[PATCH 2/2] libstdc++: Move stream initialization into compiled library [PR44952]

[Patrick Palka <ppalka@redhat.com>]

This patch moves the global object for constructing the standard streams
out from <iostream> and into the compiled library on targets that support
the init_priority attribute.  This means that <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 to force that the stream initialization runs first before any
user-defined global initializer in programs that that use a static
libstdc++.a.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look right?
Unfortunately I don't have access to a system that truly doesn't support
init priorities, so I instead tested that situation by artificially
disabling the init_priority attribute on x86_64.

	PR libstdc++/44952
	PR libstdc++/98108

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_HAS_ATTRIBUTE): Define.
	(_GLIBCXX_HAVE_ATTRIBUTE_INIT_PRIORITY): Define.
	* include/std/iostream (__ioinit): Define only if init_priority
	attribute isn't usable.
	* src/c++98/ios_init.cc (__ioinit): Define here instead if
	the init_priority is usable.
	* src/c++98/ios_base_init.h: New file.
---
 libstdc++-v3/include/bits/c++config    | 12 ++++++++++++
 libstdc++-v3/include/std/iostream      |  4 ++++
 libstdc++-v3/src/c++98/ios_base_init.h |  9 +++++++++
 libstdc++-v3/src/c++98/ios_init.cc     |  4 ++++
 4 files changed, 29 insertions(+)
 create mode 100644 libstdc++-v3/src/c++98/ios_base_init.h

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 50406066afe..f93076191d9 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -837,6 +837,18 @@ namespace __gnu_cxx
 
 #undef _GLIBCXX_HAS_BUILTIN
 
+#ifdef __has_attribute
+# define _GLIBCXX_HAS_ATTRIBUTE(B) __has_attribute(B)
+#else
+# define _GLIBCXX_HAS_ATTRIBUTE(B) 0
+#endif
+
+#if _GLIBCXX_HAS_ATTRIBUTE(init_priority)
+# define _GLIBCXX_HAVE_ATTRIBUTE_INIT_PRIORITY 1
+#endif
+
+#undef _GLIBCXX_HAS_ATTRIBUTE
+
 // Mark code that should be ignored by the compiler, but seen by Doxygen.
 #define _GLIBCXX_DOXYGEN_ONLY(X)
 
diff --git a/libstdc++-v3/include/std/iostream b/libstdc++-v3/include/std/iostream
index 70318a45891..5eaa9755d9a 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 (see src/c++98/ios_init.cc).
+#if !_GLIBCXX_HAVE_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..f3087d1da3c
--- /dev/null
+++ b/libstdc++-v3/src/c++98/ios_base_init.h
@@ -0,0 +1,9 @@
+// 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 !_GLIBCXX_HAVE_ATTRIBUTE_INIT_PRIORITY
+# error "This file should not be included for this build"
+#endif
+
+static ios_base::Init __ioinit __attribute__((init_priority(90)));
diff --git a/libstdc++-v3/src/c++98/ios_init.cc b/libstdc++-v3/src/c++98/ios_init.cc
index 1b5132f1c2d..954fa9f29cf 100644
--- a/libstdc++-v3/src/c++98/ios_init.cc
+++ b/libstdc++-v3/src/c++98/ios_init.cc
@@ -75,6 +75,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   extern wostream wclog;
 #endif
 
+#if _GLIBCXX_HAVE_ATTRIBUTE_INIT_PRIORITY
+# include "ios_base_init.h"
+#endif
+
   ios_base::Init::Init()
   {
     if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0)
-- 
2.38.1.385.g3b08839926