiled library to perform the <iostream> initialization once and
+// sufficiently early (so that it happens before any other global
+// constructor when statically linking with libstdc++.a), instead of
+// doing so in (each TU that includes) <iostream>.
+#if __has_attribute(init_priority)
+static ios_base::Init __ioinit __attribute__((init_priority(90)));
+#endif
diff --git a/libstdc++-v3/src/c++98/ios_init.cc b/libstdc++-v3/src/c++98/ios_init.cc
index 1b5132f1c2d..4016fcab785 100644
--- a/libstdc++-v3/src/c++98/ios_init.cc
+++ b/libstdc++-v3/src/c++98/ios_init.cc
@@ -75,6 +75,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   extern wostream wclog;
 #endif
 
+#include "ios_base_init.h"
+
   ios_base::Init::Init()
   {
     if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0)