libstdc++: Disable cacheline alignment for DJGPP [PR109741]

DJGPP (and maybe other targets) uses MAX_OFILE_ALIGNMENT=16 which means
that globals (and static objects) can't have alignment greater than 16.
This causes an error for the locks defined in src/c++11/shared_ptr.cc
because we try to align them to the cacheline size, to avoid false
sharing.

Add a configure check for the increased alignment, and live with false
sharing where we can't increase the alignment.

libstdc++-v3/ChangeLog:

	PR libstdc++/109741
	* acinclude.m4 (GLIBCXX_CHECK_ALIGNAS_CACHELINE): Define.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_CHECK_ALIGNAS_CACHELINE.
	* src/c++11/shared_ptr.cc (__gnu_internal::get_mutex): Do not
	align lock table if not supported. use __GCC_DESTRUCTIVE_SIZE
	instead of hardcoded 64.

libstdc++-v3/acinclude.m4

@@ -5471,6 +5471,31 @@ AC_DEFUN([GLIBCXX_ZONEINFO_DIR], [
   fi
 ])
 
+dnl
+dnl Check whether lock tables can be aligned to avoid false sharing.
+dnl
+dnl Defines:
+dnl  _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE if objects with static storage
+dnl    duration can be aligned to std::hardware_destructive_interference_size.
+dnl
+AC_DEFUN([GLIBCXX_CHECK_ALIGNAS_CACHELINE], [
+  AC_LANG_SAVE
+  AC_LANG_CPLUSPLUS
+
+  AC_MSG_CHECKING([whether static objects can be aligned to the cacheline size])
+  AC_TRY_COMPILE(, [struct alignas(__GCC_DESTRUCTIVE_SIZE) Aligned { };
+		    alignas(Aligned) static char buf[sizeof(Aligned) * 16];
+		 ], [ac_alignas_cacheline=yes], [ac_alignas_cacheline=no])
+  if test "$ac_alignas_cacheline" = yes; then
+    AC_DEFINE_UNQUOTED(_GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE, 1,
+      [Define if global objects can be aligned to
+       std::hardware_destructive_interference_size.])
+  fi
+  AC_MSG_RESULT($ac_alignas_cacheline)
+
+  AC_LANG_RESTORE
+])
+
 # Macros from the top-level gcc directory.
 m4_include([../config/gc++filt.m4])
 m4_include([../config/tls.m4])

libstdc++-v3/config.h.in

@@ -819,6 +819,10 @@
 /* Define if the compiler supports C++11 atomics. */
 #undef _GLIBCXX_ATOMIC_BUILTINS
 
+/* Define if global objects can be aligned to
+   std::hardware_destructive_interference_size. */
+#undef _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE
+
 /* Define to use concept checking code from the boost libraries. */
 #undef _GLIBCXX_CONCEPT_CHECKS
 

libstdc++-v3/configure

@@ -71957,6 +71957,54 @@ _ACEOF
   fi
 
 
+
+
+  ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether static objects can be aligned to the cacheline size" >&5
+$as_echo_n "checking whether static objects can be aligned to the cacheline size... " >&6; }
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+struct alignas(__GCC_DESTRUCTIVE_SIZE) Aligned { };
+		    alignas(Aligned) static char buf[sizeof(Aligned) * 16];
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_alignas_cacheline=yes
+else
+  ac_alignas_cacheline=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  if test "$ac_alignas_cacheline" = yes; then
+
+cat >>confdefs.h <<_ACEOF
+#define _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE 1
+_ACEOF
+
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_alignas_cacheline" >&5
+$as_echo "$ac_alignas_cacheline" >&6; }
+
+  ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+
+
 # Define documentation rules conditionally.
 
 # See if makeinfo has been installed and is modern enough

libstdc++-v3/configure.ac

@@ -538,6 +538,9 @@ GLIBCXX_EMERGENCY_EH_ALLOC
 # For src/c++20/tzdb.cc defaults.
 GLIBCXX_ZONEINFO_DIR
 
+# For src/c++11/shared_ptr.cc alignment.
+GLIBCXX_CHECK_ALIGNAS_CACHELINE
+
 # Define documentation rules conditionally.
 
 # See if makeinfo has been installed and is modern enough

libstdc++-v3/src/c++11/shared_ptr.cc

@@ -34,8 +34,12 @@ namespace __gnu_internal _GLIBCXX_VISIBILITY(hidden)
   __gnu_cxx::__mutex&
   get_mutex(unsigned char i)
   {
-    // increase alignment to put each lock on a separate cache line
-    struct alignas(64) M : __gnu_cxx::__mutex { };
+#ifdef _GLIBCXX_CAN_ALIGNAS_DESTRUCTIVE_SIZE
+    // Increase alignment to put each lock on a separate cache line.
+    struct alignas(__GCC_DESTRUCTIVE_SIZE) M : __gnu_cxx::__mutex { };
+#else
+    using M = __gnu_cxx::__mutex;
+#endif
     // Use a static buffer, so that the mutexes are not destructed
     // before potential users (or at all)
     static __attribute__ ((aligned(__alignof__(M))))